GPv3: Onion Skinning

Implements the GPv2 onion skinning functionality.

There are no functional changes exept for the `use_ghosts_always`
option, which has been removed. This was used to show onion
skinning in the final render.

Pull Request: https://projects.blender.org/blender/blender/pulls/119792

scripts/startup/bl_ui/properties_data_grease_pencil.py

@@ -231,6 +231,72 @@ class DATA_PT_grease_pencil_layer_relations(LayerDataButtonsPanel, Panel):
         col.prop_search(layer, "viewlayer_render", context.scene, "view_layers", text="View Layer")
 
 
+class DATA_PT_grease_pencil_onion_skinning(DataButtonsPanel, Panel):
+    bl_label = "Onion Skinning"
+
+    def draw(self, context):
+        grease_pencil = context.grease_pencil
+
+        layout = self.layout
+        layout.use_property_split = True
+
+        col = layout.column()
+        col.prop(grease_pencil, "onion_mode")
+        col.prop(grease_pencil, "onion_factor", text="Opacity", slider=True)
+        col.prop(grease_pencil, "onion_keyframe_type")
+
+        if grease_pencil.onion_mode == 'ABSOLUTE':
+            col = layout.column(align=True)
+            col.prop(grease_pencil, "ghost_before_range", text="Frames Before")
+            col.prop(grease_pencil, "ghost_after_range", text="Frames After")
+        elif grease_pencil.onion_mode == 'RELATIVE':
+            col = layout.column(align=True)
+            col.prop(grease_pencil, "ghost_before_range", text="Keyframes Before")
+            col.prop(grease_pencil, "ghost_after_range", text="Keyframes After")
+
+
+class DATA_PT_grease_pencil_onion_skinning_custom_colors(DataButtonsPanel, Panel):
+    bl_parent_id = "DATA_PT_grease_pencil_onion_skinning"
+    bl_label = "Custom Colors"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    def draw_header(self, context):
+        grease_pencil = context.grease_pencil
+        self.layout.prop(grease_pencil, "use_ghost_custom_colors", text="")
+
+    def draw(self, context):
+        grease_pencil = context.grease_pencil
+
+        layout = self.layout
+        layout.use_property_split = True
+        layout.enabled = grease_pencil.users <= 1 and grease_pencil.use_ghost_custom_colors
+
+        layout.prop(grease_pencil, "before_color", text="Before")
+        layout.prop(grease_pencil, "after_color", text="After")
+
+
+class DATA_PT_grease_pencil_onion_skinning_display(DataButtonsPanel, Panel):
+    bl_parent_id = "DATA_PT_grease_pencil_onion_skinning"
+    bl_label = "Display"
+    bl_options = {'DEFAULT_CLOSED'}
+
+    def draw(self, context):
+        grease_pencil = context.grease_pencil
+
+        layout = self.layout
+        layout.use_property_split = True
+        # This was done in GPv2 but it's not entirely clear why. Presumably it was
+        # to indicate that the settings will affect the onion skinning of the
+        # other users.
+        layout.enabled = grease_pencil.users <= 1
+
+        col = layout.column(align=True)
+        col.prop(grease_pencil, "use_onion_fade", text="Fade")
+        sub = layout.column()
+        sub.active = grease_pencil.onion_mode in {'RELATIVE', 'SELECTED'}
+        sub.prop(grease_pencil, "use_onion_loop", text="Show Start Frame")
+
+
 class DATA_PT_grease_pencil_settings(DataButtonsPanel, Panel):
     bl_label = "Settings"
 
@@ -257,6 +323,9 @@ classes = (
     DATA_PT_grease_pencil_layer_masks,
     DATA_PT_grease_pencil_layer_transform,
     DATA_PT_grease_pencil_layer_relations,
+    DATA_PT_grease_pencil_onion_skinning,
+    DATA_PT_grease_pencil_onion_skinning_custom_colors,
+    DATA_PT_grease_pencil_onion_skinning_display,
     DATA_PT_grease_pencil_settings,
     DATA_PT_grease_pencil_custom_props,
     GREASE_PENCIL_MT_grease_pencil_add_layer_extra,

scripts/startup/bl_ui/space_view3d.py

@@ -7861,6 +7861,8 @@ class VIEW3D_PT_overlay_grease_pencil_options(Panel):
             'OBJECT': iface_("Grease Pencil"),
         }[context.mode], translate=False)
 
+        layout.prop(overlay, "use_gpencil_onion_skin", text="Onion Skin")
+
         if ob.mode in {'EDIT'}:
             split = layout.split()
             col = split.column()