source/blender/blenkernel/BKE_grease_pencil.hh

@@ -514,6 +514,8 @@ class Layer : public ::GreasePencilLayer {
   SortedKeysIterator remove_leading_null_frames_in_range(SortedKeysIterator begin,
                                                          SortedKeysIterator end);
 
+  float4x4 parent_inverse() const;
+
   /**
    * The local transform of the layer (in layer space, not object space).
    */

source/blender/blenkernel/intern/grease_pencil.cc

@@ -911,6 +911,7 @@ Layer::Layer()
 
   this->parent = nullptr;
   this->parsubstr = nullptr;
+  unit_m4(this->parentinv);
 
   zero_v3(this->translation);
   zero_v3(this->rotation);
@@ -944,6 +945,7 @@ Layer::Layer(const Layer &other) : Layer()
 
   this->parent = other.parent;
   this->set_parent_bone_name(other.parsubstr);
+  copy_m4_m4(this->parentinv, other.parentinv);
 
   copy_v3_v3(this->translation, other.translation);
   copy_v3_v3(this->rotation, other.rotation);
@@ -1225,7 +1227,7 @@ float4x4 Layer::to_world_space(const Object &object) const
     return object.object_to_world() * this->local_transform();
   }
   const Object &parent = *this->parent;
-  return this->parent_to_world(parent) * this->local_transform();
+  return this->parent_to_world(parent) * this->parent_inverse() * this->local_transform();
 }
 
 float4x4 Layer::to_object_space(const Object &object) const
@@ -1234,7 +1236,8 @@ float4x4 Layer::to_object_space(const Object &object) const
     return this->local_transform();
   }
   const Object &parent = *this->parent;
-  return object.world_to_object() * this->parent_to_world(parent) * this->local_transform();
+  return object.world_to_object() * this->parent_to_world(parent) * this->parent_inverse() *
+         this->local_transform();
 }
 
 StringRefNull Layer::parent_bone_name() const
@@ -1263,6 +1266,11 @@ float4x4 Layer::parent_to_world(const Object &parent) const
   return parent_object_to_world;
 }
 
+float4x4 Layer::parent_inverse() const
+{
+  return float4x4_view(this->parentinv);
+}
+
 float4x4 Layer::local_transform() const
 {
   return math::from_loc_rot_scale<float4x4, math::EulerXYZ>(

source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc

@@ -586,6 +586,7 @@ void legacy_gpencil_to_grease_pencil(Main &bmain, GreasePencil &grease_pencil, b
 
     new_layer.parent = gpl->parent;
     new_layer.set_parent_bone_name(gpl->parsubstr);
+    copy_m4_m4(new_layer.parentinv, gpl->inverse);
 
     copy_v3_v3(new_layer.translation, gpl->location);
     copy_v3_v3(new_layer.rotation, gpl->rotation);

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -300,6 +300,11 @@ typedef struct GreasePencilLayer {
    */
   struct Object *parent;
   char *parsubstr;
+  /**
+   * Stores the inverse of the parent during parenting to keep the layer in its position.
+   * Also referred to as the "keep transform" parenting elsewhere.
+   */
+  float parentinv[4][4];
   /**
    * Layer transform UI settings. These should *not* be used to do any computation.
    * Use the functions is the `bke::greasepencil::Layer` class instead.