Enable SpriteFramesEditor to guess the amount of rows and columns of a sprite sheet when loading it for the first time

editor/plugins/sprite_frames_editor_plugin.cpp

@@ -504,6 +504,82 @@ void SpriteFramesEditor::_update_show_settings() {
 	}
 }
 
+void SpriteFramesEditor::_auto_slice_sprite_sheet() {
+	if (updating_split_settings) {
+		return;
+	}
+	updating_split_settings = true;
+
+	const Size2i size = split_sheet_preview->get_texture()->get_size();
+
+	const Size2i split_sheet = _estimate_sprite_sheet_size(split_sheet_preview->get_texture());
+	split_sheet_h->set_value(split_sheet.x);
+	split_sheet_v->set_value(split_sheet.y);
+	split_sheet_size_x->set_value(size.x / split_sheet.x);
+	split_sheet_size_y->set_value(size.y / split_sheet.y);
+	split_sheet_sep_x->set_value(0);
+	split_sheet_sep_y->set_value(0);
+	split_sheet_offset_x->set_value(0);
+	split_sheet_offset_y->set_value(0);
+
+	updating_split_settings = false;
+
+	frames_selected.clear();
+	selected_count = 0;
+	last_frame_selected = -1;
+	split_sheet_preview->queue_redraw();
+}
+
+bool SpriteFramesEditor::_matches_background_color(const Color &p_background_color, const Color &p_pixel_color) {
+	if ((p_background_color.a == 0 && p_pixel_color.a == 0) || p_background_color.is_equal_approx(p_pixel_color)) {
+		return true;
+	}
+
+	Color d = p_background_color - p_pixel_color;
+	// 0.04f is the threshold for how much a colour can deviate from background colour and still be considered a match. Arrived at through experimentation, can be tweaked.
+	return (d.r * d.r) + (d.g * d.g) + (d.b * d.b) + (d.a * d.a) < 0.04f;
+}
+
+Size2i SpriteFramesEditor::_estimate_sprite_sheet_size(const Ref<Texture2D> p_texture) {
+	Ref<Image> image = p_texture->get_image();
+	Size2i size = p_texture->get_size();
+
+	Color assumed_background_color = image->get_pixel(0, 0);
+	Size2i sheet_size;
+
+	bool previous_line_background = true;
+	for (int x = 0; x < size.x; x++) {
+		int y = 0;
+		while (y < size.y && _matches_background_color(assumed_background_color, image->get_pixel(x, y))) {
+			y++;
+		}
+		bool current_line_background = (y == size.y);
+		if (previous_line_background && !current_line_background) {
+			sheet_size.x++;
+		}
+		previous_line_background = current_line_background;
+	}
+
+	previous_line_background = true;
+	for (int y = 0; y < size.y; y++) {
+		int x = 0;
+		while (x < size.x && _matches_background_color(assumed_background_color, image->get_pixel(x, y))) {
+			x++;
+		}
+		bool current_line_background = (x == size.x);
+		if (previous_line_background && !current_line_background) {
+			sheet_size.y++;
+		}
+		previous_line_background = current_line_background;
+	}
+
+	if (sheet_size == Size2i(0, 0) || sheet_size == Size2i(1, 1)) {
+		sheet_size = Size2i(4, 4);
+	}
+
+	return sheet_size;
+}
+
 void SpriteFramesEditor::_prepare_sprite_sheet(const String &p_file) {
 	Ref<Texture2D> texture = ResourceLoader::load(p_file);
 	if (texture.is_null()) {
@@ -530,10 +606,11 @@ void SpriteFramesEditor::_prepare_sprite_sheet(const String &p_file) {
 			// Different texture, reset to 4x4.
 			dominant_param = PARAM_FRAME_COUNT;
 			updating_split_settings = true;
-			split_sheet_h->set_value(4);
-			split_sheet_v->set_value(4);
-			split_sheet_size_x->set_value(size.x / 4);
-			split_sheet_size_y->set_value(size.y / 4);
+			const Size2i split_sheet = Size2i(4, 4);
+			split_sheet_h->set_value(split_sheet.x);
+			split_sheet_v->set_value(split_sheet.y);
+			split_sheet_size_x->set_value(size.x / split_sheet.x);
+			split_sheet_size_y->set_value(size.y / split_sheet.y);
 			split_sheet_sep_x->set_value(0);
 			split_sheet_sep_y->set_value(0);
 			split_sheet_offset_x->set_value(0);
@@ -2290,6 +2367,11 @@ SpriteFramesEditor::SpriteFramesEditor() {
 	split_sheet_offset_hb->add_child(split_sheet_offset_vb);
 	split_sheet_settings_vb->add_child(split_sheet_offset_hb);
 
+	Button *auto_slice = memnew(Button);
+	auto_slice->set_text(TTR("Auto Slice"));
+	auto_slice->connect(SceneStringName(pressed), callable_mp(this, &SpriteFramesEditor::_auto_slice_sprite_sheet));
+	split_sheet_settings_vb->add_child(auto_slice);
+
 	split_sheet_hb->add_child(split_sheet_settings_vb);
 
 	file_split_sheet = memnew(EditorFileDialog);

editor/plugins/sprite_frames_editor_plugin.h

@@ -234,6 +234,9 @@ class SpriteFramesEditor : public HSplitContainer {
 	void drop_data_fw(const Point2 &p_point, const Variant &p_data, Control *p_from);
 
 	void _open_sprite_sheet();
+	void _auto_slice_sprite_sheet();
+	bool _matches_background_color(const Color &p_background_color, const Color &p_pixel_color);
+	Size2i _estimate_sprite_sheet_size(const Ref<Texture2D> p_texture);
 	void _prepare_sprite_sheet(const String &p_file);
 	int _sheet_preview_position_to_frame_index(const Vector2 &p_position);
 	void _sheet_preview_draw();