render: add support for clearing render passes

introduces a new `wlr_render_pass_clear` function, allowing
render passes to be cleared with a solid color.

include/wlr/render/interface.h

@@ -62,6 +62,8 @@ struct wlr_render_pass_impl {
 	/* Implementers are also guaranteed that options->box is nonempty */
 	void (*add_rect)(struct wlr_render_pass *pass,
 		const struct wlr_render_rect_options *options);
+	void (*clear)(struct wlr_render_pass *pass,
+		const struct wlr_render_clear_options *options);
 };
 
 struct wlr_render_timer {

include/wlr/render/pass.h

@@ -157,4 +157,20 @@ struct wlr_render_rect_options {
 void wlr_render_pass_add_rect(struct wlr_render_pass *render_pass,
 	const struct wlr_render_rect_options *options);
 
+struct wlr_render_clear_options {
+	/* Clear color */
+	struct wlr_render_color color;
+	/* Clip region, leave NULL to disable clipping */
+	const pixman_region32_t *clip;
+};
+
+/**
+ * Clear the render target with a solid color.
+ *
+ * This is optimized for GPU hardware (e.g., glClear + glScissor) and is more
+ * efficient than drawing a rectangle.
+ */
+void wlr_render_pass_clear(struct wlr_render_pass *render_pass,
+	const struct wlr_render_clear_options *options);
+
 #endif

render/gles2/pass.c

@@ -275,10 +275,41 @@ static void render_pass_add_rect(struct wlr_render_pass *wlr_pass,
 	pop_gles2_debug(renderer);
 }
 
+static void render_pass_clear(struct wlr_render_pass *wlr_pass,
+		const struct wlr_render_clear_options *options) {
+	struct wlr_gles2_render_pass *pass = get_render_pass(wlr_pass);
+	struct wlr_gles2_renderer *renderer = pass->buffer->renderer;
+
+	push_gles2_debug(renderer);
+
+	const struct wlr_render_color *color = &options->color;
+	glClearColor(color->r, color->g, color->b, color->a);
+
+	if (options->clip) {
+		int rects_len;
+		const pixman_box32_t *rects = pixman_region32_rectangles(options->clip, &rects_len);
+
+		glEnable(GL_SCISSOR_TEST);
+		for (int i = 0; i < rects_len; i++) {
+			const pixman_box32_t *rect = &rects[i];
+			int height = pass->buffer->buffer->height;
+			glScissor(rect->x1, height - rect->y2,
+				rect->x2 - rect->x1, rect->y2 - rect->y1);
+			glClear(GL_COLOR_BUFFER_BIT);
+		}
+		glDisable(GL_SCISSOR_TEST);
+	} else {
+		glClear(GL_COLOR_BUFFER_BIT);
+	}
+
+	pop_gles2_debug(renderer);
+}
+
 static const struct wlr_render_pass_impl render_pass_impl = {
 	.submit = render_pass_submit,
 	.add_texture = render_pass_add_texture,
 	.add_rect = render_pass_add_rect,
+	.clear = render_pass_clear,
 };
 
 static const char *reset_status_str(GLenum status) {

render/pass.c

@@ -4,7 +4,7 @@
 
 void wlr_render_pass_init(struct wlr_render_pass *render_pass,
 		const struct wlr_render_pass_impl *impl) {
-	assert(impl->submit && impl->add_texture && impl->add_rect);
+	assert(impl->submit && impl->add_texture && impl->add_rect && impl->clear);
 	*render_pass = (struct wlr_render_pass){
 		.impl = impl,
 	};
@@ -34,6 +34,11 @@ void wlr_render_pass_add_rect(struct wlr_render_pass *render_pass,
 	render_pass->impl->add_rect(render_pass, options);
 }
 
+void wlr_render_pass_clear(struct wlr_render_pass *render_pass,
+		const struct wlr_render_clear_options *options) {
+	render_pass->impl->clear(render_pass, options);
+}
+
 void wlr_render_texture_options_get_src_box(const struct wlr_render_texture_options *options,
 		struct wlr_fbox *box) {
 	*box = options->src_box;

render/pixman/pass.c

@@ -225,10 +225,23 @@ static void render_pass_add_rect(struct wlr_render_pass *wlr_pass,
 	pixman_image_unref(fill);
 }
 
+static void render_pass_clear(struct wlr_render_pass *wlr_pass,
+		const struct wlr_render_clear_options *options) {
+	struct wlr_pixman_render_pass *pass = get_render_pass(wlr_pass);
+
+	render_pass_add_rect(wlr_pass, &(struct wlr_render_rect_options){
+		.box = { .width = pass->buffer->buffer->width, .height = pass->buffer->buffer->height },
+		.color = options->color,
+		.clip = options->clip,
+		.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
+	});
+}
+
 static const struct wlr_render_pass_impl render_pass_impl = {
 	.submit = render_pass_submit,
 	.add_texture = render_pass_add_texture,
 	.add_rect = render_pass_add_rect,
+	.clear = render_pass_clear,
 };
 
 struct wlr_pixman_render_pass *begin_pixman_render_pass(

render/vulkan/pass.c

@@ -943,13 +943,60 @@ static void render_pass_add_texture(struct wlr_render_pass *wlr_pass,
 	}
 }
 
+static void render_pass_clear(struct wlr_render_pass *wlr_pass,
+		const struct wlr_render_clear_options *options) {
+	struct wlr_vk_render_pass *pass = get_render_pass(wlr_pass);
+	VkCommandBuffer cb = pass->command_buffer->vk;
+
+	float linear_color[] = {
+		color_to_linear_premult(options->color.r, options->color.a),
+		color_to_linear_premult(options->color.g, options->color.a),
+		color_to_linear_premult(options->color.b, options->color.a),
+		options->color.a,
+	};
+
+	VkClearAttachment clear_att = {
+		.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT,
+		.colorAttachment = 0,
+		.clearValue.color.float32 = {
+			linear_color[0],
+			linear_color[1],
+			linear_color[2],
+			linear_color[3],
+		},
+	};
+
+	pixman_region32_t clip;
+	get_clip_region(pass, options->clip, &clip);
+
+	int clip_rects_len;
+	const pixman_box32_t *clip_rects = pixman_region32_rectangles(&clip, &clip_rects_len);
+
+	VkClearRect clear_rect = {
+		.layerCount = 1,
+	};
+	for (int i = 0; i < clip_rects_len; i++) {
+		convert_pixman_box_to_vk_rect(&clip_rects[i], &clear_rect.rect);
+		vkCmdClearAttachments(cb, 1, &clear_att, 1, &clear_rect);
+		struct wlr_box box = {
+			.x = clip_rects[i].x1,
+			.y = clip_rects[i].y1,
+			.width = clip_rects[i].x2 - clip_rects[i].x1,
+			.height = clip_rects[i].y2 - clip_rects[i].y1,
+		};
+		render_pass_mark_box_updated(pass, &box);
+	}
+
+	pixman_region32_fini(&clip);
+}
+
 static const struct wlr_render_pass_impl render_pass_impl = {
 	.submit = render_pass_submit,
 	.add_rect = render_pass_add_rect,
 	.add_texture = render_pass_add_texture,
+	.clear = render_pass_clear,
 };
 
-
 void vk_color_transform_destroy(struct wlr_addon *addon) {
 	struct wlr_vk_renderer *renderer = (struct wlr_vk_renderer *)addon->owner;
 	struct wlr_vk_color_transform *transform = wl_container_of(addon, transform, addon);

types/output/cursor.c

@@ -268,9 +268,8 @@ static struct wlr_buffer *render_cursor_buffer(struct wlr_output_cursor *cursor)
 	enum wl_output_transform transform = wlr_output_transform_invert(cursor->transform);
 	transform = wlr_output_transform_compose(transform, output->transform);
 
-	wlr_render_pass_add_rect(pass, &(struct wlr_render_rect_options){
+	wlr_render_pass_clear(pass, &(struct wlr_render_clear_options){
-		.box = { .width = buffer->width, .height = buffer->height },
+		.color = { 0, 0, 0, 0 },
-		.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
 	});
 	wlr_render_pass_add_texture(pass, &(struct wlr_render_texture_options){
 		.texture = texture,
@@ -535,4 +534,4 @@ bool output_cursor_refresh_color_transform(struct wlr_output_cursor *output_curs
 	wlr_color_transform_unref(transforms[0]);
 	wlr_color_transform_unref(transforms[1]);
 	return output_cursor->color_transform != NULL;
-}
+}

types/output/render.c

@@ -63,9 +63,8 @@ static struct wlr_buffer *output_acquire_empty_buffer(struct wlr_output *output,
 		return NULL;
 	}
 
-	wlr_render_pass_add_rect(pass, &(struct wlr_render_rect_options){
+	wlr_render_pass_clear(pass, &(struct wlr_render_clear_options){
 		.color = { 0, 0, 0, 0 },
-		.blend_mode = WLR_RENDER_BLEND_MODE_NONE,
 	});
 
 	if (!wlr_render_pass_submit(pass)) {