Fix T79517: Data Transfer modifier fails in edit-mode

Referenced by issue #79517, Data Transfer modifier not showing transferred uvs in editmode (when the "on_cage" modifier option is used)

source/blender/blenkernel/intern/mesh.c

@@ -95,7 +95,15 @@ static void mesh_copy_data(Main *bmain, ID *id_dst, const ID *id_src, const int
     /* This is a direct copy of a main mesh, so for now it has the same topology. */
     mesh_dst->runtime.deformed_only = true;
   }
-  /* XXX WHAT? Why? Comment, please! And pretty sure this is not valid for regular Mesh copying? */
+  /* This option is set for run-time meshes that have been copied from the current objects mode.
+   * Currently this is used for edit-mesh although it could be used for sculpt or other
+   * kinds of data specific to an objects mode.
+   *
+   * The flag signals that the mesh hasn't been modified from the data that generated it,
+   * allowing us to use the object-mode data for drawing.
+   *
+   * While this could be the callers responsibility, keep here since it's
+   * highly unlikely we want to create a duplicate and not use it for drawing. */
   mesh_dst->runtime.is_original = false;
 
   /* Only do tessface if we have no polys. */

source/blender/makesdna/DNA_mesh_types.h

@@ -109,9 +109,12 @@ typedef struct Mesh_Runtime {
   /** Set by modifier stack if only deformed from original. */
   char deformed_only;
   /**
-   * Copied from edit-mesh (hint, draw with editmesh data).
-   * In the future we may leave the mesh-data empty
-   * since its not needed if we can use edit-mesh data. */
+   * Copied from edit-mesh (hint, draw with edit-mesh data when true).
+   *
+   * Modifiers that edit the mesh data in-place must set this to false
+   * (most #eModifierTypeType_NonGeometrical modifiers). Otherwise the edit-mesh
+   * data will be used for drawing, missing changes from modifiers. See T79517.
+   */
   char is_original;
 
   /** #eMeshWrapperType and others. */

source/blender/modifiers/intern/MOD_datatransfer.c

@@ -204,29 +204,31 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   BKE_reports_init(&reports, RPT_STORE);
 
   /* Note: no islands precision for now here. */
-  BKE_object_data_transfer_ex(ctx->depsgraph,
-                              scene,
-                              ob_source,
-                              ctx->object,
-                              result,
-                              dtmd->data_types,
-                              false,
-                              dtmd->vmap_mode,
-                              dtmd->emap_mode,
-                              dtmd->lmap_mode,
-                              dtmd->pmap_mode,
-                              space_transform,
-                              false,
-                              max_dist,
-                              dtmd->map_ray_radius,
-                              0.0f,
-                              dtmd->layers_select_src,
-                              dtmd->layers_select_dst,
-                              dtmd->mix_mode,
-                              dtmd->mix_factor,
-                              dtmd->defgrp_name,
-                              invert_vgroup,
-                              &reports);
+  if (BKE_object_data_transfer_ex(ctx->depsgraph,
+                                  scene,
+                                  ob_source,
+                                  ctx->object,
+                                  result,
+                                  dtmd->data_types,
+                                  false,
+                                  dtmd->vmap_mode,
+                                  dtmd->emap_mode,
+                                  dtmd->lmap_mode,
+                                  dtmd->pmap_mode,
+                                  space_transform,
+                                  false,
+                                  max_dist,
+                                  dtmd->map_ray_radius,
+                                  0.0f,
+                                  dtmd->layers_select_src,
+                                  dtmd->layers_select_dst,
+                                  dtmd->mix_mode,
+                                  dtmd->mix_factor,
+                                  dtmd->defgrp_name,
+                                  invert_vgroup,
+                                  &reports)) {
+    result->runtime.is_original = false;
+  }
 
   if (BKE_reports_contain(&reports, RPT_ERROR)) {
     const char *report_str = BKE_reports_string(&reports, RPT_ERROR);

source/blender/modifiers/intern/MOD_normal_edit.c

@@ -637,6 +637,8 @@ static Mesh *normalEditModifier_do(NormalEditModifierData *enmd,
   CustomData_free_layers(pdata, CD_NORMAL, num_polys);
   MEM_SAFE_FREE(loopnors);
 
+  result->runtime.is_original = false;
+
   return result;
 }
 

source/blender/modifiers/intern/MOD_uvproject.c

@@ -308,6 +308,8 @@ static Mesh *uvprojectModifier_do(UVProjectModifierData *umd,
     }
   }
 
+  mesh->runtime.is_original = false;
+
   /* Mark tessellated CD layers as dirty. */
   mesh->runtime.cd_dirty_vert |= CD_MASK_TESSLOOPNORMAL;
 

source/blender/modifiers/intern/MOD_uvwarp.c

@@ -233,6 +233,8 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   /* XXX TODO is this still needed? */
   //  me_eval->dirty |= DM_DIRTY_TESS_CDLAYERS;
 
+  mesh->runtime.is_original = false;
+
   return mesh;
 }
 

source/blender/modifiers/intern/MOD_weighted_normal.c

@@ -677,6 +677,9 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
 
   /* Currently Modifier stack assumes there is no poly normal data passed around... */
   CustomData_free_layers(pdata, CD_NORMAL, numPolys);
+
+  result->runtime.is_original = false;
+
   return result;
 }
 

source/blender/modifiers/intern/MOD_weightvgedit.c

@@ -312,6 +312,8 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   MEM_freeN(new_w);
   MEM_freeN(dw);
 
+  mesh->runtime.is_original = false;
+
   /* Return the vgroup-modified mesh. */
   return mesh;
 }

source/blender/modifiers/intern/MOD_weightvgmix.c

@@ -456,6 +456,8 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   MEM_freeN(dw2);
   MEM_SAFE_FREE(indices);
 
+  mesh->runtime.is_original = false;
+
   /* Return the vgroup-modified mesh. */
   return mesh;
 }

source/blender/modifiers/intern/MOD_weightvgproximity.c

@@ -637,6 +637,8 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   TIMEIT_END(perf);
 #endif
 
+  mesh->runtime.is_original = false;
+
   /* Return the vgroup-modified mesh. */
   return mesh;
 }