Fix T83705: GPencil - Duplicate strokes of destination layer when merge layer

If the destination layer hadn't keyframe, a new keyframe was added and later the merge layer strokes were added, but this could change the animation because the new frame replaced the old drawings of the target layer.

Now, before merge the layer, all keyframes are added in the target layer in order to keep the drawings.

1 files changed, 13 insertions, 8 deletions

diff --git a/source/blender/editors/gpencil/gpencil_data.c b/source/blender/editors/gpencil/gpencil_data.c
index 33a1469beab..203e86e9e15 100644
--- a/source/blender/editors/gpencil/gpencil_data.c
+++ b/source/blender/editors/gpencil/gpencil_data.c
@@ -1330,19 +1330,24 @@ static int gpencil_merge_layer_exec(bContext *C, wmOperator *op)
     BLI_ghash_insert(gh_frames_dst, POINTER_FROM_INT(gpf_dst->framenum), gpf_dst);
   }
 
-  /* Read all frames from merge layer and add any missing in destination layer. */
+  /* Read all frames from merge layer and add any missing in destination layer,
+   * copying all previous strokes to keep the image equals. Need to do it in a separated
+   * loop to avoid strokes acumulation. */
   LISTBASE_FOREACH (bGPDframe *, gpf_src, &gpl_src->frames) {
     /* Try to find frame in destination layer hash table. */
     bGPDframe *gpf_dst = BLI_ghash_lookup(gh_frames_dst, POINTER_FROM_INT(gpf_src->framenum));
     if (!gpf_dst) {
-      gpf_dst = BKE_gpencil_frame_addnew(gpl_dst, gpf_src->framenum);
-      /* Duplicate strokes into destination frame. */
-      if (gpf_dst) {
-        BKE_gpencil_frame_copy_strokes(gpf_src, gpf_dst);
-      }
+      gpf_dst = BKE_gpencil_layer_frame_get(gpl_dst, gpf_src->framenum, GP_GETFRAME_ADD_COPY);
+      BLI_ghash_insert(gh_frames_dst, POINTER_FROM_INT(gpf_src->framenum), gpf_dst);
     }
-    else {
-      /* Add to tail all strokes. */
+  }
+
+  /* Read all frames from merge layer and add strokes. */
+  LISTBASE_FOREACH (bGPDframe *, gpf_src, &gpl_src->frames) {
+    /* Try to find frame in destination layer hash table. */
+    bGPDframe *gpf_dst = BLI_ghash_lookup(gh_frames_dst, POINTER_FROM_INT(gpf_src->framenum));
+    /* Add to tail all strokes. */
+    if (gpf_dst) {
       BLI_movelisttolist(&gpf_dst->strokes, &gpf_src->strokes);
     }
   }