source/blender/blenkernel/BKE_grease_pencil.hh

@@ -82,6 +82,17 @@ class TreeNode : public ::GreasePencilLayerTreeNode, NonMovable {
   Layer &as_layer_for_write();
 };
 
+/**
+ * A layer mask stores a reference to a layer that will mask other layers.
+ */
+class LayerMask : public ::GreasePencilLayerMask {
+ public:
+  LayerMask();
+  explicit LayerMask(StringRefNull name);
+  LayerMask(const LayerMask &other);
+  ~LayerMask();
+};
+
 /**
  * A layer maps drawings to scene frames. It can be thought of as one independent channel in the
  * timeline.
@@ -114,7 +125,15 @@ class Layer : public TreeNode, public ::GreasePencilLayer {
    * drawings, then the last referenced drawing is held for the rest of the duration.
    */
   Map<int, GreasePencilFrame> frames_;
+  /**
+   * Caches a sorted vector of the keys of `frames_`.
+   */
   mutable SharedCache<Vector<int>> sorted_keys_cache_;
+  /**
+   * A vector of LayerMask. This layer will be masked by the layers referenced in the masks.
+   * A layer can have zero or more layer masks.
+   */
+  Vector<LayerMask> masks_;
 
  public:
   Layer();
@@ -128,6 +147,12 @@ class Layer : public TreeNode, public ::GreasePencilLayer {
   const Map<int, GreasePencilFrame> &frames() const;
   Map<int, GreasePencilFrame> &frames_for_write();
 
+  /**
+   * \returns the layer masks.
+   */
+  const Vector<LayerMask> &masks() const;
+  Vector<LayerMask> &masks_for_write();
+
   /**
    * Inserts the frame into the layer. Fails if there exists a frame at \a frame_number already.
    * \returns true on success.

source/blender/blenkernel/intern/grease_pencil.cc

@@ -298,19 +298,45 @@ Layer &TreeNode::as_layer_for_write()
   return *static_cast<Layer *>(this);
 }
 
-Layer::Layer() : TreeNode::TreeNode(GP_LAYER_TREE_LEAF), frames_()
+LayerMask::LayerMask()
+{
+  this->layer_name = nullptr;
+}
+
+LayerMask::LayerMask(StringRefNull name)
+{
+  this->layer_name = BLI_strdup(name.c_str());
+}
+
+LayerMask::LayerMask(const LayerMask &other) : LayerMask()
+{
+  if (other.layer_name) {
+    this->layer_name = BLI_strdup(other.layer_name);
+  }
+}
+
+LayerMask::~LayerMask()
+{
+  if (this->layer_name) {
+    MEM_freeN(this->layer_name);
+  }
+}
+
+Layer::Layer() : TreeNode::TreeNode(GP_LAYER_TREE_LEAF), frames_(), masks_()
 {
   this->parsubstr = nullptr;
   this->viewlayer_name = nullptr;
 }
 
-Layer::Layer(StringRefNull name) : TreeNode::TreeNode(GP_LAYER_TREE_LEAF, name), frames_()
+Layer::Layer(StringRefNull name)
+    : TreeNode::TreeNode(GP_LAYER_TREE_LEAF, name), frames_(), masks_()
 {
   this->parsubstr = nullptr;
   this->viewlayer_name = nullptr;
 }
 
-Layer::Layer(const Layer &other) : TreeNode::TreeNode(other), frames_(other.frames_)
+Layer::Layer(const Layer &other)
+    : TreeNode::TreeNode(other), frames_(other.frames_), masks_(other.masks_)
 {
   this->parsubstr = nullptr;
   this->viewlayer_name = nullptr;
@@ -343,6 +369,16 @@ Map<int, GreasePencilFrame> &Layer::frames_for_write()
   return this->frames_;
 }
 
+const Vector<LayerMask> &Layer::masks() const
+{
+  return this->masks_;
+}
+
+Vector<LayerMask> &Layer::masks_for_write()
+{
+  return this->masks_;
+}
+
 bool Layer::insert_frame(int frame_number, GreasePencilFrame &frame)
 {
   this->sorted_keys_cache_.tag_dirty();
@@ -1094,6 +1130,13 @@ static void save_layer_to_storage(const blender::bke::greasepencil::Layer &node,
   if (node.viewlayer_name) {
     new_leaf->layer.viewlayer_name = BLI_strdup(node.viewlayer_name);
   }
+  BLI_listbase_clear(&new_leaf->layer.masks_storage);
+  for (const bke::greasepencil::LayerMask &mask : node.masks()) {
+    GreasePencilLayerMask *new_mask = MEM_cnew<GreasePencilLayerMask>(__func__);
+    new_mask->layer_name = BLI_strdup(mask.layer_name);
+    new_mask->flag = mask.flag;
+    BLI_addtail(&new_leaf->layer.masks_storage, new_mask);
+  }
   new_leaf->layer.opacity = node.opacity;
   copy_v4_v4(new_leaf->layer.tint_color, node.tint_color);
   copy_v3_v3(new_leaf->layer.location, node.location);
@@ -1162,6 +1205,12 @@ static void read_layer_from_storage(blender::bke::greasepencil::LayerGroup &curr
   if (node_leaf->layer.viewlayer_name) {
     new_layer.viewlayer_name = BLI_strdup(node_leaf->layer.viewlayer_name);
   }
+  Vector<LayerMask> masks = new_layer.masks_for_write();
+  LISTBASE_FOREACH (bGPDlayer_Mask *, mask, &node_leaf->layer.masks_storage) {
+    LayerMask new_mask = LayerMask(mask->name);
+    new_mask.flag = mask->flag;
+    masks.append(std::move(new_mask));
+  }
   new_layer.opacity = node_leaf->layer.opacity;
   copy_v4_v4(new_layer.tint_color, node_leaf->layer.tint_color);
   copy_v3_v3(new_layer.location, node_leaf->layer.location);
@@ -1236,6 +1285,10 @@ void GreasePencil::read_layer_tree_storage(BlendDataReader *reader)
         BLO_read_data_address(reader, &node_leaf->layer.frames_storage.values);
         BLO_read_data_address(reader, &node_leaf->layer.parsubstr);
         BLO_read_data_address(reader, &node_leaf->layer.viewlayer_name);
+        BLO_read_list(reader, &node_leaf->layer.masks_storage);
+        LISTBASE_FOREACH (GreasePencilLayerMask *, mask, &node_leaf->layer.masks_storage) {
+          BLO_read_data_address(reader, &mask->layer_name);
+        }
         break;
       }
       case GP_LAYER_TREE_GROUP: {
@@ -1267,6 +1320,10 @@ void GreasePencil::write_layer_tree_storage(BlendWriter *writer)
                                node_leaf->layer.frames_storage.values);
         BLO_write_string(writer, node_leaf->layer.parsubstr);
         BLO_write_string(writer, node_leaf->layer.viewlayer_name);
+        BLO_write_struct_list(writer, GreasePencilLayerMask, &node_leaf->layer.masks_storage);
+        LISTBASE_FOREACH (GreasePencilLayerMask *, mask, &node_leaf->layer.masks_storage) {
+          BLO_write_string(writer, mask->layer_name);
+        }
         break;
       }
       case GP_LAYER_TREE_GROUP: {
@@ -1302,6 +1359,12 @@ void GreasePencil::free_layer_tree_storage()
         if (node_leaf->layer.viewlayer_name) {
           MEM_freeN(node_leaf->layer.viewlayer_name);
         }
+        LISTBASE_FOREACH_MUTABLE (GreasePencilLayerMask *, mask, &node_leaf->layer.masks_storage) {
+          if (mask->layer_name) {
+            MEM_freeN(mask->layer_name);
+          }
+          MEM_freeN(mask);
+        }
         MEM_freeN(node_leaf);
         break;
       }

source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc

@@ -219,6 +219,13 @@ void legacy_gpencil_to_grease_pencil(Main &bmain, GreasePencil &grease_pencil, b
     if (viewlayername_len > 0) {
       new_layer.viewlayer_name = BLI_strdupn(gpl->viewlayername, viewlayername_len);
     }
+    /* Convert the layer masks. */
+    Vector<LayerMask> masks = new_layer.masks_for_write();
+    LISTBASE_FOREACH (bGPDlayer_Mask *, mask, &gpl->mask_layers) {
+      LayerMask new_mask = LayerMask(mask->name);
+      new_mask.flag = mask->flag;
+      masks.append(std::move(new_mask));
+    }
     new_layer.opacity = gpl->opacity;
     copy_v4_v4(new_layer.tint_color, gpl->tintcolor);
     copy_v3_v3(new_layer.location, gpl->location);

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -139,6 +139,31 @@ typedef struct GreasePencilLayerFramesMapStorage {
   char _pad[4];
 } GreasePencilLayerFramesMapStorage;
 
+/**
+ * Flag for layer masks. #GreasePencilLayerMask.flag
+ */
+typedef enum GreasePencilLayerMaskFlag {
+  GP_LAYER_MASK_HIDE = (1 << 0),
+  GP_LAYER_MASK_INVERT = (1 << 1),
+} GreasePencilLayerMaskFlag;
+
+/**
+ * A grease pencil layer mask stores the name of a layer that is the mask.
+ */
+typedef struct GreasePencilLayerMask {
+  struct GreasePencilLayerMask *next, *prev;
+  /**
+   * The name of the layer that is the mask.
+   * \note Null-terminated.
+   */
+  char *layer_name;
+  /**
+   * Layer mask flag. See `GreasePencilLayerMaskFlag`.
+   */
+  uint16_t flag;
+  char _pad[6];
+} GreasePencilLayerMask;
+
 /**
  * Type of parent of a layer. #GreasePencilLayer.parent_type
  */
@@ -193,6 +218,10 @@ typedef struct GreasePencilLayer {
    * \note Null-terminated.
    */
   char *viewlayer_name;
+  /**
+   * List of `GreasePencilLayerMask`.
+   */
+  ListBase masks_storage;
   /**
    * Opacity of the layer.
    */