Fix T66040: Make Data Transfer' Mix Factor multiply with vgroup weights when given, instead of ignoring it.

Brings behavior of those options in line with the VGroup editing
modifiers ones e.g.

4 files changed, 21 insertions, 3 deletions

diff --git a/source/blender/blenkernel/BKE_blender_version.h b/source/blender/blenkernel/BKE_blender_version.h
index 062e185eb23..ced9f4a3153 100644
--- a/source/blender/blenkernel/BKE_blender_version.h
+++ b/source/blender/blenkernel/BKE_blender_version.h
@@ -27,7 +27,7 @@
  * \note Use #STRINGIFY() rather than defining with quotes.
  */
 #define BLENDER_VERSION 281
-#define BLENDER_SUBVERSION 0
+#define BLENDER_SUBVERSION 1
 /** Several breakages with 280, e.g. collections vs layers. */
 #define BLENDER_MINVERSION 280
 #define BLENDER_MINSUBVERSION 0
diff --git a/source/blender/blenkernel/intern/customdata.c b/source/blender/blenkernel/intern/customdata.c
index 1fb29745dba..3c7ac5d8d2e 100644
--- a/source/blender/blenkernel/intern/customdata.c
+++ b/source/blender/blenkernel/intern/customdata.c
@@ -4756,7 +4756,8 @@ void CustomData_data_transfer(const MeshPairRemap *me_remap,
 
   for (i = 0; i < totelem; i++, data_dst = POINTER_OFFSET(data_dst, data_step), mapit++) {
     const int sources_num = mapit->sources_num;
-    const float mix_factor = laymap->mix_weights ? laymap->mix_weights[i] : laymap->mix_factor;
+    const float mix_factor = laymap->mix_factor *
+                             (laymap->mix_weights ? laymap->mix_weights[i] : 1.0f);
     int j;
 
     if (!sources_num) {
diff --git a/source/blender/blenloader/intern/versioning_280.c b/source/blender/blenloader/intern/versioning_280.c
index cb2b64957f0..1a79b7c9b5a 100644
--- a/source/blender/blenloader/intern/versioning_280.c
+++ b/source/blender/blenloader/intern/versioning_280.c
@@ -3533,6 +3533,22 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
     }
   }
 
+  if (!MAIN_VERSION_ATLEAST(bmain, 281, 1)) {
+    LISTBASE_FOREACH (Object *, ob, &bmain->objects) {
+      for (ModifierData *md = ob->modifiers.first; md; md = md->next) {
+        if (md->type == eModifierType_DataTransfer) {
+          /* Now datatransfer's mix factor is multiplied with weights when any,
+           * instead of being ignored,
+           * we need to take care of that to keep 'old' files compatible. */
+          DataTransferModifierData *dtmd = (DataTransferModifierData *)md;
+          if (dtmd->defgrp_name[0] != '\0') {
+            dtmd->mix_factor = 1.0f;
+          }
+        }
+      }
+    }
+  }
+
   {
     /* Versioning code until next subversion bump goes here. */
   }
diff --git a/source/blender/makesrna/intern/rna_modifier.c b/source/blender/makesrna/intern/rna_modifier.c
index a04e0e6422f..7e1ff01a7c3 100644
--- a/source/blender/makesrna/intern/rna_modifier.c
+++ b/source/blender/makesrna/intern/rna_modifier.c
@@ -5668,7 +5668,8 @@ static void rna_def_modifier_datatransfer(BlenderRNA *brna)
       0.0f,
       1.0f,
       "Mix Factor",
-      "Factor to use when applying data to destination (exact behavior depends on mix mode)",
+      "Factor to use when applying data to destination (exact behavior depends on mix mode, "
+      "multiplied with weights from vertex group when defined)",
       0.0f,
       1.0f);
   RNA_def_property_update(prop, 0, "rna_Modifier_update");