marlin/Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

/**
 * Marlin 3D Printer Firmware
 * Copyright (c) 2020 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
 *
 * Based on Sprinter and grbl.
 * Copyright (c) 2011 Camiel Gubbels / Erik van der Zalm
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 */

#include "../../../inc/MarlinConfig.h"

#if ENABLED(AUTO_BED_LEVELING_UBL)

  #include "../bedlevel.h"

  #include "../../../MarlinCore.h"
  #include "../../../HAL/shared/eeprom_api.h"
  #include "../../../libs/hex_print.h"
  #include "../../../module/settings.h"
  #include "../../../lcd/ultralcd.h"
  #include "../../../module/stepper.h"
  #include "../../../module/planner.h"
  #include "../../../module/motion.h"
  #include "../../../module/probe.h"
  #include "../../../gcode/gcode.h"
  #include "../../../libs/least_squares_fit.h"

  #if HAS_MULTI_HOTEND
    #include "../../../module/tool_change.h"
  #endif

  #define DEBUG_OUT ENABLED(DEBUG_LEVELING_FEATURE)
  #include "../../../core/debug_out.h"

  #if ENABLED(EXTENSIBLE_UI)
    #include "../../../lcd/extui/ui_api.h"
  #endif

  #include <math.h>

  #define UBL_G29_P31

  #if HAS_LCD_MENU

    bool unified_bed_leveling::lcd_map_control = false;

    void unified_bed_leveling::steppers_were_disabled() {
      if (lcd_map_control) {
        lcd_map_control = false;
        ui.defer_status_screen(false);
      }
    }

    void ubl_map_screen();

  #endif

  #define SIZE_OF_LITTLE_RAISE 1
  #define BIG_RAISE_NOT_NEEDED 0

  int    unified_bed_leveling::g29_verbose_level,
         unified_bed_leveling::g29_phase_value,
         unified_bed_leveling::g29_repetition_cnt,
         unified_bed_leveling::g29_storage_slot = 0,
         unified_bed_leveling::g29_map_type;
  bool   unified_bed_leveling::g29_c_flag;
  float  unified_bed_leveling::g29_card_thickness = 0,
         unified_bed_leveling::g29_constant = 0;
  xy_bool_t unified_bed_leveling::xy_seen;
  xy_pos_t unified_bed_leveling::g29_pos;

  #if HAS_BED_PROBE
    int  unified_bed_leveling::g29_grid_size;
  #endif

  /**
   *   G29: Unified Bed Leveling by Roxy
   *
   *   Parameters understood by this leveling system:
   *
   *   A     Activate   Activate the Unified Bed Leveling system.
   *
   *   B #   Business   Use the 'Business Card' mode of the Manual Probe subsystem with P2.
   *                    Note: A non-compressible Spark Gap feeler gauge is recommended over a business card.
   *                    In this mode of G29 P2, a business or index card is used as a shim that the nozzle can
   *                    grab onto as it is lowered. In principle, the nozzle-bed distance is the same when the
   *                    same resistance is felt in the shim. You can omit the numerical value on first invocation
   *                    of G29 P2 B to measure shim thickness. Subsequent use of 'B' will apply the previously-
   *                    measured thickness by default.
   *
   *   C     Continue   G29 P1 C continues the generation of a partially-constructed Mesh without invalidating
   *                    previous measurements.
   *
   *   C                G29 P2 C tells the Manual Probe subsystem to not use the current nozzle
   *                    location in its search for the closest unmeasured Mesh Point. Instead, attempt to
   *                    start at one end of the uprobed points and Continue sequentially.
   *
   *                    G29 P3 C specifies the Constant for the fill. Otherwise, uses a "reasonable" value.
   *
   *   C     Current    G29 Z C uses the Current location (instead of bed center or nearest edge).
   *
   *   D     Disable    Disable the Unified Bed Leveling system.
   *
   *   E     Stow_probe Stow the probe after each sampled point.
   *
   *   F #   Fade       Fade the amount of Mesh Based Compensation over a specified height. At the
   *                    specified height, no correction is applied and natural printer kenimatics take over. If no
   *                    number is specified for the command, 10mm is assumed to be reasonable.
   *
   *   H #   Height     With P2, 'H' specifies the Height to raise the nozzle after each manual probe of the bed.
   *                    If omitted, the nozzle will raise by Z_CLEARANCE_BETWEEN_PROBES.
   *
   *   H #   Offset     With P4, 'H' specifies the Offset above the mesh height to place the nozzle.
   *                    If omitted, Z_CLEARANCE_BETWEEN_PROBES will be used.
   *
   *   I #   Invalidate Invalidate the specified number of Mesh Points near the given 'X' 'Y'. If X or Y are omitted,
   *                    the nozzle location is used. If no 'I' value is given, only the point nearest to the location
   *                    is invalidated. Use 'T' to produce a map afterward. This command is useful to invalidate a
   *                    portion of the Mesh so it can be adjusted using other UBL tools. When attempting to invalidate
   *                    an isolated bad mesh point, the 'T' option shows the nozzle position in the Mesh with (#). You
   *                    can move the nozzle around and use this feature to select the center of the area (or cell) to
   *                    invalidate.
   *
   *   J #   Grid       Perform a Grid Based Leveling of the current Mesh using a grid with n points on a side.
   *                    Not specifying a grid size will invoke the 3-Point leveling function.
   *
   *   L     Load       Load Mesh from the previously activated location in the EEPROM.
   *
   *   L #   Load       Load Mesh from the specified location in the EEPROM. Set this location as activated
   *                    for subsequent Load and Store operations.
   *
   *   The P or Phase commands are used for the bulk of the work to setup a Mesh. In general, your Mesh will
   *   start off being initialized with a G29 P0 or a G29 P1. Further refinement of the Mesh happens with
   *   each additional Phase that processes it.
   *
   *   P0    Phase 0    Zero Mesh Data and turn off the Mesh Compensation System. This reverts the
   *                    3D Printer to the same state it was in before the Unified Bed Leveling Compensation
   *                    was turned on. Setting the entire Mesh to Zero is a special case that allows
   *                    a subsequent G or T leveling operation for backward compatibility.
   *
   *   P1    Phase 1    Invalidate entire Mesh and continue with automatic generation of the Mesh data using
   *                    the Z-Probe. Usually the probe can't reach all areas that the nozzle can reach. For delta
   *                    printers only the areas where the probe and nozzle can both reach will be automatically probed.
   *
   *                    Unreachable points will be handled in Phase 2 and Phase 3.
   *
   *                    Use 'C' to leave the previous mesh intact and automatically probe needed points. This allows you
   *                    to invalidate parts of the Mesh but still use Automatic Probing.
   *
   *                    The 'X' and 'Y' parameters prioritize where to try and measure points. If omitted, the current
   *                    probe position is used.
   *
   *                    Use 'T' (Topology) to generate a report of mesh generation.
   *
   *                    P1 will suspend Mesh generation if the controller button is held down. Note that you may need
   *                    to press and hold the switch for several seconds if moves are underway.
   *
   *   P2    Phase 2    Probe unreachable points.
   *
   *                    Use 'H' to set the height between Mesh points. If omitted, Z_CLEARANCE_BETWEEN_PROBES is used.
   *                    Smaller values will be quicker. Move the nozzle down till it barely touches the bed. Make sure the
   *                    nozzle is clean and unobstructed. Use caution and move slowly. This can damage your printer!
   *                    (Uses SIZE_OF_LITTLE_RAISE mm if the nozzle is moving less than BIG_RAISE_NOT_NEEDED mm.)
   *
   *                    The 'H' value can be negative if the Mesh dips in a large area. Press and hold the
   *                    controller button to terminate the current Phase 2 command. You can then re-issue "G29 P 2"
   *                    with an 'H' parameter more suitable for the area you're manually probing. Note that the command
   *                    tries to start in a corner of the bed where movement will be predictable. Override the distance
   *                    calculation location with the X and Y parameters. You can print a Mesh Map (G29 T) to see where
   *                    the mesh is invalidated and where the nozzle needs to move to complete the command. Use 'C' to
   *                    indicate that the search should be based on the current position.
   *
   *                    The 'B' parameter for this command is described above. It places the manual probe subsystem into
   *                    Business Card mode where the thickness of a business card is measured and then used to accurately
   *                    set the nozzle height in all manual probing for the duration of the command. A Business card can
   *                    be used, but you'll get better results with a flexible Shim that doesn't compress. This makes it
   *                    easier to produce similar amounts of force and get more accurate measurements. Google if you're
   *                    not sure how to use a shim.
   *
   *                    The 'T' (Map) parameter helps track Mesh building progress.
   *
   *                    NOTE: P2 requires an LCD controller!
   *
   *   P3    Phase 3    Fill the unpopulated regions of the Mesh with a fixed value. There are two different paths to
   *                    go down:
   *
   *                    - If a 'C' constant is specified, the closest invalid mesh points to the nozzle will be filled,
   *                      and a repeat count can then also be specified with 'R'.
   *
   *                    - Leaving out 'C' invokes Smart Fill, which scans the mesh from the edges inward looking for
   *                      invalid mesh points. Adjacent points are used to determine the bed slope. If the bed is sloped
   *                      upward from the invalid point, it takes the value of the nearest point. If sloped downward, it's
   *                      replaced by a value that puts all three points in a line. This version of G29 P3 is a quick, easy
   *                      and (usually) safe way to populate unprobed mesh regions before continuing to G26 Mesh Validation
   *                      Pattern. Note that this populates the mesh with unverified values. Pay attention and use caution.
   *
   *   P4    Phase 4    Fine tune the Mesh. The Delta Mesh Compensation System assumes the existence of
   *                    an LCD Panel. It is possible to fine tune the mesh without an LCD Panel using
   *                    G42 and M421. See the UBL documentation for further details.
   *
   *                    Phase 4 is meant to be used with G26 Mesh Validation to fine tune the mesh by direct editing
   *                    of Mesh Points. Raise and lower points to fine tune the mesh until it gives consistently reliable
   *                    adhesion.
   *
   *                    P4 moves to the closest Mesh Point (and/or the given X Y), raises the nozzle above the mesh height
   *                    by the given 'H' offset (or default 0), and waits while the controller is used to adjust the nozzle
   *                    height. On click the displayed height is saved in the mesh.
   *
   *                    Start Phase 4 at a specific location with X and Y. Adjust a specific number of Mesh Points with
   *                    the 'R' (Repeat) parameter. (If 'R' is left out, the whole matrix is assumed.) This command can be
   *                    terminated early (e.g., after editing the area of interest) by pressing and holding the encoder button.
   *
   *                    The general form is G29 P4 [R points] [X position] [Y position]
   *
   *                    The H [offset] parameter is useful if a shim is used to fine-tune the mesh. For a 0.4mm shim the
   *                    command would be G29 P4 H0.4. The nozzle is moved to the shim height, you adjust height to the shim,
   *                    and on click the height minus the shim thickness will be saved in the mesh.
   *
   *                    !!Use with caution, as a very poor mesh could cause the nozzle to crash into the bed!!
   *
   *                    NOTE:  P4 is not available unless you have LCD support enabled!
   *
   *   P5    Phase 5    Find Mean Mesh Height and Standard Deviation. Typically, it is easier to use and
   *                    work with the Mesh if it is Mean Adjusted. You can specify a C parameter to
   *                    Correct the Mesh to a 0.00 Mean Height. Adding a C parameter will automatically
   *                    execute a G29 P6 C <mean height>.
   *
   *   P6    Phase 6    Shift Mesh height. The entire Mesh's height is adjusted by the height specified
   *                    with the C parameter. Being able to adjust the height of a Mesh is useful tool. It
   *                    can be used to compensate for poorly calibrated Z-Probes and other errors. Ideally,
   *                    you should have the Mesh adjusted for a Mean Height of 0.00 and the Z-Probe measuring
   *                    0.000 at the Z Home location.
   *
   *   Q     Test       Load specified Test Pattern to assist in checking correct operation of system. This
   *                    command is not anticipated to be of much value to the typical user. It is intended
   *                    for developers to help them verify correct operation of the Unified Bed Leveling System.
   *
   *   R #   Repeat     Repeat this command the specified number of times. If no number is specified the
   *                    command will be repeated GRID_MAX_POINTS_X * GRID_MAX_POINTS_Y times.
   *
   *   S     Store      Store the current Mesh in the Activated area of the EEPROM. It will also store the
   *                    current state of the Unified Bed Leveling system in the EEPROM.
   *
   *   S #   Store      Store the current Mesh at the specified location in EEPROM. Activate this location
   *                    for subsequent Load and Store operations. Valid storage slot numbers begin at 0 and
   *                    extend to a limit related to the available EEPROM storage.
   *
   *   S -1  Store      Print the current Mesh as G-code that can be used to restore the mesh anytime.
   *
   *   T     Topology   Display the Mesh Map Topology.
   *                    'T' can be used alone (e.g., G29 T) or in combination with most of the other commands.
   *                    This option works with all Phase commands (e.g., G29 P4 R 5 T X 50 Y100 C -.1 O)
   *                    This parameter can also specify a Map Type. T0 (the default) is user-readable. T1
   *                    is suitable to paste into a spreadsheet for a 3D graph of the mesh.
   *
   *   U     Unlevel    Perform a probe of the outer perimeter to assist in physically leveling unlevel beds.
   *                    Only used for G29 P1 T U. This speeds up the probing of the edge of the bed. Useful
   *                    when the entire bed doesn't need to be probed because it will be adjusted.
   *
   *   V #   Verbosity  Set the verbosity level (0-4) for extra details. (Default 0)
   *
   *   X #              X Location for this command
   *
   *   Y #              Y Location for this command
   *
   * With UBL_DEVEL_DEBUGGING:
   *
   *   K #  Kompare     Kompare current Mesh with stored Mesh #, replacing current Mesh with the result.
   *                    This command literally performs a diff between two Meshes.
   *
   *   Q-1  Dump EEPROM Dump the UBL contents stored in EEPROM as HEX format. Useful for developers to help
   *                    verify correct operation of the UBL.
   *
   *   W    What?       Display valuable UBL data.
   *
   *
   *   Release Notes:
   *   You MUST do M502, M500 to initialize the storage. Failure to do this will cause all
   *   kinds of problems. Enabling EEPROM Storage is required.
   *
   *   When you do a G28 and G29 P1 to automatically build your first mesh, you are going to notice that
   *   UBL probes points increasingly further from the starting location. (The starting location defaults
   *   to the center of the bed.) In contrast, ABL and MBL follow a zigzag pattern. The spiral pattern is
   *   especially better for Delta printers, since it populates the center of the mesh first, allowing for
   *   a quicker test print to verify settings. You don't need to populate the entire mesh to use it.
   *   After all, you don't want to spend a lot of time generating a mesh only to realize the resolution
   *   or probe offsets are incorrect. Mesh-generation gathers points starting closest to the nozzle unless
   *   an (X,Y) coordinate pair is given.
   *
   *   Unified Bed Leveling uses a lot of EEPROM storage to hold its data, and it takes some effort to get
   *   the mesh just right. To prevent this valuable data from being destroyed as the EEPROM structure
   *   evolves, UBL stores all mesh data at the end of EEPROM.
   *
   *   UBL is founded on Edward Patel's Mesh Bed Leveling code. A big 'Thanks!' to him and the creators of
   *   3-Point and Grid Based leveling. Combining their contributions we now have the functionality and
   *   features of all three systems combined.
   */

  void unified_bed_leveling::G29() {

    bool probe_deployed = false;
    if (g29_parameter_parsing()) return; // Abort on parameter error

    const int8_t p_val = parser.intval('P', -1);
    const bool may_move = p_val == 1 || p_val == 2 || p_val == 4 || parser.seen('J');
    TERN_(HAS_MULTI_HOTEND, const uint8_t old_tool_index = active_extruder);

    // Check for commands that require the printer to be homed
    if (may_move) {
      planner.synchronize();
      if (axes_should_home()) gcode.home_all_axes();
      TERN_(HAS_MULTI_HOTEND, if (active_extruder) tool_change(0));
    }

    // Invalidate Mesh Points. This command is a little bit asymmetrical because
    // it directly specifies the repetition count and does not use the 'R' parameter.
    if (parser.seen('I')) {
      uint8_t cnt = 0;
      g29_repetition_cnt = parser.has_value() ? parser.value_int() : 1;
      if (g29_repetition_cnt >= GRID_MAX_POINTS) {
        set_all_mesh_points_to_value(NAN);
      }
      else {
        while (g29_repetition_cnt--) {
          if (cnt > 20) { cnt = 0; idle(); }
          const mesh_index_pair closest = find_closest_mesh_point_of_type(REAL, g29_pos);
          const xy_int8_t &cpos = closest.pos;
          if (cpos.x < 0) {
            // No more REAL mesh points to invalidate, so we ASSUME the user
            // meant to invalidate the ENTIRE mesh, which cannot be done with
            // find_closest_mesh_point loop which only returns REAL points.
            set_all_mesh_points_to_value(NAN);
            SERIAL_ECHOLNPGM("Entire Mesh invalidated.\n");
            break;            // No more invalid Mesh Points to populate
          }
          z_values[cpos.x][cpos.y] = NAN;
          TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(cpos, 0.0f));
          cnt++;
        }
      }
      SERIAL_ECHOLNPGM("Locations invalidated.\n");
    }

    if (parser.seen('Q')) {
      const int test_pattern = parser.has_value() ? parser.value_int() : -99;
      if (!WITHIN(test_pattern, -1, 2)) {
        SERIAL_ECHOLNPGM("Invalid test_pattern value. (-1 to 2)\n");
        return;
      }
      SERIAL_ECHOLNPGM("Loading test_pattern values.\n");
      switch (test_pattern) {

        #if ENABLED(UBL_DEVEL_DEBUGGING)
          case -1:
            g29_eeprom_dump();
            break;
        #endif

        case 0:
          GRID_LOOP(x, y) {                                     // Create a bowl shape similar to a poorly-calibrated Delta
            const float p1 = 0.5f * (GRID_MAX_POINTS_X) - x,
                        p2 = 0.5f * (GRID_MAX_POINTS_Y) - y;
            z_values[x][y] += 2.0f * HYPOT(p1, p2);
            TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
          }
          break;

        case 1:
          LOOP_L_N(x, GRID_MAX_POINTS_X) {                     // Create a diagonal line several Mesh cells thick that is raised
            z_values[x][x] += 9.999f;
            z_values[x][x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1] += 9.999f; // We want the altered line several mesh points thick
            #if ENABLED(EXTENSIBLE_UI)
              ExtUI::onMeshUpdate(x, x, z_values[x][x]);
              ExtUI::onMeshUpdate(x, (x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1), z_values[x][x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1]);
            #endif

          }
          break;

        case 2:
          // Allow the user to specify the height because 10mm is a little extreme in some cases.
          for (uint8_t x = (GRID_MAX_POINTS_X) / 3; x < 2 * (GRID_MAX_POINTS_X) / 3; x++)     // Create a rectangular raised area in
            for (uint8_t y = (GRID_MAX_POINTS_Y) / 3; y < 2 * (GRID_MAX_POINTS_Y) / 3; y++) { // the center of the bed
              z_values[x][y] += parser.seen('C') ? g29_constant : 9.99f;
              TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
            }
          break;
      }
    }

    #if HAS_BED_PROBE

      if (parser.seen('J')) {
        save_ubl_active_state_and_disable();
        tilt_mesh_based_on_probed_grid(g29_grid_size == 0); // Zero size does 3-Point
        restore_ubl_active_state_and_leave();
        #if ENABLED(UBL_G29_J_RECENTER)
          do_blocking_move_to_xy(0.5f * ((MESH_MIN_X) + (MESH_MAX_X)), 0.5f * ((MESH_MIN_Y) + (MESH_MAX_Y)));
        #endif
        report_current_position();
        probe_deployed = true;
      }

    #endif // HAS_BED_PROBE

    if (parser.seen('P')) {
      if (WITHIN(g29_phase_value, 0, 1) && storage_slot == -1) {
        storage_slot = 0;
        SERIAL_ECHOLNPGM("Default storage slot 0 selected.");
      }

      switch (g29_phase_value) {
        case 0:
          //
          // Zero Mesh Data
          //
          reset();
          SERIAL_ECHOLNPGM("Mesh zeroed.");
          break;

        #if HAS_BED_PROBE

          case 1: {
            //
            // Invalidate Entire Mesh and Automatically Probe Mesh in areas that can be reached by the probe
            //
            if (!parser.seen('C')) {
              invalidate();
              SERIAL_ECHOLNPGM("Mesh invalidated. Probing mesh.");
            }
            if (g29_verbose_level > 1) {
              SERIAL_ECHOPAIR("Probing around (", g29_pos.x);
              SERIAL_CHAR(',');
              SERIAL_DECIMAL(g29_pos.y);
              SERIAL_ECHOLNPGM(").\n");
            }
            const xy_pos_t near_probe_xy = g29_pos + probe.offset_xy;
            probe_entire_mesh(near_probe_xy, parser.seen('T'), parser.seen('E'), parser.seen('U'));

            report_current_position();
            probe_deployed = true;
          } break;

        #endif // HAS_BED_PROBE

        case 2: {
          #if HAS_LCD_MENU
            //
            // Manually Probe Mesh in areas that can't be reached by the probe
            //
            SERIAL_ECHOLNPGM("Manually probing unreachable points.");
            do_z_clearance(Z_CLEARANCE_BETWEEN_PROBES);

            if (parser.seen('C') && !xy_seen) {

              /**
               * Use a good default location for the path.
               * The flipped > and < operators in these comparisons is intentional.
               * It should cause the probed points to follow a nice path on Cartesian printers.
               * It may make sense to have Delta printers default to the center of the bed.
               * Until that is decided, this can be forced with the X and Y parameters.
               */
              g29_pos.set(
                #if IS_KINEMATIC
                  X_HOME_POS, Y_HOME_POS
                #else
                  probe.offset_xy.x > 0 ? X_BED_SIZE : 0,
                  probe.offset_xy.y < 0 ? Y_BED_SIZE : 0
                #endif
              );
            }

            if (parser.seen('B')) {
              g29_card_thickness = parser.has_value() ? parser.value_float() : measure_business_card_thickness(float(Z_CLEARANCE_BETWEEN_PROBES));
              if (ABS(g29_card_thickness) > 1.5f) {
                SERIAL_ECHOLNPGM("?Error in Business Card measurement.");
                return;
              }
              probe_deployed = true;
            }

            if (!position_is_reachable(g29_pos)) {
              SERIAL_ECHOLNPGM("XY outside printable radius.");
              return;
            }

            const float height = parser.floatval('H', Z_CLEARANCE_BETWEEN_PROBES);
            manually_probe_remaining_mesh(g29_pos, height, g29_card_thickness, parser.seen('T'));

            SERIAL_ECHOLNPGM("G29 P2 finished.");

            report_current_position();

          #else

            SERIAL_ECHOLNPGM("?P2 is only available when an LCD is present.");
            return;

          #endif
        } break;

        case 3: {
          /**
           * Populate invalid mesh areas. Proceed with caution.
           * Two choices are available:
           *   - Specify a constant with the 'C' parameter.
           *   - Allow 'G29 P3' to choose a 'reasonable' constant.
           */

          if (g29_c_flag) {
            if (g29_repetition_cnt >= GRID_MAX_POINTS) {
              set_all_mesh_points_to_value(g29_constant);
            }
            else {
              while (g29_repetition_cnt--) {  // this only populates reachable mesh points near
                const mesh_index_pair closest = find_closest_mesh_point_of_type(INVALID, g29_pos);
                const xy_int8_t &cpos = closest.pos;
                if (cpos.x < 0) {
                  // No more REAL INVALID mesh points to populate, so we ASSUME
                  // user meant to populate ALL INVALID mesh points to value
                  GRID_LOOP(x, y) if (isnan(z_values[x][y])) z_values[x][y] = g29_constant;
                  break; // No more invalid Mesh Points to populate
                }
                else {
                  z_values[cpos.x][cpos.y] = g29_constant;
                  TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(cpos, g29_constant));
                }
              }
            }
          }
          else {
            const float cvf = parser.value_float();
            switch ((int)truncf(cvf * 10.0f) - 30) {   // 3.1 -> 1
              #if ENABLED(UBL_G29_P31)
                case 1: {

                  // P3.1  use least squares fit to fill missing mesh values
                  // P3.10 zero weighting for distance, all grid points equal, best fit tilted plane
                  // P3.11 10X weighting for nearest grid points versus farthest grid points
                  // P3.12 100X distance weighting
                  // P3.13 1000X distance weighting, approaches simple average of nearest points

                  const float weight_power  = (cvf - 3.10f) * 100.0f,  // 3.12345 -> 2.345
                              weight_factor = weight_power ? POW(10.0f, weight_power) : 0;
                  smart_fill_wlsf(weight_factor);
                }
                break;
              #endif
              case 0:   // P3 or P3.0
              default:  // and anything P3.x that's not P3.1
                smart_fill_mesh();  // Do a 'Smart' fill using nearby known values
                break;
            }
          }
          break;
        }

        case 4: // Fine Tune (i.e., Edit) the Mesh
          #if HAS_LCD_MENU
            fine_tune_mesh(g29_pos, parser.seen('T'));
          #else
            SERIAL_ECHOLNPGM("?P4 is only available when an LCD is present.");
            return;
          #endif
          break;

        case 5: adjust_mesh_to_mean(g29_c_flag, g29_constant); break;

        case 6: shift_mesh_height(); break;
      }
    }

    #if ENABLED(UBL_DEVEL_DEBUGGING)

      //
      // Much of the 'What?' command can be eliminated. But until we are fully debugged, it is
      // good to have the extra information. Soon... we prune this to just a few items
      //
      if (parser.seen('W')) g29_what_command();

      //
      // When we are fully debugged, this may go away. But there are some valid
      // use cases for the users. So we can wait and see what to do with it.
      //

      if (parser.seen('K')) // Kompare Current Mesh Data to Specified Stored Mesh
        g29_compare_current_mesh_to_stored_mesh();

    #endif // UBL_DEVEL_DEBUGGING


    //
    // Load a Mesh from the EEPROM
    //

    if (parser.seen('L')) {     // Load Current Mesh Data
      g29_storage_slot = parser.has_value() ? parser.value_int() : storage_slot;

      int16_t a = settings.calc_num_meshes();

      if (!a) {
        SERIAL_ECHOLNPGM("?EEPROM storage not available.");
        return;
      }

      if (!WITHIN(g29_storage_slot, 0, a - 1)) {
        SERIAL_ECHOLNPAIR("?Invalid storage slot.\n?Use 0 to ", a - 1);
        return;
      }

      settings.load_mesh(g29_storage_slot);
      storage_slot = g29_storage_slot;

      SERIAL_ECHOLNPGM("Done.");
    }

    //
    // Store a Mesh in the EEPROM
    //

    if (parser.seen('S')) {     // Store (or Save) Current Mesh Data
      g29_storage_slot = parser.has_value() ? parser.value_int() : storage_slot;

      if (g29_storage_slot == -1)                     // Special case, the user wants to 'Export' the mesh to the
        return report_current_mesh();                 // host program to be saved on the user's computer

      int16_t a = settings.calc_num_meshes();

      if (!a) {
        SERIAL_ECHOLNPGM("?EEPROM storage not available.");
        goto LEAVE;
      }

      if (!WITHIN(g29_storage_slot, 0, a - 1)) {
        SERIAL_ECHOLNPAIR("?Invalid storage slot.\n?Use 0 to ", a - 1);
        goto LEAVE;
      }

      settings.store_mesh(g29_storage_slot);
      storage_slot = g29_storage_slot;

      SERIAL_ECHOLNPGM("Done.");
    }

    if (parser.seen('T'))
      display_map(g29_map_type);

    LEAVE:

    #if HAS_LCD_MENU
      ui.reset_alert_level();
      ui.quick_feedback();
      ui.reset_status();
      ui.release();
    #endif

    #ifdef Z_PROBE_END_SCRIPT
      if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR("Z Probe End Script: ", Z_PROBE_END_SCRIPT);
      if (probe_deployed) {
        planner.synchronize();
        gcode.process_subcommands_now_P(PSTR(Z_PROBE_END_SCRIPT));
      }
    #else
      UNUSED(probe_deployed);
    #endif

    TERN_(HAS_MULTI_HOTEND, tool_change(old_tool_index));
    return;
  }

  void unified_bed_leveling::adjust_mesh_to_mean(const bool cflag, const float value) {
    float sum = 0;
    int n = 0;
    GRID_LOOP(x, y)
      if (!isnan(z_values[x][y])) {
        sum += z_values[x][y];
        n++;
      }

    const float mean = sum / n;

    //
    // Sum the squares of difference from mean
    //
    float sum_of_diff_squared = 0;
    GRID_LOOP(x, y)
      if (!isnan(z_values[x][y]))
        sum_of_diff_squared += sq(z_values[x][y] - mean);

    SERIAL_ECHOLNPAIR("# of samples: ", n);
    SERIAL_ECHOLNPAIR_F("Mean Mesh Height: ", mean, 6);

    const float sigma = SQRT(sum_of_diff_squared / (n + 1));
    SERIAL_ECHOLNPAIR_F("Standard Deviation: ", sigma, 6);

    if (cflag)
      GRID_LOOP(x, y)
        if (!isnan(z_values[x][y])) {
          z_values[x][y] -= mean + value;
          TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
        }
  }

  void unified_bed_leveling::shift_mesh_height() {
    GRID_LOOP(x, y)
      if (!isnan(z_values[x][y])) {
        z_values[x][y] += g29_constant;
        TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
      }
  }

  #if HAS_BED_PROBE
    /**
     * Probe all invalidated locations of the mesh that can be reached by the probe.
     * This attempts to fill in locations closest to the nozzle's start location first.
     */
    void unified_bed_leveling::probe_entire_mesh(const xy_pos_t &near, const bool do_ubl_mesh_map, const bool stow_probe, const bool do_furthest) {
      probe.deploy(); // Deploy before ui.capture() to allow for PAUSE_BEFORE_DEPLOY_STOW

      TERN_(HAS_LCD_MENU, ui.capture());

      save_ubl_active_state_and_disable();  // No bed level correction so only raw data is obtained
      uint8_t count = GRID_MAX_POINTS;

      mesh_index_pair best;
      TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(best.pos, ExtUI::MESH_START));
      do {
        if (do_ubl_mesh_map) display_map(g29_map_type);

        const int point_num = (GRID_MAX_POINTS) - count + 1;
        SERIAL_ECHOLNPAIR("\nProbing mesh point ", point_num, "/", int(GRID_MAX_POINTS), ".\n");
        TERN_(HAS_DISPLAY, ui.status_printf_P(0, PSTR(S_FMT " %i/%i"), GET_TEXT(MSG_PROBING_MESH), point_num, int(GRID_MAX_POINTS)));

        #if HAS_LCD_MENU
          if (ui.button_pressed()) {
            ui.quick_feedback(false); // Preserve button state for click-and-hold
            SERIAL_ECHOLNPGM("\nMesh only partially populated.\n");
            ui.wait_for_release();
            ui.quick_feedback();
            ui.release();
            probe.stow(); // Release UI before stow to allow for PAUSE_BEFORE_DEPLOY_STOW
            return restore_ubl_active_state_and_leave();
          }
        #endif

        best = do_furthest
          ? find_furthest_invalid_mesh_point()
          : find_closest_mesh_point_of_type(INVALID, near, true);

        if (best.pos.x >= 0) {    // mesh point found and is reachable by probe
          TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(best.pos, ExtUI::PROBE_START));
          const float measured_z = probe.probe_at_point(
                        best.meshpos(),
                        stow_probe ? PROBE_PT_STOW : PROBE_PT_RAISE, g29_verbose_level
                      );
          z_values[best.pos.x][best.pos.y] = measured_z;
          #if ENABLED(EXTENSIBLE_UI)
            ExtUI::onMeshUpdate(best.pos, ExtUI::PROBE_FINISH);
            ExtUI::onMeshUpdate(best.pos, measured_z);
          #endif
        }
        SERIAL_FLUSH(); // Prevent host M105 buffer overrun.

      } while (best.pos.x >= 0 && --count);

      TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(best.pos, ExtUI::MESH_FINISH));

      // Release UI during stow to allow for PAUSE_BEFORE_DEPLOY_STOW
      TERN_(HAS_LCD_MENU, ui.release());
      probe.stow();
      TERN_(HAS_LCD_MENU, ui.capture());

      probe.move_z_after_probing();

      restore_ubl_active_state_and_leave();

      do_blocking_move_to_xy(
        constrain(near.x - probe.offset_xy.x, MESH_MIN_X, MESH_MAX_X),
        constrain(near.y - probe.offset_xy.y, MESH_MIN_Y, MESH_MAX_Y)
      );
    }

  #endif // HAS_BED_PROBE

  #if HAS_LCD_MENU

    typedef void (*clickFunc_t)();

    bool click_and_hold(const clickFunc_t func=nullptr) {
      if (ui.button_pressed()) {
        ui.quick_feedback(false);         // Preserve button state for click-and-hold
        const millis_t nxt = millis() + 1500UL;
        while (ui.button_pressed()) {     // Loop while the encoder is pressed. Uses hardware flag!
          idle();                         // idle, of course
          if (ELAPSED(millis(), nxt)) {   // After 1.5 seconds
            ui.quick_feedback();
            if (func) (*func)();
            ui.wait_for_release();
            return true;
          }
        }
      }
      serial_delay(15);
      return false;
    }

    void unified_bed_leveling::move_z_with_encoder(const float &multiplier) {
      ui.wait_for_release();
      while (!ui.button_pressed()) {
        idle();
        gcode.reset_stepper_timeout(); // Keep steppers powered
        if (encoder_diff) {
          do_blocking_move_to_z(current_position.z + float(encoder_diff) * multiplier);
          encoder_diff = 0;
        }
      }
    }

    float unified_bed_leveling::measure_point_with_encoder() {
      KEEPALIVE_STATE(PAUSED_FOR_USER);
      move_z_with_encoder(0.01f);
      return current_position.z;
    }

    static void echo_and_take_a_measurement() { SERIAL_ECHOLNPGM(" and take a measurement."); }

    float unified_bed_leveling::measure_business_card_thickness(float in_height) {
      ui.capture();
      save_ubl_active_state_and_disable();   // Disable bed level correction for probing

      do_blocking_move_to(0.5f * (MESH_MAX_X - (MESH_MIN_X)), 0.5f * (MESH_MAX_Y - (MESH_MIN_Y)), in_height);
        //, _MIN(planner.settings.max_feedrate_mm_s[X_AXIS], planner.settings.max_feedrate_mm_s[Y_AXIS]) * 0.5f);
      planner.synchronize();

      SERIAL_ECHOPGM("Place shim under nozzle");
      LCD_MESSAGEPGM(MSG_UBL_BC_INSERT);
      ui.return_to_status();
      echo_and_take_a_measurement();

      const float z1 = measure_point_with_encoder();
      do_blocking_move_to_z(current_position.z + SIZE_OF_LITTLE_RAISE);
      planner.synchronize();

      SERIAL_ECHOPGM("Remove shim");
      LCD_MESSAGEPGM(MSG_UBL_BC_REMOVE);
      echo_and_take_a_measurement();

      const float z2 = measure_point_with_encoder();
      do_blocking_move_to_z(current_position.z + Z_CLEARANCE_BETWEEN_PROBES);

      const float thickness = ABS(z1 - z2);

      if (g29_verbose_level > 1) {
        SERIAL_ECHOPAIR_F("Business Card is ", thickness, 4);
        SERIAL_ECHOLNPGM("mm thick.");
      }

      restore_ubl_active_state_and_leave();

      return thickness;
    }

    void unified_bed_leveling::manually_probe_remaining_mesh(const xy_pos_t &pos, const float &z_clearance, const float &thick, const bool do_ubl_mesh_map) {
      ui.capture();

      save_ubl_active_state_and_disable();  // No bed level correction so only raw data is obtained
      do_blocking_move_to_xy_z(current_position, z_clearance);

      ui.return_to_status();

      mesh_index_pair location;
      const xy_int8_t &lpos = location.pos;
      do {
        location = find_closest_mesh_point_of_type(INVALID, pos);
        // It doesn't matter if the probe can't reach the NAN location. This is a manual probe.
        if (!location.valid()) continue;

        const xyz_pos_t ppos = {
          mesh_index_to_xpos(lpos.x),
          mesh_index_to_ypos(lpos.y),
          Z_CLEARANCE_BETWEEN_PROBES
        };

        if (!position_is_reachable(ppos)) break; // SHOULD NOT OCCUR (find_closest_mesh_point only returns reachable points)

        LCD_MESSAGEPGM(MSG_UBL_MOVING_TO_NEXT);

        do_blocking_move_to(ppos);
        do_z_clearance(z_clearance);

        KEEPALIVE_STATE(PAUSED_FOR_USER);
        ui.capture();

        if (do_ubl_mesh_map) display_map(g29_map_type);  // show user where we're probing

        serialprintPGM(parser.seen('B') ? GET_TEXT(MSG_UBL_BC_INSERT) : GET_TEXT(MSG_UBL_BC_INSERT2));

        const float z_step = 0.01f;                         // existing behavior: 0.01mm per click, occasionally step
        //const float z_step = planner.steps_to_mm[Z_AXIS]; // approx one step each click

        move_z_with_encoder(z_step);

        if (click_and_hold()) {
          SERIAL_ECHOLNPGM("\nMesh only partially populated.");
          do_z_clearance(Z_CLEARANCE_DEPLOY_PROBE);
          return restore_ubl_active_state_and_leave();
        }

        z_values[lpos.x][lpos.y] = current_position.z - thick;
        TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(location, z_values[lpos.x][lpos.y]));

        if (g29_verbose_level > 2)
          SERIAL_ECHOLNPAIR_F("Mesh Point Measured at: ", z_values[lpos.x][lpos.y], 6);
        SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
      } while (location.valid());

      if (do_ubl_mesh_map) display_map(g29_map_type);  // show user where we're probing

      restore_ubl_active_state_and_leave();
      do_blocking_move_to_xy_z(pos, Z_CLEARANCE_DEPLOY_PROBE);
    }

    inline void set_message_with_feedback(PGM_P const msg_P) {
      ui.set_status_P(msg_P);
      ui.quick_feedback();
    }

    void abort_fine_tune() {
      ui.return_to_status();
      do_z_clearance(Z_CLEARANCE_BETWEEN_PROBES);
      set_message_with_feedback(GET_TEXT(MSG_EDITING_STOPPED));
    }

    void unified_bed_leveling::fine_tune_mesh(const xy_pos_t &pos, const bool do_ubl_mesh_map) {
      if (!parser.seen('R'))      // fine_tune_mesh() is special. If no repetition count flag is specified
        g29_repetition_cnt = 1;   // do exactly one mesh location. Otherwise use what the parser decided.

      #if ENABLED(UBL_MESH_EDIT_MOVES_Z)
        const float h_offset = parser.seenval('H') ? parser.value_linear_units() : 0;
        if (!WITHIN(h_offset, 0, 10)) {
          SERIAL_ECHOLNPGM("Offset out of bounds. (0 to 10mm)\n");
          return;
        }
      #endif

      mesh_index_pair location;

      if (!position_is_reachable(pos)) {
        SERIAL_ECHOLNPGM("(X,Y) outside printable radius.");
        return;
      }

      save_ubl_active_state_and_disable();

      LCD_MESSAGEPGM(MSG_UBL_FINE_TUNE_MESH);
      ui.capture();                                               // Take over control of the LCD encoder

      do_blocking_move_to_xy_z(pos, Z_CLEARANCE_BETWEEN_PROBES);  // Move to the given XY with probe clearance

      TERN_(UBL_MESH_EDIT_MOVES_Z, do_blocking_move_to_z(h_offset));  // Move Z to the given 'H' offset

      MeshFlags done_flags{0};
      const xy_int8_t &lpos = location.pos;
      do {
        location = find_closest_mesh_point_of_type(SET_IN_BITMAP, pos, false, &done_flags);

        if (lpos.x < 0) break;                              // Stop when there are no more reachable points

        done_flags.mark(lpos);                              // Mark this location as 'adjusted' so a new
                                                            // location is used on the next loop
        const xyz_pos_t raw = {
          mesh_index_to_xpos(lpos.x),
          mesh_index_to_ypos(lpos.y),
          Z_CLEARANCE_BETWEEN_PROBES
        };

        if (!position_is_reachable(raw)) break;             // SHOULD NOT OCCUR (find_closest_mesh_point_of_type only returns reachable)

        do_blocking_move_to(raw);                           // Move the nozzle to the edit point with probe clearance

        TERN_(UBL_MESH_EDIT_MOVES_Z, do_blocking_move_to_z(h_offset)); // Move Z to the given 'H' offset before editing

        KEEPALIVE_STATE(PAUSED_FOR_USER);

        if (do_ubl_mesh_map) display_map(g29_map_type);     // Display the current point

        #if IS_TFTGLCD_PANEL
          ui.ubl_plot(lpos.x, lpos.y);   // update plot screen
        #endif

        ui.refresh();

        float new_z = z_values[lpos.x][lpos.y];
        if (isnan(new_z)) new_z = 0;                        // Invalid points begin at 0
        new_z = FLOOR(new_z * 1000) * 0.001f;               // Chop off digits after the 1000ths place

        lcd_mesh_edit_setup(new_z);

        SET_SOFT_ENDSTOP_LOOSE(true);

        do {
          idle();
          new_z = lcd_mesh_edit();
          TERN_(UBL_MESH_EDIT_MOVES_Z, do_blocking_move_to_z(h_offset + new_z)); // Move the nozzle as the point is edited
          SERIAL_FLUSH();                                   // Prevent host M105 buffer overrun.
        } while (!ui.button_pressed());

        SET_SOFT_ENDSTOP_LOOSE(false);

        if (!lcd_map_control) ui.return_to_status();        // Just editing a single point? Return to status

        if (click_and_hold(abort_fine_tune)) break;         // Button held down? Abort editing

        z_values[lpos.x][lpos.y] = new_z;                   // Save the updated Z value
        TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(location, new_z));

        serial_delay(20);                                   // No switch noise
        ui.refresh();

      } while (lpos.x >= 0 && --g29_repetition_cnt > 0);

      if (do_ubl_mesh_map) display_map(g29_map_type);
      restore_ubl_active_state_and_leave();

      do_blocking_move_to_xy_z(pos, Z_CLEARANCE_BETWEEN_PROBES);

      LCD_MESSAGEPGM(MSG_UBL_DONE_EDITING_MESH);
      SERIAL_ECHOLNPGM("Done Editing Mesh");

      if (lcd_map_control)
        ui.goto_screen(ubl_map_screen);
      else
        ui.return_to_status();
    }

  #endif // HAS_LCD_MENU

  bool unified_bed_leveling::g29_parameter_parsing() {
    bool err_flag = false;

    TERN_(HAS_LCD_MENU, set_message_with_feedback(GET_TEXT(MSG_UBL_DOING_G29)));

    g29_constant = 0;
    g29_repetition_cnt = 0;

    if (parser.seen('R')) {
      g29_repetition_cnt = parser.has_value() ? parser.value_int() : GRID_MAX_POINTS;
      NOMORE(g29_repetition_cnt, GRID_MAX_POINTS);
      if (g29_repetition_cnt < 1) {
        SERIAL_ECHOLNPGM("?(R)epetition count invalid (1+).\n");
        return UBL_ERR;
      }
    }

    g29_verbose_level = parser.seen('V') ? parser.value_int() : 0;
    if (!WITHIN(g29_verbose_level, 0, 4)) {
      SERIAL_ECHOLNPGM("?(V)erbose level implausible (0-4).\n");
      err_flag = true;
    }

    if (parser.seen('P')) {
      const int pv = parser.value_int();
      #if !HAS_BED_PROBE
        if (pv == 1) {
          SERIAL_ECHOLNPGM("G29 P1 requires a probe.\n");
          err_flag = true;
        }
        else
      #endif
        {
          g29_phase_value = pv;
          if (!WITHIN(g29_phase_value, 0, 6)) {
            SERIAL_ECHOLNPGM("?(P)hase value invalid (0-6).\n");
            err_flag = true;
          }
        }
    }

    if (parser.seen('J')) {
      #if HAS_BED_PROBE
        g29_grid_size = parser.has_value() ? parser.value_int() : 0;
        if (g29_grid_size && !WITHIN(g29_grid_size, 2, 9)) {
          SERIAL_ECHOLNPGM("?Invalid grid size (J) specified (2-9).\n");
          err_flag = true;
        }
      #else
        SERIAL_ECHOLNPGM("G29 J action requires a probe.\n");
        err_flag = true;
      #endif
    }

    xy_seen.x = parser.seenval('X');
    float sx = xy_seen.x ? parser.value_float() : current_position.x;
    xy_seen.y = parser.seenval('Y');
    float sy = xy_seen.y ? parser.value_float() : current_position.y;

    if (xy_seen.x != xy_seen.y) {
      SERIAL_ECHOLNPGM("Both X & Y locations must be specified.\n");
      err_flag = true;
    }

    // If X or Y are not valid, use center of the bed values
    if (!WITHIN(sx, X_MIN_BED, X_MAX_BED)) sx = X_CENTER;
    if (!WITHIN(sy, Y_MIN_BED, Y_MAX_BED)) sy = Y_CENTER;

    if (err_flag) return UBL_ERR;

    g29_pos.set(sx, sy);

    /**
     * Activate or deactivate UBL
     * Note: UBL's G29 restores the state set here when done.
     *       Leveling is being enabled here with old data, possibly
     *       none. Error handling should disable for safety...
     */
    if (parser.seen('A')) {
      if (parser.seen('D')) {
        SERIAL_ECHOLNPGM("?Can't activate and deactivate at the same time.\n");
        return UBL_ERR;
      }
      set_bed_leveling_enabled(true);
      report_state();
    }
    else if (parser.seen('D')) {
      set_bed_leveling_enabled(false);
      report_state();
    }

    // Set global 'C' flag and its value
    if ((g29_c_flag = parser.seen('C')))
      g29_constant = parser.value_float();

    #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
      if (parser.seenval('F')) {
        const float fh = parser.value_float();
        if (!WITHIN(fh, 0, 100)) {
          SERIAL_ECHOLNPGM("?(F)ade height for Bed Level Correction not plausible.\n");
          return UBL_ERR;
        }
        set_z_fade_height(fh);
      }
    #endif

    g29_map_type = parser.intval('T');
    if (!WITHIN(g29_map_type, 0, 2)) {
      SERIAL_ECHOLNPGM("Invalid map type.\n");
      return UBL_ERR;
    }
    return UBL_OK;
  }

  static uint8_t ubl_state_at_invocation = 0;

  #if ENABLED(UBL_DEVEL_DEBUGGING)
    static uint8_t ubl_state_recursion_chk = 0;
  #endif

  void unified_bed_leveling::save_ubl_active_state_and_disable() {
    #if ENABLED(UBL_DEVEL_DEBUGGING)
      ubl_state_recursion_chk++;
      if (ubl_state_recursion_chk != 1) {
        SERIAL_ECHOLNPGM("save_ubl_active_state_and_disabled() called multiple times in a row.");
        TERN_(HAS_LCD_MENU, set_message_with_feedback(GET_TEXT(MSG_UBL_SAVE_ERROR)));
        return;
      }
    #endif
    ubl_state_at_invocation = planner.leveling_active;
    set_bed_leveling_enabled(false);
  }

  void unified_bed_leveling::restore_ubl_active_state_and_leave() {
    TERN_(HAS_LCD_MENU, ui.release());
    #if ENABLED(UBL_DEVEL_DEBUGGING)
      if (--ubl_state_recursion_chk) {
        SERIAL_ECHOLNPGM("restore_ubl_active_state_and_leave() called too many times.");
        TERN_(HAS_LCD_MENU, set_message_with_feedback(GET_TEXT(MSG_UBL_RESTORE_ERROR)));
        return;
      }
    #endif
    set_bed_leveling_enabled(ubl_state_at_invocation);
  }

  mesh_index_pair unified_bed_leveling::find_furthest_invalid_mesh_point() {

    bool found_a_NAN = false, found_a_real = false;

    mesh_index_pair farthest { -1, -1, -99999.99 };

    GRID_LOOP(i, j) {
      if (!isnan(z_values[i][j])) continue;  // Skip valid mesh points

      // Skip unreachable points
      if (!probe.can_reach(mesh_index_to_xpos(i), mesh_index_to_ypos(j)))
        continue;

      found_a_NAN = true;

      xy_int8_t near { -1, -1 };
      float d1, d2 = 99999.9f;
      GRID_LOOP(k, l) {
        if (isnan(z_values[k][l])) continue;

        found_a_real = true;

        // Add in a random weighting factor that scrambles the probing of the
        // last half of the mesh (when every unprobed mesh point is one index
        // from a probed location).

        d1 = HYPOT(i - k, j - l) + (1.0f / ((millis() % 47) + 13));

        if (d1 < d2) {    // Invalid mesh point (i,j) is closer to the defined point (k,l)
          d2 = d1;
          near.set(i, j);
        }
      }

      //
      // At this point d2 should have the near defined mesh point to invalid mesh point (i,j)
      //

      if (found_a_real && near.x >= 0 && d2 > farthest.distance) {
        farthest.pos = near;      // Found an invalid location farther from the defined mesh point
        farthest.distance = d2;
      }
    } // GRID_LOOP

    if (!found_a_real && found_a_NAN) {        // if the mesh is totally unpopulated, start the probing
      farthest.pos.set((GRID_MAX_POINTS_X) / 2, (GRID_MAX_POINTS_Y) / 2);
      farthest.distance = 1;
    }
    return farthest;
  }

  mesh_index_pair unified_bed_leveling::find_closest_mesh_point_of_type(const MeshPointType type, const xy_pos_t &pos, const bool probe_relative/*=false*/, MeshFlags *done_flags/*=nullptr*/) {
    mesh_index_pair closest;
    closest.invalidate();
    closest.distance = -99999.9f;

    // Get the reference position, either nozzle or probe
    const xy_pos_t ref = probe_relative ? pos + probe.offset_xy : pos;

    float best_so_far = 99999.99f;

    GRID_LOOP(i, j) {
      if ( (type == (isnan(z_values[i][j]) ? INVALID : REAL))
        || (type == SET_IN_BITMAP && !done_flags->marked(i, j))
      ) {
        // Found a Mesh Point of the specified type!
        const xy_pos_t mpos = { mesh_index_to_xpos(i), mesh_index_to_ypos(j) };

        // If using the probe as the reference there are some unreachable locations.
        // Also for round beds, there are grid points outside the bed the nozzle can't reach.
        // Prune them from the list and ignore them till the next Phase (manual nozzle probing).

        if (!(probe_relative ? probe.can_reach(mpos) : position_is_reachable(mpos)))
          continue;

        // Reachable. Check if it's the best_so_far location to the nozzle.

        const xy_pos_t diff = current_position - mpos;
        const float distance = (ref - mpos).magnitude() + diff.magnitude() * 0.1f;

        // factor in the distance from the current location for the normal case
        // so the nozzle isn't running all over the bed.
        if (distance < best_so_far) {
          best_so_far = distance;   // Found a closer location with the desired value type.
          closest.pos.set(i, j);
          closest.distance = best_so_far;
        }
      }
    } // GRID_LOOP

    return closest;
  }

  /**
   * 'Smart Fill': Scan from the outward edges of the mesh towards the center.
   * If an invalid location is found, use the next two points (if valid) to
   * calculate a 'reasonable' value for the unprobed mesh point.
   */

  bool unified_bed_leveling::smart_fill_one(const uint8_t x, const uint8_t y, const int8_t xdir, const int8_t ydir) {
    const float v = z_values[x][y];
    if (isnan(v)) {                           // A NAN...
      const int8_t dx = x + xdir, dy = y + ydir;
      const float v1 = z_values[dx][dy];
      if (!isnan(v1)) {                       // ...next to a pair of real values?
        const float v2 = z_values[dx + xdir][dy + ydir];
        if (!isnan(v2)) {
          z_values[x][y] = v1 < v2 ? v1 : v1 + v1 - v2;
          TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
          return true;
        }
      }
    }
    return false;
  }

  typedef struct { uint8_t sx, ex, sy, ey; bool yfirst; } smart_fill_info;

  void unified_bed_leveling::smart_fill_mesh() {
    static const smart_fill_info
      info0 PROGMEM = { 0, GRID_MAX_POINTS_X,      0, GRID_MAX_POINTS_Y - 2,  false },  // Bottom of the mesh looking up
      info1 PROGMEM = { 0, GRID_MAX_POINTS_X,      GRID_MAX_POINTS_Y - 1, 0,  false },  // Top of the mesh looking down
      info2 PROGMEM = { 0, GRID_MAX_POINTS_X - 2,  0, GRID_MAX_POINTS_Y,      true  },  // Left side of the mesh looking right
      info3 PROGMEM = { GRID_MAX_POINTS_X - 1, 0,  0, GRID_MAX_POINTS_Y,      true  };  // Right side of the mesh looking left
    static const smart_fill_info * const info[] PROGMEM = { &info0, &info1, &info2, &info3 };

    LOOP_L_N(i, COUNT(info)) {
      const smart_fill_info *f = (smart_fill_info*)pgm_read_ptr(&info[i]);
      const int8_t sx = pgm_read_byte(&f->sx), sy = pgm_read_byte(&f->sy),
                   ex = pgm_read_byte(&f->ex), ey = pgm_read_byte(&f->ey);
      if (pgm_read_byte(&f->yfirst)) {
        const int8_t dir = ex > sx ? 1 : -1;
        for (uint8_t y = sy; y != ey; ++y)
          for (uint8_t x = sx; x != ex; x += dir)
            if (smart_fill_one(x, y, dir, 0)) break;
      }
      else {
        const int8_t dir = ey > sy ? 1 : -1;
         for (uint8_t x = sx; x != ex; ++x)
          for (uint8_t y = sy; y != ey; y += dir)
            if (smart_fill_one(x, y, 0, dir)) break;
      }
    }
  }

  #if HAS_BED_PROBE

    //#define VALIDATE_MESH_TILT

    #include "../../../libs/vector_3.h"

    void unified_bed_leveling::tilt_mesh_based_on_probed_grid(const bool do_3_pt_leveling) {
      const float x_min = probe.min_x(), x_max = probe.max_x(),
                  y_min = probe.min_y(), y_max = probe.max_y(),
                  dx = (x_max - x_min) / (g29_grid_size - 1),
                  dy = (y_max - y_min) / (g29_grid_size - 1);

      xy_float_t points[3];
      probe.get_three_points(points);

      float measured_z;
      bool abort_flag = false;

      #ifdef VALIDATE_MESH_TILT
        float z1, z2, z3;  // Needed for algorithm validation below
      #endif

      struct linear_fit_data lsf_results;
      incremental_LSF_reset(&lsf_results);

      if (do_3_pt_leveling) {
        SERIAL_ECHOLNPGM("Tilting mesh (1/3)");
        TERN_(HAS_DISPLAY, ui.status_printf_P(0, PSTR(S_FMT " 1/3"), GET_TEXT(MSG_LCD_TILTING_MESH)));

        measured_z = probe.probe_at_point(points[0], PROBE_PT_RAISE, g29_verbose_level);
        if (isnan(measured_z))
          abort_flag = true;
        else {
          measured_z -= get_z_correction(points[0]);
          #ifdef VALIDATE_MESH_TILT
            z1 = measured_z;
          #endif
          if (g29_verbose_level > 3) {
            serial_spaces(16);
            SERIAL_ECHOLNPAIR("Corrected_Z=", measured_z);
          }
          incremental_LSF(&lsf_results, points[0], measured_z);
        }

        if (!abort_flag) {
          SERIAL_ECHOLNPGM("Tilting mesh (2/3)");
          TERN_(HAS_DISPLAY, ui.status_printf_P(0, PSTR(S_FMT " 2/3"), GET_TEXT(MSG_LCD_TILTING_MESH)));

          measured_z = probe.probe_at_point(points[1], PROBE_PT_RAISE, g29_verbose_level);
          #ifdef VALIDATE_MESH_TILT
            z2 = measured_z;
          #endif
          if (isnan(measured_z))
            abort_flag = true;
          else {
            measured_z -= get_z_correction(points[1]);
            if (g29_verbose_level > 3) {
              serial_spaces(16);
              SERIAL_ECHOLNPAIR("Corrected_Z=", measured_z);
            }
            incremental_LSF(&lsf_results, points[1], measured_z);
          }
        }

        if (!abort_flag) {
          SERIAL_ECHOLNPGM("Tilting mesh (3/3)");
          TERN_(HAS_DISPLAY, ui.status_printf_P(0, PSTR(S_FMT " 3/3"), GET_TEXT(MSG_LCD_TILTING_MESH)));

          measured_z = probe.probe_at_point(points[2], PROBE_PT_STOW, g29_verbose_level);
          #ifdef VALIDATE_MESH_TILT
            z3 = measured_z;
          #endif
          if (isnan(measured_z))
            abort_flag = true;
          else {
            measured_z -= get_z_correction(points[2]);
            if (g29_verbose_level > 3) {
              serial_spaces(16);
              SERIAL_ECHOLNPAIR("Corrected_Z=", measured_z);
            }
            incremental_LSF(&lsf_results, points[2], measured_z);
          }
        }

        probe.stow();
        probe.move_z_after_probing();

        if (abort_flag) {
          SERIAL_ECHOLNPGM("?Error probing point. Aborting operation.");
          return;
        }
      }
      else { // !do_3_pt_leveling

        bool zig_zag = false;

        const uint16_t total_points = sq(g29_grid_size);
        uint16_t point_num = 1;

        xy_pos_t rpos;
        LOOP_L_N(ix, g29_grid_size) {
          rpos.x = x_min + ix * dx;
          LOOP_L_N(iy, g29_grid_size) {
            rpos.y = y_min + dy * (zig_zag ? g29_grid_size - 1 - iy : iy);

            if (!abort_flag) {
              SERIAL_ECHOLNPAIR("Tilting mesh point ", point_num, "/", total_points, "\n");
              TERN_(HAS_DISPLAY, ui.status_printf_P(0, PSTR(S_FMT " %i/%i"), GET_TEXT(MSG_LCD_TILTING_MESH), point_num, total_points));

              measured_z = probe.probe_at_point(rpos, parser.seen('E') ? PROBE_PT_STOW : PROBE_PT_RAISE, g29_verbose_level); // TODO: Needs error handling

              abort_flag = isnan(measured_z);

              #if ENABLED(DEBUG_LEVELING_FEATURE)
                if (DEBUGGING(LEVELING)) {
                  const xy_pos_t lpos = rpos.asLogical();
                  DEBUG_CHAR('(');
                  DEBUG_ECHO_F(rpos.x, 7);
                  DEBUG_CHAR(',');
                  DEBUG_ECHO_F(rpos.y, 7);
                  DEBUG_ECHOPAIR_F(")   logical: (", lpos.x, 7);
                  DEBUG_CHAR(',');
                  DEBUG_ECHO_F(lpos.y, 7);
                  DEBUG_ECHOPAIR_F(")   measured: ", measured_z, 7);
                  DEBUG_ECHOPAIR_F("   correction: ", get_z_correction(rpos), 7);
                }
              #endif

              measured_z -= get_z_correction(rpos) /* + probe.offset.z */ ;

              if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR_F("   final >>>---> ", measured_z, 7);

              if (g29_verbose_level > 3) {
                serial_spaces(16);
                SERIAL_ECHOLNPAIR("Corrected_Z=", measured_z);
              }
              incremental_LSF(&lsf_results, rpos, measured_z);
            }

            point_num++;
          }

          zig_zag ^= true;
        }
      }
      probe.stow();
      probe.move_z_after_probing();

      if (abort_flag || finish_incremental_LSF(&lsf_results)) {
        SERIAL_ECHOPGM("Could not complete LSF!");
        return;
      }

      vector_3 normal = vector_3(lsf_results.A, lsf_results.B, 1).get_normal();

      if (g29_verbose_level > 2) {
        SERIAL_ECHOPAIR_F("bed plane normal = [", normal.x, 7);
        SERIAL_CHAR(',');
        SERIAL_ECHO_F(normal.y, 7);
        SERIAL_CHAR(',');
        SERIAL_ECHO_F(normal.z, 7);
        SERIAL_ECHOLNPGM("]");
      }

      matrix_3x3 rotation = matrix_3x3::create_look_at(vector_3(lsf_results.A, lsf_results.B, 1));

      GRID_LOOP(i, j) {
        float mx = mesh_index_to_xpos(i),
              my = mesh_index_to_ypos(j),
              mz = z_values[i][j];

        if (DEBUGGING(LEVELING)) {
          DEBUG_ECHOPAIR_F("before rotation = [", mx, 7);
          DEBUG_CHAR(',');
          DEBUG_ECHO_F(my, 7);
          DEBUG_CHAR(',');
          DEBUG_ECHO_F(mz, 7);
          DEBUG_ECHOPGM("]   ---> ");
          DEBUG_DELAY(20);
        }

        apply_rotation_xyz(rotation, mx, my, mz);

        if (DEBUGGING(LEVELING)) {
          DEBUG_ECHOPAIR_F("after rotation = [", mx, 7);
          DEBUG_CHAR(',');
          DEBUG_ECHO_F(my, 7);
          DEBUG_CHAR(',');
          DEBUG_ECHO_F(mz, 7);
          DEBUG_ECHOLNPGM("]");
          DEBUG_DELAY(20);
        }

        z_values[i][j] = mz - lsf_results.D;
        TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(i, j, z_values[i][j]));
      }

      if (DEBUGGING(LEVELING)) {
        rotation.debug(PSTR("rotation matrix:\n"));
        DEBUG_ECHOPAIR_F("LSF Results A=", lsf_results.A, 7);
        DEBUG_ECHOPAIR_F("  B=", lsf_results.B, 7);
        DEBUG_ECHOLNPAIR_F("  D=", lsf_results.D, 7);
        DEBUG_DELAY(55);

        DEBUG_ECHOPAIR_F("bed plane normal = [", normal.x, 7);
        DEBUG_CHAR(',');
        DEBUG_ECHO_F(normal.y, 7);
        DEBUG_CHAR(',');
        DEBUG_ECHO_F(normal.z, 7);
        DEBUG_ECHOLNPGM("]");
        DEBUG_EOL();

        /**
         * Use the code below to check the validity of the mesh tilting algorithm.
         * 3-Point Mesh Tilt uses the same algorithm as grid-based tilting, but only
         * three points are used in the calculation. This guarantees that each probed point
         * has an exact match when get_z_correction() for that location is calculated.
         * The Z error between the probed point locations and the get_z_correction()
         * numbers for those locations should be 0.
         */
        #ifdef VALIDATE_MESH_TILT
          auto d_from = []{ DEBUG_ECHOPGM("D from "); };
          auto normed = [&](const xy_pos_t &pos, const float &zadd) {
            return normal.x * pos.x + normal.y * pos.y + zadd;
          };
          auto debug_pt = [](PGM_P const pre, const xy_pos_t &pos, const float &zadd) {
            d_from(); serialprintPGM(pre);
            DEBUG_ECHO_F(normed(pos, zadd), 6);
            DEBUG_ECHOLNPAIR_F("   Z error = ", zadd - get_z_correction(pos), 6);
          };
          debug_pt(PSTR("1st point: "), probe_pt[0], normal.z * z1);
          debug_pt(PSTR("2nd point: "), probe_pt[1], normal.z * z2);
          debug_pt(PSTR("3rd point: "), probe_pt[2], normal.z * z3);
          d_from(); DEBUG_ECHOPGM("safe home with Z=");
          DEBUG_ECHOLNPAIR_F("0 : ", normed(safe_homing_xy, 0), 6);
          d_from(); DEBUG_ECHOPGM("safe home with Z=");
          DEBUG_ECHOLNPAIR_F("mesh value ", normed(safe_homing_xy, get_z_correction(safe_homing_xy)), 6);
          DEBUG_ECHOPAIR("   Z error = (", Z_SAFE_HOMING_X_POINT, ",", Z_SAFE_HOMING_Y_POINT);
          DEBUG_ECHOLNPAIR_F(") = ", get_z_correction(safe_homing_xy), 6);
        #endif
      } // DEBUGGING(LEVELING)

    }

  #endif // HAS_BED_PROBE

  #if ENABLED(UBL_G29_P31)
    void unified_bed_leveling::smart_fill_wlsf(const float &weight_factor) {

      // For each undefined mesh point, compute a distance-weighted least squares fit
      // from all the originally populated mesh points, weighted toward the point
      // being extrapolated so that nearby points will have greater influence on
      // the point being extrapolated.  Then extrapolate the mesh point from WLSF.

      static_assert((GRID_MAX_POINTS_Y) <= 16, "GRID_MAX_POINTS_Y too big");
      uint16_t bitmap[GRID_MAX_POINTS_X] = { 0 };
      struct linear_fit_data lsf_results;

      SERIAL_ECHOPGM("Extrapolating mesh...");

      const float weight_scaled = weight_factor * _MAX(MESH_X_DIST, MESH_Y_DIST);

      GRID_LOOP(jx, jy) if (!isnan(z_values[jx][jy])) SBI(bitmap[jx], jy);

      xy_pos_t ppos;
      LOOP_L_N(ix, GRID_MAX_POINTS_X) {
        ppos.x = mesh_index_to_xpos(ix);
        LOOP_L_N(iy, GRID_MAX_POINTS_Y) {
          ppos.y = mesh_index_to_ypos(iy);
          if (isnan(z_values[ix][iy])) {
            // undefined mesh point at (ppos.x,ppos.y), compute weighted LSF from original valid mesh points.
            incremental_LSF_reset(&lsf_results);
            xy_pos_t rpos;
            LOOP_L_N(jx, GRID_MAX_POINTS_X) {
              rpos.x = mesh_index_to_xpos(jx);
              LOOP_L_N(jy, GRID_MAX_POINTS_Y) {
                if (TEST(bitmap[jx], jy)) {
                  rpos.y = mesh_index_to_ypos(jy);
                  const float rz = z_values[jx][jy],
                               w = 1.0f + weight_scaled / (rpos - ppos).magnitude();
                  incremental_WLSF(&lsf_results, rpos, rz, w);
                }
              }
            }
            if (finish_incremental_LSF(&lsf_results)) {
              SERIAL_ECHOLNPGM("Insufficient data");
              return;
            }
            const float ez = -lsf_results.D - lsf_results.A * ppos.x - lsf_results.B * ppos.y;
            z_values[ix][iy] = ez;
            TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(ix, iy, z_values[ix][iy]));
            idle(); // housekeeping
          }
        }
      }

      SERIAL_ECHOLNPGM("done");
    }
  #endif // UBL_G29_P31

  #if ENABLED(UBL_DEVEL_DEBUGGING)
    /**
     * Much of the 'What?' command can be eliminated. But until we are fully debugged, it is
     * good to have the extra information. Soon... we prune this to just a few items
     */
    void unified_bed_leveling::g29_what_command() {
      report_state();

      if (storage_slot == -1)
        SERIAL_ECHOPGM("No Mesh Loaded.");
      else
        SERIAL_ECHOPAIR("Mesh ", storage_slot, " Loaded.");
      SERIAL_EOL();
      serial_delay(50);

      #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
        SERIAL_ECHOLNPAIR_F("Fade Height M420 Z", planner.z_fade_height, 4);
      #endif

      adjust_mesh_to_mean(g29_c_flag, g29_constant);

      #if HAS_BED_PROBE
        SERIAL_ECHOLNPAIR_F("Probe Offset M851 Z", probe.offset.z, 7);
      #endif

      SERIAL_ECHOLNPAIR("MESH_MIN_X  " STRINGIFY(MESH_MIN_X) "=", MESH_MIN_X); serial_delay(50);
      SERIAL_ECHOLNPAIR("MESH_MIN_Y  " STRINGIFY(MESH_MIN_Y) "=", MESH_MIN_Y); serial_delay(50);
      SERIAL_ECHOLNPAIR("MESH_MAX_X  " STRINGIFY(MESH_MAX_X) "=", MESH_MAX_X); serial_delay(50);
      SERIAL_ECHOLNPAIR("MESH_MAX_Y  " STRINGIFY(MESH_MAX_Y) "=", MESH_MAX_Y); serial_delay(50);
      SERIAL_ECHOLNPAIR("GRID_MAX_POINTS_X  ", GRID_MAX_POINTS_X);             serial_delay(50);
      SERIAL_ECHOLNPAIR("GRID_MAX_POINTS_Y  ", GRID_MAX_POINTS_Y);             serial_delay(50);
      SERIAL_ECHOLNPAIR("MESH_X_DIST  ", MESH_X_DIST);
      SERIAL_ECHOLNPAIR("MESH_Y_DIST  ", MESH_Y_DIST);                         serial_delay(50);

      SERIAL_ECHOPGM("X-Axis Mesh Points at: ");
      LOOP_L_N(i, GRID_MAX_POINTS_X) {
        SERIAL_ECHO_F(LOGICAL_X_POSITION(mesh_index_to_xpos(i)), 3);
        SERIAL_ECHOPGM("  ");
        serial_delay(25);
      }
      SERIAL_EOL();

      SERIAL_ECHOPGM("Y-Axis Mesh Points at: ");
      LOOP_L_N(i, GRID_MAX_POINTS_Y) {
        SERIAL_ECHO_F(LOGICAL_Y_POSITION(mesh_index_to_ypos(i)), 3);
        SERIAL_ECHOPGM("  ");
        serial_delay(25);
      }
      SERIAL_EOL();

      #if HAS_KILL
        SERIAL_ECHOLNPAIR("Kill pin on :", int(KILL_PIN), "  state:", int(kill_state()));
      #endif

      SERIAL_EOL();
      serial_delay(50);

      #if ENABLED(UBL_DEVEL_DEBUGGING)
        SERIAL_ECHOLNPAIR("ubl_state_at_invocation :", ubl_state_at_invocation, "\nubl_state_recursion_chk :", ubl_state_recursion_chk);
        serial_delay(50);

        SERIAL_ECHOLNPAIR("Meshes go from ", hex_address((void*)settings.meshes_start_index()), " to ", hex_address((void*)settings.meshes_end_index()));
        serial_delay(50);

        SERIAL_ECHOLNPAIR("sizeof(ubl) :  ", (int)sizeof(ubl));         SERIAL_EOL();
        SERIAL_ECHOLNPAIR("z_value[][] size: ", (int)sizeof(z_values)); SERIAL_EOL();
        serial_delay(25);

        SERIAL_ECHOLNPAIR("EEPROM free for UBL: ", hex_address((void*)(settings.meshes_end_index() - settings.meshes_start_index())));
        serial_delay(50);

        SERIAL_ECHOLNPAIR("EEPROM can hold ", settings.calc_num_meshes(), " meshes.\n");
        serial_delay(25);
      #endif // UBL_DEVEL_DEBUGGING

      if (!sanity_check()) {
        echo_name();
        SERIAL_ECHOLNPGM(" sanity checks passed.");
      }
    }

    /**
     * When we are fully debugged, the EEPROM dump command will get deleted also. But
     * right now, it is good to have the extra information. Soon... we prune this.
     */
    void unified_bed_leveling::g29_eeprom_dump() {
      uint8_t cccc;

      SERIAL_ECHO_MSG("EEPROM Dump:");
      persistentStore.access_start();
      for (uint16_t i = 0; i < persistentStore.capacity(); i += 16) {
        if (!(i & 0x3)) idle();
        print_hex_word(i);
        SERIAL_ECHOPGM(": ");
        for (uint16_t j = 0; j < 16; j++) {
          persistentStore.read_data(i + j, &cccc, sizeof(uint8_t));
          print_hex_byte(cccc);
          SERIAL_CHAR(' ');
        }
        SERIAL_EOL();
      }
      SERIAL_EOL();
      persistentStore.access_finish();
    }

    /**
     * When we are fully debugged, this may go away. But there are some valid
     * use cases for the users. So we can wait and see what to do with it.
     */
    void unified_bed_leveling::g29_compare_current_mesh_to_stored_mesh() {
      const int16_t a = settings.calc_num_meshes();

      if (!a) {
        SERIAL_ECHOLNPGM("?EEPROM storage not available.");
        return;
      }

      if (!parser.has_value() || !WITHIN(g29_storage_slot, 0, a - 1)) {
        SERIAL_ECHOLNPAIR("?Invalid storage slot.\n?Use 0 to ", a - 1);
        return;
      }

      g29_storage_slot = parser.value_int();

      float tmp_z_values[GRID_MAX_POINTS_X][GRID_MAX_POINTS_Y];
      settings.load_mesh(g29_storage_slot, &tmp_z_values);

      SERIAL_ECHOLNPAIR("Subtracting mesh in slot ", g29_storage_slot, " from current mesh.");

      GRID_LOOP(x, y) {
        z_values[x][y] -= tmp_z_values[x][y];
        TERN_(EXTENSIBLE_UI, ExtUI::onMeshUpdate(x, y, z_values[x][y]));
      }
    }

  #endif // UBL_DEVEL_DEBUGGING

#endif // AUTO_BED_LEVELING_UBL