bed_mesh: enable relative offset bed meshes (#1323)

Adds parameter for bed meshes to be adjusted relative to a probed point
during calibration. This allows the probe z offset to be ignored for
probes that are not stable over time (for example, the thermal drift of
inductive probes). An endstop other than the probe is then necessary to
determine the bed-nozzle offset.

Signed-off-by: Matt Baker <baker.matt.j@gmail.com>

config/example-extras.cfg

@@ -226,6 +226,10 @@
 #   may be applied to change the amount of slope interpolated.
 #   Larger numbers will increase the amount of slope, which
 #   results in more curvature in the mesh. Default is .2.
+#relative_reference_index:
+#   A point index in the mesh to reference all z values to. Enabling
+#   this parameter produces a mesh relative to the probed z position
+#   at the provided index.
 
 
 # Tool to help adjust bed leveling screws. One may define a

klippy/extras/bed_mesh.py

@@ -186,6 +186,7 @@ class BedMeshCalibrate:
         self.printer = config.get_printer()
         self.name = config.get_name()
         self.radius = None
+        self.relative_reference_index = None
         self.bedmesh = bedmesh
         self.probed_z_table = None
         self.build_map = False
@@ -267,8 +268,17 @@ class BedMeshCalibrate:
                         points.append((pos_x, pos_y))
             pos_y += y_dist
         logging.info('bed_mesh: generated points')
-        for p in points:
-            logging.info("(%.1f, %.1f)" % (p[0], p[1]))
+        for i, p in enumerate(points):
+            logging.info("%d: (%.1f, %.1f)" % (i, p[0], p[1]))
+        rref_index = config.get('relative_reference_index', None)
+        if rref_index is not None:
+            rref_index = int(rref_index)
+            if rref_index < 0 or rref_index >= len(points):
+                raise config.error("bed_mesh: relative reference index %d "
+                    "is out of bounds" % (rref_index))
+            logging.info("bed_mesh: relative_reference_index %d is (%.2f, %.2f)"
+                % (rref_index, points[rref_index][0], points[rref_index][1]))
+            self.relative_reference_index = rref_index
         return points
     def _init_probe_params(self, config, points):
         self.probe_params['min_x'] = min(points, key=lambda p: p[0])[0]
@@ -411,6 +421,11 @@ class BedMeshCalibrate:
         x_cnt = self.probe_params['x_count']
         y_cnt = self.probe_params['y_count']
 
+        if self.relative_reference_index is not None:
+            # zero out probe z offset and
+            # set offset relative to reference index
+            z_offset = positions[self.relative_reference_index][2]
+
         self.probed_z_table = []
         row = []
         prev_pos = positions[0]