Fix T85051: Add bisect distance as a parameter to the mirror modifier

The `bisect_distance` in the mirror modifier was hard-coded to `0.001`.
This would result in some unexpected behavior like vertices close
to the mirror plane being deleted or merged.

The fix now adds a parameter to the mirror modifier to expose the
bisect distance to the user. The default is set to the previous
hard-coded value to not "change" previous files.

Ref D10201

source/blender/blenkernel/BKE_blender_version.h

@@ -39,7 +39,7 @@ extern "C" {
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 18
+#define BLENDER_FILE_SUBVERSION 19
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and show a warning if the file

source/blender/blenkernel/intern/mesh_mirror.c

@@ -51,7 +51,7 @@ Mesh *BKE_mesh_mirror_bisect_on_mirror_plane_for_modifier(MirrorModifierData *mm
                               (axis == 1 && mmd->flag & MOD_MIR_BISECT_FLIP_AXIS_Y) ||
                               (axis == 2 && mmd->flag & MOD_MIR_BISECT_FLIP_AXIS_Z));
 
-  const float bisect_distance = 0.001f;
+  const float bisect_distance = mmd->bisect_threshold;
 
   Mesh *result;
   BMesh *bm;

source/blender/blenloader/intern/versioning_290.c

@@ -2059,6 +2059,19 @@ void blo_do_versions_290(FileData *fd, Library *UNUSED(lib), Main *bmain)
     }
   }
 
+  /* Set default value for the new bisect_threshold parameter in the mirror modifier. */
+  if (!MAIN_VERSION_ATLEAST(bmain, 293, 19)) {
+    LISTBASE_FOREACH (Object *, ob, &bmain->objects) {
+      LISTBASE_FOREACH (ModifierData *, md, &ob->modifiers) {
+        if (md->type == eModifierType_Mirror) {
+          MirrorModifierData *mmd = (MirrorModifierData *)md;
+          /* This was the previous hard-coded value. */
+          mmd->bisect_threshold = 0.001f;
+        }
+      }
+    }
+  }
+
   /**
    * Versioning code until next subversion bump goes here.
    *

source/blender/makesdna/DNA_modifier_defaults.h

@@ -429,6 +429,7 @@
   { \
     .flag = MOD_MIR_AXIS_X | MOD_MIR_VGROUP, \
     .tolerance = 0.001f, \
+    .bisect_threshold = 0.001f, \
     .uv_offset = {0.0f, 0.0f}, \
     .uv_offset_copy = {0.0f, 0.0f}, \
     .mirror_ob = NULL, \

source/blender/makesdna/DNA_modifier_types.h

@@ -368,6 +368,8 @@ typedef struct MirrorModifierData {
   short axis DNA_DEPRECATED;
   short flag;
   float tolerance;
+  float bisect_threshold;
+  char _pad[4];
   float uv_offset[2];
   float uv_offset_copy[2];
   struct Object *mirror_ob;

source/blender/makesrna/intern/rna_modifier.c

@@ -2226,6 +2226,14 @@ static void rna_def_modifier_mirror(BlenderRNA *brna)
       prop, "Merge Distance", "Distance within which mirrored vertices are merged");
   RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
+  prop = RNA_def_property(srna, "bisect_threshold", PROP_FLOAT, PROP_DISTANCE);
+  RNA_def_property_float_sdna(prop, NULL, "bisect_threshold");
+  RNA_def_property_range(prop, 0, FLT_MAX);
+  RNA_def_property_ui_range(prop, 0, 1, 0.01, 6);
+  RNA_def_property_ui_text(
+      prop, "Bisect Distance", "Distance from the bisect plane within which vertices are removed");
+  RNA_def_property_update(prop, 0, "rna_Modifier_update");
+
   prop = RNA_def_property(srna, "mirror_object", PROP_POINTER, PROP_NONE);
   RNA_def_property_pointer_sdna(prop, NULL, "mirror_ob");
   RNA_def_property_ui_text(prop, "Mirror Object", "Object to use as mirror");

source/blender/modifiers/intern/MOD_mirror.c

@@ -165,6 +165,13 @@ static void panel_draw(const bContext *UNUSED(C), Panel *panel)
   uiLayoutSetActive(sub, RNA_boolean_get(ptr, "use_mirror_merge"));
   uiItemR(sub, ptr, "merge_threshold", 0, "", ICON_NONE);
 
+  bool is_bisect_set[3];
+  RNA_boolean_get_array(ptr, "use_bisect_axis", is_bisect_set);
+
+  sub = uiLayoutRow(col, true);
+  uiLayoutSetActive(sub, is_bisect_set[0] || is_bisect_set[1] || is_bisect_set[2]);
+  uiItemR(sub, ptr, "bisect_threshold", 0, IFACE_("Bisect Distance"), ICON_NONE);
+
   modifier_panel_end(layout, ptr);
 }