Implement onion skinning for the animation editor

editor/editor_settings.cpp

@@ -462,6 +462,8 @@ void EditorSettings::_load_defaults(Ref<ConfigFile> p_extra_config) {
 
 	_initial_set("editors/animation/autorename_animation_tracks", true);
 	_initial_set("editors/animation/confirm_insert_track", true);
+	_initial_set("editors/animation/onion_layers_past_color", Color(1, 0, 0));
+	_initial_set("editors/animation/onion_layers_future_color", Color(0, 1, 0));
 
 	_initial_set("docks/property_editor/texture_preview_width", 48);
 	_initial_set("docks/property_editor/auto_refresh_interval", 0.3);

editor/plugins/animation_player_editor_plugin.cpp

@@ -36,6 +36,12 @@
 #include "os/keyboard.h"
 #include "project_settings.h"
 
+// For onion skinning
+#include "editor/plugins/canvas_item_editor_plugin.h"
+#include "editor/plugins/spatial_editor_plugin.h"
+#include "scene/main/viewport.h"
+#include "servers/visual_server.h"
+
 void AnimationPlayerEditor::_node_removed(Node *p_node) {
 
 	if (player && player == p_node) {
@@ -98,6 +104,8 @@ void AnimationPlayerEditor::_notification(int p_what) {
 
 			tool_anim->get_popup()->connect("id_pressed", this, "_animation_tool_menu");
 
+			onion_skinning->get_popup()->connect("id_pressed", this, "_onion_skinning_menu");
+
 			blend_editor.next->connect("item_selected", this, "_blend_editor_next_changed");
 
 			get_tree()->connect("node_removed", this, "_node_removed");
@@ -132,6 +140,7 @@ void AnimationPlayerEditor::_notification(int p_what) {
 			resource_edit_anim->set_icon(get_icon("EditResource", "EditorIcons"));
 			pin->set_icon(get_icon("Pin", "EditorIcons"));
 			tool_anim->set_icon(get_icon("Tools", "EditorIcons"));
+			onion_skinning->set_icon(get_icon("Onion", "EditorIcons"));
 
 		} break;
 	}
@@ -809,6 +818,7 @@ void AnimationPlayerEditor::_update_player() {
 	resource_edit_anim->set_disabled(animlist.size() == 0);
 	save_anim->set_disabled(animlist.size() == 0);
 	tool_anim->set_disabled(player == NULL);
+	onion_skinning->set_disabled(player == NULL);
 
 	int active_idx = -1;
 	for (List<StringName>::Element *E = animlist.front(); E; E = E->next()) {
@@ -855,6 +865,9 @@ void AnimationPlayerEditor::_update_player() {
 
 void AnimationPlayerEditor::edit(AnimationPlayer *p_player) {
 
+	if (onion.enabled)
+		_start_onion_skinning();
+
 	if (player && pin->is_pressed())
 		return; //ignore, pinned
 	player = p_player;
@@ -869,6 +882,55 @@ void AnimationPlayerEditor::edit(AnimationPlayer *p_player) {
 	}
 }
 
+void AnimationPlayerEditor::forward_force_draw_over_viewport(Control *p_overlay) {
+
+	if (!onion.can_overlay)
+		return;
+
+	// Can happen on viewport resize, at least
+	if (!_are_onion_layers_valid())
+		return;
+
+	RID ci = p_overlay->get_canvas_item();
+	Rect2 src_rect = p_overlay->get_global_rect();
+	// Re-flip since captures are already flipped
+	src_rect.position.y = onion.capture_size.y - (src_rect.position.y + src_rect.size.y);
+	src_rect.size.y *= -1;
+
+	Rect2 dst_rect = Rect2(Point2(), p_overlay->get_size());
+
+	float alpha_step = 1.0 / (onion.steps + 1);
+
+	int cidx = 0;
+	if (onion.past) {
+		float alpha = 0;
+		do {
+			alpha += alpha_step;
+
+			if (onion.captures_valid[cidx]) {
+				VS::get_singleton()->canvas_item_add_texture_rect_region(
+						ci, dst_rect, VS::get_singleton()->viewport_get_texture(onion.captures[cidx]), src_rect, Color(1, 1, 1, alpha));
+			}
+
+			cidx++;
+		} while (cidx < onion.steps);
+	}
+	if (onion.future) {
+		float alpha = 1;
+		int base_cidx = cidx;
+		do {
+			alpha -= alpha_step;
+
+			if (onion.captures_valid[cidx]) {
+				VS::get_singleton()->canvas_item_add_texture_rect_region(
+						ci, dst_rect, VS::get_singleton()->viewport_get_texture(onion.captures[cidx]), src_rect, Color(1, 1, 1, alpha));
+			}
+
+			cidx++;
+		} while (cidx < base_cidx + onion.steps); // In case there's the present capture at the end, skip it
+	}
+}
+
 void AnimationPlayerEditor::_animation_duplicate() {
 
 	if (!animation->get_item_count())
@@ -1100,6 +1162,70 @@ void AnimationPlayerEditor::_animation_save_menu(int p_option) {
 	}
 }
 
+void AnimationPlayerEditor::_onion_skinning_menu(int p_option) {
+
+	PopupMenu *menu = onion_skinning->get_popup();
+	int idx = menu->get_item_index(p_option);
+
+	switch (p_option) {
+
+		case ONION_SKINNING_ENABLE: {
+
+			onion.enabled = !onion.enabled;
+			menu->set_item_checked(idx, onion.enabled);
+
+			if (onion.enabled)
+				_start_onion_skinning();
+			else
+				_stop_onion_skinning();
+
+		} break;
+
+		case ONION_SKINNING_PAST: {
+
+			// Ensure at least one of past/future is checjed
+			onion.past = onion.future ? !onion.past : true;
+			menu->set_item_checked(idx, onion.past);
+		} break;
+
+		case ONION_SKINNING_FUTURE: {
+
+			// Ensure at least one of past/future is checjed
+			onion.future = onion.past ? !onion.future : true;
+			menu->set_item_checked(idx, onion.future);
+		} break;
+
+		case ONION_SKINNING_1_STEP: // Fall-through
+		case ONION_SKINNING_2_STEPS:
+		case ONION_SKINNING_3_STEPS: {
+
+			onion.steps = (p_option - ONION_SKINNING_1_STEP) + 1;
+			int one_frame_idx = menu->get_item_index(ONION_SKINNING_1_STEP);
+			for (int i = 0; i <= ONION_SKINNING_LAST_STEPS_OPTION - ONION_SKINNING_1_STEP; i++) {
+				menu->set_item_checked(one_frame_idx + i, onion.steps == i + 1);
+			}
+		} break;
+
+		case ONION_SKINNING_DIFFERENCES_ONLY: {
+
+			onion.differences_only = !onion.differences_only;
+			menu->set_item_checked(idx, onion.differences_only);
+		} break;
+
+		case ONION_SKINNING_FORCE_WHITE_MODULATE: {
+
+			onion.force_white_modulate = !onion.force_white_modulate;
+			menu->set_item_checked(idx, onion.force_white_modulate);
+		} break;
+
+		case ONION_SKINNING_INCLUDE_GIZMOS: {
+
+			onion.include_gizmos = !onion.include_gizmos;
+			menu->set_item_checked(idx, onion.include_gizmos);
+		} break;
+	}
+}
+
 void AnimationPlayerEditor::_unhandled_key_input(const Ref<InputEvent> &p_ev) {
 
 	Ref<InputEventKey> k = p_ev;
@@ -1126,6 +1252,237 @@ void AnimationPlayerEditor::_unhandled_key_input(const Ref<InputEvent> &p_ev) {
 	}
 }
 
+void AnimationPlayerEditor::_editor_visibility_changed() {
+
+	if (is_visible()) {
+		_start_onion_skinning();
+	}
+}
+
+bool AnimationPlayerEditor::_are_onion_layers_valid() {
+
+	ERR_FAIL_COND_V(!onion.past && !onion.future, false);
+
+	Point2 capture_size = get_tree()->get_root()->get_size();
+	return onion.captures.size() == onion.get_needed_capture_count() && onion.capture_size == capture_size;
+}
+
+void AnimationPlayerEditor::_allocate_onion_layers() {
+
+	_free_onion_layers();
+
+	int captures = onion.get_needed_capture_count();
+	Point2 capture_size = get_tree()->get_root()->get_size();
+
+	onion.captures.resize(captures);
+	onion.captures_valid.resize(captures);
+
+	for (int i = 0; i < captures; i++) {
+		bool is_present = onion.differences_only && i == captures - 1;
+
+		// Each capture is a viewport with a canvas item attached that renders a full-size rect with the contents of the main viewport
+		onion.captures[i] = VS::get_singleton()->viewport_create();
+		VS::get_singleton()->viewport_set_usage(onion.captures[i], VS::VIEWPORT_USAGE_2D);
+		VS::get_singleton()->viewport_set_size(onion.captures[i], capture_size.width, capture_size.height);
+		VS::get_singleton()->viewport_set_update_mode(onion.captures[i], VS::VIEWPORT_UPDATE_ALWAYS);
+		VS::get_singleton()->viewport_set_transparent_background(onion.captures[i], !is_present);
+		VS::get_singleton()->viewport_set_vflip(onion.captures[i], true);
+		VS::get_singleton()->viewport_attach_canvas(onion.captures[i], onion.capture.canvas);
+	}
+
+	// Reset the capture canvas item to the current root viewport texture (defensive)
+	VS::get_singleton()->canvas_item_clear(onion.capture.canvas_item);
+	VS::get_singleton()->canvas_item_add_texture_rect(onion.capture.canvas_item, Rect2(Point2(), capture_size), get_tree()->get_root()->get_texture()->get_rid());
+
+	onion.capture_size = capture_size;
+}
+
+void AnimationPlayerEditor::_free_onion_layers() {
+
+	for (int i = 0; i < onion.captures.size(); i++) {
+		if (onion.captures[i].is_valid()) {
+			VS::get_singleton()->free(onion.captures[i]);
+		}
+	}
+	onion.captures.clear();
+	onion.captures_valid.clear();
+}
+
+void AnimationPlayerEditor::_prepare_onion_layers_1() {
+
+	// This would be called per viewport and we want to act once only
+	int64_t frame = get_tree()->get_frame();
+	if (frame == onion.last_frame)
+		return;
+
+	if (!onion.enabled || !is_processing() || !is_visible() || !get_player()) {
+		_stop_onion_skinning();
+		return;
+	}
+
+	onion.last_frame = frame;
+
+	// Refresh viewports with no onion layers overlaid
+	onion.can_overlay = false;
+	plugin->update_overlays();
+
+	if (player->is_playing())
+		return;
+
+	// And go to next step afterwards
+	call_deferred("_prepare_onion_layers_2");
+}
+
+void AnimationPlayerEditor::_prepare_onion_layers_2() {
+
+	Ref<Animation> anim = player->get_animation(player->get_current_animation());
+	if (!anim.is_valid())
+		return;
+
+	if (!_are_onion_layers_valid())
+		_allocate_onion_layers();
+
+	// Hide superfluous elements that would make the overlay unnecessary cluttered
+	Dictionary canvas_edit_state;
+	Dictionary spatial_edit_state;
+	if (SpatialEditor::get_singleton()->is_visible()) {
+		// 3D
+		spatial_edit_state = SpatialEditor::get_singleton()->get_state();
+		Dictionary new_state = spatial_edit_state.copy();
+		new_state["show_grid"] = false;
+		new_state["show_origin"] = false;
+		Array orig_vp = spatial_edit_state["viewports"];
+		Array vp;
+		vp.resize(4);
+		for (int i = 0; i < vp.size(); i++) {
+			Dictionary d = ((Dictionary)orig_vp[i]).copy();
+			d["use_environment"] = false;
+			d["doppler"] = false;
+			d["gizmos"] = onion.include_gizmos ? d["gizmos"] : Variant(false);
+			d["information"] = false;
+			vp[i] = d;
+		}
+		new_state["viewports"] = vp;
+		// TODO: Save/restore only affected entries
+		SpatialEditor::get_singleton()->set_state(new_state);
+	} else { // CanvasItemEditor
+		// 2D
+		canvas_edit_state = CanvasItemEditor::get_singleton()->get_state();
+		Dictionary new_state = canvas_edit_state.copy();
+		new_state["show_grid"] = false;
+		new_state["show_rulers"] = false;
+		new_state["show_guides"] = false;
+		new_state["show_helpers"] = false;
+		// TODO: Save/restore only affected entries
+		CanvasItemEditor::get_singleton()->set_state(new_state);
+	}
+
+	// Tweak the root viewport to ensure it's rendered before our target
+	RID root_vp = get_tree()->get_root()->get_viewport_rid();
+	Rect2 root_vp_screen_rect = get_tree()->get_root()->get_attach_to_screen_rect();
+	VS::get_singleton()->viewport_attach_to_screen(root_vp, Rect2());
+	VS::get_singleton()->viewport_set_update_mode(root_vp, VS::VIEWPORT_UPDATE_ALWAYS);
+
+	RID present_rid;
+	if (onion.differences_only) {
+		// Capture present scene as it is
+		VS::get_singleton()->canvas_item_set_material(onion.capture.canvas_item, RID());
+		present_rid = onion.captures[onion.captures.size() - 1];
+		VS::get_singleton()->viewport_set_active(present_rid, true);
+		VS::get_singleton()->viewport_set_parent_viewport(root_vp, present_rid);
+		VS::get_singleton()->draw(false);
+		VS::get_singleton()->viewport_set_active(present_rid, false);
+	}
+
+	// Backup current animation state
+	AnimatedValuesBackup values_backup = player->backup_animated_values();
+	float cpos = player->get_current_animation_position();
+
+	// Render every past/future step with the capture shader
+
+	VS::get_singleton()->canvas_item_set_material(onion.capture.canvas_item, onion.capture.material->get_rid());
+	onion.capture.material->set_shader_param("bkg_color", GLOBAL_GET("rendering/environment/default_clear_color"));
+	onion.capture.material->set_shader_param("differences_only", onion.differences_only);
+	onion.capture.material->set_shader_param("present", onion.differences_only ? VS::get_singleton()->viewport_get_texture(present_rid) : RID());
+
+	int step_off_a = onion.past ? -onion.steps : 0;
+	int step_off_b = onion.future ? onion.steps : 0;
+	int cidx = 0;
+	onion.capture.material->set_shader_param("dir_color", onion.force_white_modulate ? Color(1, 1, 1) : Color(EDITOR_GET("editors/animation/onion_layers_past_color")));
+	for (int step_off = step_off_a; step_off <= step_off_b; step_off++) {
+
+		if (step_off == 0) {
+			// Skip present step and switch to the color of future
+			if (!onion.force_white_modulate)
+				onion.capture.material->set_shader_param("dir_color", EDITOR_GET("editors/animation/onion_layers_future_color"));
+			continue;
+		}
+
+		float pos = cpos + step_off * anim->get_step();
+
+		bool valid = anim->has_loop() || pos >= 0 && pos <= anim->get_length();
+		onion.captures_valid[cidx] = valid;
+		if (valid) {
+			player->seek(pos, true);
+			get_tree()->flush_transform_notifications(); // Needed for transforms of Spatials
+			values_backup.update_skeletons(); // Needed for Skeletons
+
+			VS::get_singleton()->viewport_set_active(onion.captures[cidx], true);
+			VS::get_singleton()->viewport_set_parent_viewport(root_vp, onion.captures[cidx]);
+			VS::get_singleton()->draw(false);
+			VS::get_singleton()->viewport_set_active(onion.captures[cidx], false);
+		}
+
+		cidx++;
+	}
+
+	// Restore root viewport
+	VS::get_singleton()->viewport_set_parent_viewport(root_vp, RID());
+	VS::get_singleton()->viewport_attach_to_screen(root_vp, root_vp_screen_rect);
+	VS::get_singleton()->viewport_set_update_mode(root_vp, VS::VIEWPORT_UPDATE_WHEN_VISIBLE);
+
+	// Restore animation state
+	// (Seeking with update=true wouldn't do the trick because the current value of the properties
+	// may not match their value for the current point in the animation)
+	player->seek(cpos, false);
+	player->restore_animated_values(values_backup);
+
+	// Restor state of main editors
+	if (SpatialEditor::get_singleton()->is_visible()) {
+		// 3D
+		SpatialEditor::get_singleton()->set_state(spatial_edit_state);
+	} else { // CanvasItemEditor
+		// 2D
+		CanvasItemEditor::get_singleton()->set_state(canvas_edit_state);
+	}
+
+	// Update viewports with skin layers overlaid for the actual engine loop render
+	onion.can_overlay = true;
+	plugin->update_overlays();
+}
+
+void AnimationPlayerEditor::_start_onion_skinning() {
+
+	// FIXME: Using "idle_frame" makes onion layers update one frame behing the current
+	if (!get_tree()->is_connected("idle_frame", this, "call_deferred")) {
+		get_tree()->connect("idle_frame", this, "call_deferred", varray("_prepare_onion_layers_1"));
+	}
+}
+
+void AnimationPlayerEditor::_stop_onion_skinning() {
+
+	if (get_tree()->is_connected("idle_frame", this, "call_deferred")) {
+
+		get_tree()->disconnect("idle_frame", this, "call_deferred");
+
+		_free_onion_layers();
+
+		// Clean up the overlay
+		onion.can_overlay = false;
+		plugin->update_overlays();
+	}
+}
+
 void AnimationPlayerEditor::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("_gui_input"), &AnimationPlayerEditor::_gui_input);
@@ -1165,6 +1522,10 @@ void AnimationPlayerEditor::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("_unhandled_key_input"), &AnimationPlayerEditor::_unhandled_key_input);
 	ClassDB::bind_method(D_METHOD("_animation_tool_menu"), &AnimationPlayerEditor::_animation_tool_menu);
 	ClassDB::bind_method(D_METHOD("_animation_save_menu"), &AnimationPlayerEditor::_animation_save_menu);
+	ClassDB::bind_method(D_METHOD("_onion_skinning_menu"), &AnimationPlayerEditor::_onion_skinning_menu);
+	ClassDB::bind_method(D_METHOD("_editor_visibility_changed"), &AnimationPlayerEditor::_editor_visibility_changed);
+	ClassDB::bind_method(D_METHOD("_prepare_onion_layers_1"), &AnimationPlayerEditor::_prepare_onion_layers_1);
+	ClassDB::bind_method(D_METHOD("_prepare_onion_layers_2"), &AnimationPlayerEditor::_prepare_onion_layers_2);
 }
 
 AnimationPlayerEditor *AnimationPlayerEditor::singleton = NULL;
@@ -1173,8 +1534,10 @@ AnimationPlayer *AnimationPlayerEditor::get_player() const {
 
 	return player;
 }
-AnimationPlayerEditor::AnimationPlayerEditor(EditorNode *p_editor) {
+
+AnimationPlayerEditor::AnimationPlayerEditor(EditorNode *p_editor, AnimationPlayerEditorPlugin *p_plugin) {
 	editor = p_editor;
+	plugin = p_plugin;
 	singleton = this;
 
 	updating = false;
@@ -1301,6 +1664,29 @@ AnimationPlayerEditor::AnimationPlayerEditor(EditorNode *p_editor) {
 	//tool_anim->get_popup()->add_item("Edit Anim Resource",TOOL_PASTE_ANIM);
 	hb->add_child(tool_anim);
 
+	onion_skinning = memnew(MenuButton);
+	//onion_skinning->set_flat(false);
+	onion_skinning->set_tooltip(TTR("Onion Skinning"));
+	onion_skinning->get_popup()->add_check_shortcut(ED_SHORTCUT("animation_player_editor/onion_skinning", TTR("Enable Onion Skinning")), ONION_SKINNING_ENABLE);
+	onion_skinning->get_popup()->add_separator();
+	onion_skinning->get_popup()->add_item(TTR("Directions"), -1);
+	onion_skinning->get_popup()->set_item_disabled(onion_skinning->get_popup()->get_item_count() - 1, true);
+	onion_skinning->get_popup()->add_check_item(TTR("Past"), ONION_SKINNING_PAST);
+	onion_skinning->get_popup()->set_item_checked(onion_skinning->get_popup()->get_item_count() - 1, true);
+	onion_skinning->get_popup()->add_check_item(TTR("Future"), ONION_SKINNING_FUTURE);
+	onion_skinning->get_popup()->add_separator();
+	onion_skinning->get_popup()->add_item(TTR("Depth"), -1);
+	onion_skinning->get_popup()->set_item_disabled(onion_skinning->get_popup()->get_item_count() - 1, true);
+	onion_skinning->get_popup()->add_check_item(TTR("1 step"), ONION_SKINNING_1_STEP);
+	onion_skinning->get_popup()->set_item_checked(onion_skinning->get_popup()->get_item_count() - 1, true);
+	onion_skinning->get_popup()->add_check_item(TTR("2 steps"), ONION_SKINNING_2_STEPS);
+	onion_skinning->get_popup()->add_check_item(TTR("3 steps"), ONION_SKINNING_3_STEPS);
+	onion_skinning->get_popup()->add_separator();
+	onion_skinning->get_popup()->add_check_item(TTR("Differences Only"), ONION_SKINNING_DIFFERENCES_ONLY);
+	onion_skinning->get_popup()->add_check_item(TTR("Force White Modulate"), ONION_SKINNING_FORCE_WHITE_MODULATE);
+	onion_skinning->get_popup()->add_check_item(TTR("Include Gizmos (3D)"), ONION_SKINNING_INCLUDE_GIZMOS);
+	hb->add_child(onion_skinning);
+
 	pin = memnew(ToolButton);
 	pin->set_toggle_mode(true);
 	hb->add_child(pin);
@@ -1387,6 +1773,68 @@ AnimationPlayerEditor::AnimationPlayerEditor(EditorNode *p_editor) {
 	key_editor->connect("animation_step_changed", this, "_animation_key_editor_anim_step_changed");
 
 	_update_player();
+
+	// Onion skinning
+
+	key_editor->connect("visibility_changed", this, "_editor_visibility_changed");
+
+	onion.enabled = false;
+	onion.past = true;
+	onion.future = false;
+	onion.steps = 1;
+	onion.differences_only = false;
+	onion.force_white_modulate = false;
+	onion.include_gizmos = false;
+
+	onion.last_frame = 0;
+	onion.can_overlay = false;
+	onion.capture_size = Size2();
+	onion.capture.canvas = VS::get_singleton()->canvas_create();
+	onion.capture.canvas_item = VS::get_singleton()->canvas_item_create();
+	VS::get_singleton()->canvas_item_set_parent(onion.capture.canvas_item, onion.capture.canvas);
+
+	onion.capture.material = Ref<ShaderMaterial>(memnew(ShaderMaterial));
+
+	onion.capture.shader = Ref<Shader>(memnew(Shader));
+	onion.capture.shader->set_code(" \
+		shader_type canvas_item; \
+		\
+        uniform vec4 bkg_color; \
+		uniform vec4 dir_color; \
+		uniform bool differences_only; \
+		uniform sampler2D present; \
+		\
+		float zero_if_equal(vec4 a, vec4 b) { \
+			return smoothstep(0.0, 0.005, length(a.rgb - b.rgb) / sqrt(3.0)); \
+		} \
+		\
+		void fragment() { \
+			vec4 capture_samp = texture(TEXTURE, UV); \
+			vec4 present_samp = texture(present, UV); \
+			float bkg_mask = zero_if_equal(capture_samp, bkg_color); \
+			float diff_mask = 1.0 - zero_if_equal(present_samp, bkg_color); \
+			diff_mask = min(1.0, diff_mask + float(!differences_only)); \
+			COLOR = vec4(capture_samp.rgb * dir_color.rgb, bkg_mask * diff_mask); \
+		} \
+	");
+	VS::get_singleton()->material_set_shader(onion.capture.material->get_rid(), onion.capture.shader->get_rid());
+}
+
+AnimationPlayerEditor::~AnimationPlayerEditor() {
+
+	_free_onion_layers();
+	VS::get_singleton()->free(onion.capture.canvas);
+	VS::get_singleton()->free(onion.capture.canvas_item);
+}
+
+void AnimationPlayerEditorPlugin::_notification(int p_what) {
+
+	switch (p_what) {
+		case NOTIFICATION_ENTER_TREE: {
+
+			set_force_draw_over_forwarding_enabled();
+		} break;
+	}
 }
 
 void AnimationPlayerEditorPlugin::edit(Object *p_object) {
@@ -1420,7 +1868,7 @@ void AnimationPlayerEditorPlugin::make_visible(bool p_visible) {
 AnimationPlayerEditorPlugin::AnimationPlayerEditorPlugin(EditorNode *p_node) {
 
 	editor = p_node;
-	anim_editor = memnew(AnimationPlayerEditor(editor));
+	anim_editor = memnew(AnimationPlayerEditor(editor, this));
 	anim_editor->set_undo_redo(editor->get_undo_redo());
 
 	editor->add_bottom_panel_item(TTR("Animation"), anim_editor);

editor/plugins/animation_player_editor_plugin.h

@@ -42,11 +42,13 @@
 	@author Juan Linietsky <reduzio@gmail.com>
 */
 class AnimationKeyEditor;
+class AnimationPlayerEditorPlugin;
 class AnimationPlayerEditor : public VBoxContainer {
 
 	GDCLASS(AnimationPlayerEditor, VBoxContainer);
 
 	EditorNode *editor;
+	AnimationPlayerEditorPlugin *plugin;
 	AnimationPlayer *player;
 
 	enum {
@@ -55,6 +57,19 @@ class AnimationPlayerEditor : public VBoxContainer {
 		TOOL_EDIT_RESOURCE
 	};
 
+	enum {
+		ONION_SKINNING_ENABLE,
+		ONION_SKINNING_PAST,
+		ONION_SKINNING_FUTURE,
+		ONION_SKINNING_1_STEP,
+		ONION_SKINNING_2_STEPS,
+		ONION_SKINNING_3_STEPS,
+		ONION_SKINNING_LAST_STEPS_OPTION = ONION_SKINNING_3_STEPS,
+		ONION_SKINNING_DIFFERENCES_ONLY,
+		ONION_SKINNING_FORCE_WHITE_MODULATE,
+		ONION_SKINNING_INCLUDE_GIZMOS,
+	};
+
 	enum {
 		ANIM_SAVE,
 		ANIM_SAVE_AS
@@ -84,6 +99,7 @@ class AnimationPlayerEditor : public VBoxContainer {
 	Button *blend_anim;
 	Button *remove_anim;
 	MenuButton *tool_anim;
+	MenuButton *onion_skinning;
 	ToolButton *pin;
 	SpinBox *frame;
 	LineEdit *scale;
@@ -115,6 +131,36 @@ class AnimationPlayerEditor : public VBoxContainer {
 
 	AnimationKeyEditor *key_editor;
 
+	// Onion skinning
+	struct {
+		// Settings
+		bool enabled;
+		bool past;
+		bool future;
+		int steps;
+		bool differences_only;
+		bool force_white_modulate;
+		bool include_gizmos;
+
+		int get_needed_capture_count() const {
+			// 'Differences only' needs a capture of the present
+			return (past && future ? 2 * steps : steps) + (differences_only ? 1 : 0);
+		}
+
+		// Rendering
+		int64_t last_frame;
+		int can_overlay;
+		Size2 capture_size;
+		Vector<RID> captures;
+		Vector<bool> captures_valid;
+		struct {
+			RID canvas;
+			RID canvas_item;
+			Ref<ShaderMaterial> material;
+			Ref<Shader> shader;
+		} capture;
+	} onion;
+
 	void _select_anim_by_name(const String &p_anim);
 	void _play_pressed();
 	void _play_from_pressed();
@@ -161,8 +207,19 @@ class AnimationPlayerEditor : public VBoxContainer {
 	void _unhandled_key_input(const Ref<InputEvent> &p_ev);
 	void _animation_tool_menu(int p_option);
 	void _animation_save_menu(int p_option);
+	void _onion_skinning_menu(int p_option);
+
+	void _editor_visibility_changed();
+	bool _are_onion_layers_valid();
+	void _allocate_onion_layers();
+	void _free_onion_layers();
+	void _prepare_onion_layers_1();
+	void _prepare_onion_layers_2();
+	void _start_onion_skinning();
+	void _stop_onion_skinning();
 
 	AnimationPlayerEditor();
+	~AnimationPlayerEditor();
 
 protected:
 	void _notification(int p_what);
@@ -182,7 +239,9 @@ public:
 
 	void set_undo_redo(UndoRedo *p_undo_redo) { undo_redo = p_undo_redo; }
 	void edit(AnimationPlayer *p_player);
-	AnimationPlayerEditor(EditorNode *p_editor);
+	void forward_force_draw_over_viewport(Control *p_overlay);
+
+	AnimationPlayerEditor(EditorNode *p_editor, AnimationPlayerEditorPlugin *p_plugin);
 };
 
 class AnimationPlayerEditorPlugin : public EditorPlugin {
@@ -192,6 +251,9 @@ class AnimationPlayerEditorPlugin : public EditorPlugin {
 	AnimationPlayerEditor *anim_editor;
 	EditorNode *editor;
 
+protected:
+	void _notification(int p_what);
+
 public:
 	virtual Dictionary get_state() const { return anim_editor->get_state(); }
 	virtual void set_state(const Dictionary &p_state) { anim_editor->set_state(p_state); }
@@ -202,6 +264,8 @@ public:
 	virtual bool handles(Object *p_object) const;
 	virtual void make_visible(bool p_visible);
 
+	virtual void forward_force_draw_over_viewport(Control *p_overlay) { anim_editor->forward_force_draw_over_viewport(p_overlay); }
+
 	AnimationPlayerEditorPlugin(EditorNode *p_node);
 	~AnimationPlayerEditorPlugin();
 };

scene/animation/animation_player.cpp

@@ -33,6 +33,17 @@
 #include "message_queue.h"
 #include "scene/scene_string_names.h"
 
+#ifdef TOOLS_ENABLED
+void AnimatedValuesBackup::update_skeletons() {
+
+	for (int i = 0; i < entries.size(); i++) {
+		if (entries[i].bone_idx != -1) {
+			Object::cast_to<Skeleton>(entries[i].object)->notification(Skeleton::NOTIFICATION_UPDATE_SKELETON);
+		}
+	}
+}
+#endif
+
 bool AnimationPlayer::_set(const StringName &p_name, const Variant &p_value) {
 
 	String name = p_name;

scene/animation/animation_player.h

@@ -50,6 +50,9 @@ class AnimatedValuesBackup {
 	Vector<Entry> entries;
 
 	friend class AnimationPlayer;
+
+public:
+	void update_skeletons();
 };
 #endif
 

scene/main/scene_tree.cpp

@@ -127,7 +127,7 @@ void SceneTree::remove_from_group(const StringName &p_group, Node *p_node) {
 		group_map.erase(E);
 }
 
-void SceneTree::_flush_transform_notifications() {
+void SceneTree::flush_transform_notifications() {
 
 	SelfList<Node> *n = xform_change_list.first();
 	while (n) {
@@ -448,7 +448,7 @@ bool SceneTree::iteration(float p_time) {
 
 	current_frame++;
 
-	_flush_transform_notifications();
+	flush_transform_notifications();
 
 	MainLoop::iteration(p_time);
 	physics_process_time = p_time;
@@ -459,7 +459,7 @@ bool SceneTree::iteration(float p_time) {
 	_notify_group_pause("physics_process", Node::NOTIFICATION_PHYSICS_PROCESS);
 	_flush_ugc();
 	MessageQueue::get_singleton()->flush(); //small little hack
-	_flush_transform_notifications();
+	flush_transform_notifications();
 	call_group_flags(GROUP_CALL_REALTIME, "_viewports", "update_worlds");
 	root_lock--;
 
@@ -487,7 +487,7 @@ bool SceneTree::idle(float p_time) {
 
 	MessageQueue::get_singleton()->flush(); //small little hack
 
-	_flush_transform_notifications();
+	flush_transform_notifications();
 
 	_notify_group_pause("idle_process_internal", Node::NOTIFICATION_INTERNAL_PROCESS);
 	_notify_group_pause("idle_process", Node::NOTIFICATION_PROCESS);
@@ -503,7 +503,7 @@ bool SceneTree::idle(float p_time) {
 
 	_flush_ugc();
 	MessageQueue::get_singleton()->flush(); //small little hack
-	_flush_transform_notifications(); //transforms after world update, to avoid unnecessary enter/exit notifications
+	flush_transform_notifications(); //transforms after world update, to avoid unnecessary enter/exit notifications
 	call_group_flags(GROUP_CALL_REALTIME, "_viewports", "update_worlds");
 
 	root_lock--;

scene/main/scene_tree.h

@@ -157,7 +157,6 @@ private:
 	Map<UGCall, Vector<Variant> > unique_group_calls;
 	bool ugc_locked;
 	void _flush_ugc();
-	void _flush_transform_notifications();
 
 	_FORCE_INLINE_ void _update_group_order(Group &g);
 	void _update_listener();
@@ -344,6 +343,8 @@ public:
 	void notify_group(const StringName &p_group, int p_notification);
 	void set_group(const StringName &p_group, const String &p_name, const Variant &p_value);
 
+	void flush_transform_notifications();
+
 	virtual void input_text(const String &p_text);
 	virtual void input_event(const Ref<InputEvent> &p_event);
 	virtual void init();