Fix: GPencil animated layer transforms evaluate wrong when identity

Due to (optimization) checks in in `BKE_gpencil_prepare_eval_data` &
`BKE_gpencil_update_layer_transforms`, updates were skipped if animation
reached exact identity transforms.

Now check if the matrix has changed additionally to gain proper updates.
Unsure if this is the cheapest way to check for the animated state of
layer transforms tbh, but I see similar checks elsewhere.

Fixes T101164.

Maniphest Tasks: T101164

Differential Revision: https://developer.blender.org/D16018

2 files changed, 13 insertions, 3 deletions

diff --git a/source/blender/blenkernel/intern/gpencil.c b/source/blender/blenkernel/intern/gpencil.c
index 13cee6d9e6a..ec1818ce260 100644
--- a/source/blender/blenkernel/intern/gpencil.c
+++ b/source/blender/blenkernel/intern/gpencil.c
@@ -2991,11 +2991,14 @@ void BKE_gpencil_update_layer_transforms(const Depsgraph *depsgraph, Object *ob)
         changed = !equals_m4m4(gpl->inverse, cur_mat);
       }
 
-      /* Calc local layer transform. */
+      /* Calc local layer transform. Early out if we have non-animated zero transforms. */
       bool transformed = ((!is_zero_v3(gpl->location)) || (!is_zero_v3(gpl->rotation)) ||
                           (!is_one_v3(gpl->scale)));
+      float tmp_mat[4][4];
+      loc_eul_size_to_mat4(tmp_mat, gpl->location, gpl->rotation, gpl->scale);
+      transformed |= !equals_m4m4(gpl->layer_mat, tmp_mat);
       if (transformed) {
-        loc_eul_size_to_mat4(gpl->layer_mat, gpl->location, gpl->rotation, gpl->scale);
+        copy_m4_m4(gpl->layer_mat, tmp_mat);
       }
 
       /* only redo if any change */
diff --git a/source/blender/blenkernel/intern/gpencil_modifier.c b/source/blender/blenkernel/intern/gpencil_modifier.c
index 3222689ef57..af8f2b8d92d 100644
--- a/source/blender/blenkernel/intern/gpencil_modifier.c
+++ b/source/blender/blenkernel/intern/gpencil_modifier.c
@@ -712,7 +712,14 @@ void BKE_gpencil_prepare_eval_data(Depsgraph *depsgraph, Scene *scene, Object *o
       do_parent = true;
       break;
     }
-    if ((!is_zero_v3(gpl->location)) || (!is_zero_v3(gpl->rotation)) || (!is_one_v3(gpl->scale))) {
+
+    /* Only do layer transformations for non-zero or animated transforms. */
+    bool transformed = ((!is_zero_v3(gpl->location)) || (!is_zero_v3(gpl->rotation)) ||
+                        (!is_one_v3(gpl->scale)));
+    float tmp_mat[4][4];
+    loc_eul_size_to_mat4(tmp_mat, gpl->location, gpl->rotation, gpl->scale);
+    transformed |= !equals_m4m4(gpl->layer_mat, tmp_mat);
+    if (transformed) {
       do_transform = true;
       break;
     }