Fix #124725: GPv3: Masked layers not rendered correctly with onion skinning

The issue was that the masks for the current frame would be
rendered for a different frame.
Unfortunately, we can't easily render masks for the onion
skinned frames correctly at the moment.

The fix makes it so that we render the mask only for the current frame.

Pull Request: https://projects.blender.org/blender/blender/pulls/129878

source/blender/draw/engines/gpencil/gpencil_cache_utils.cc

@@ -515,15 +515,17 @@ GPENCIL_tLayer *gpencil_layer_cache_add(GPENCIL_PrivateData *pd,
   return tgp_layer;
 }
 
-GPENCIL_tLayer *gpencil_layer_cache_get(GPENCIL_tObject *tgp_ob, int number)
+GPENCIL_tLayer *grease_pencil_layer_cache_get(GPENCIL_tObject *tgp_ob,
+                                              int layer_id,
+                                              const bool skip_onion)
 {
-  if (number >= 0) {
-    GPENCIL_tLayer *layer = tgp_ob->layers.first;
-    while (layer != nullptr) {
-      if (layer->layer_id == number) {
-        return layer;
-      }
-      layer = layer->next;
+  BLI_assert(layer_id >= 0);
+  for (GPENCIL_tLayer *layer = tgp_ob->layers.first; layer != nullptr; layer = layer->next) {
+    if (skip_onion && layer->is_onion) {
+      continue;
+    }
+    if (layer->layer_id == layer_id) {
+      return layer;
     }
   }
   return nullptr;
@@ -576,6 +578,7 @@ GPENCIL_tLayer *grease_pencil_layer_cache_add(GPENCIL_PrivateData *pd,
   GPENCIL_tLayer *tgp_layer = static_cast<GPENCIL_tLayer *>(BLI_memblock_alloc(pd->gp_layer_pool));
   BLI_LINKS_APPEND(&tgp_ob->layers, tgp_layer);
   tgp_layer->layer_id = *grease_pencil.get_layer_index(layer);
+  tgp_layer->is_onion = onion_id != 0;
   tgp_layer->mask_bits = nullptr;
   tgp_layer->mask_invert_bits = nullptr;
   tgp_layer->blend_ps = nullptr;

source/blender/draw/engines/gpencil/gpencil_engine.h

@@ -109,6 +109,8 @@ typedef struct GPENCIL_tLayer {
   BLI_bitmap *mask_invert_bits;
   /** Index in the layer list. Used as id for masking. */
   int layer_id;
+  /** True if this pass is part of the onion skinning. */
+  bool is_onion;
 } GPENCIL_tLayer;
 
 typedef struct GPENCIL_tObject {
@@ -333,7 +335,9 @@ GPENCIL_tLayer *gpencil_layer_cache_add(GPENCIL_PrivateData *pd,
                                         const bGPDlayer *gpl,
                                         const bGPDframe *gpf,
                                         GPENCIL_tObject *tgp_ob);
-GPENCIL_tLayer *gpencil_layer_cache_get(GPENCIL_tObject *tgp_ob, int number);
+GPENCIL_tLayer *grease_pencil_layer_cache_get(GPENCIL_tObject *tgp_ob,
+                                              int layer_id,
+                                              bool skip_onion);
 
 GPENCIL_tLayer *grease_pencil_layer_cache_add(GPENCIL_PrivateData *pd,
                                               const Object *ob,

source/blender/draw/engines/gpencil/gpencil_engine_c.cc

@@ -775,7 +775,7 @@ static void gpencil_draw_mask(GPENCIL_Data *vedata, GPENCIL_tObject *ob, GPENCIL
       GPU_framebuffer_clear_color_depth(fbl->mask_fb, clear_col, clear_depth);
     }
 
-    GPENCIL_tLayer *mask_layer = gpencil_layer_cache_get(ob, i);
+    GPENCIL_tLayer *mask_layer = grease_pencil_layer_cache_get(ob, i, true);
     /* When filtering by view-layer, the mask could be null and must be ignored. */
     if (mask_layer == nullptr) {
       continue;