render: introduce blend mode

Allow callers to pick the blend mode when rendering a rect. The
"none" mode can be used to disable blending and clear rects.

include/wlr/render/wlr_renderer.h

@@ -177,6 +177,16 @@ struct wlr_render_pass *wlr_renderer_begin_buffer_pass(
  */
 bool wlr_render_pass_submit(struct wlr_render_pass *render_pass);
 
+/**
+ * Blend modes.
+ */
+enum wlr_render_blend_mode {
+	/* Pre-multiplied alpha (default) */
+	WLR_RENDER_BLEND_MODE_PREMULTIPLIED,
+	/* Blending is disabled */
+	WLR_RENDER_BLEND_MODE_NONE,
+};
+
 struct wlr_render_texture_options {
 	/* Source texture */
 	struct wlr_texture *texture;
@@ -215,6 +225,8 @@ struct wlr_render_rect_options {
 	struct wlr_render_color color;
 	/* Clip region, leave NULL to disable clipping */
 	const pixman_region32_t *clip;
+	/* Blend mode */
+	enum wlr_render_blend_mode blend_mode;
 };
 
 /**

render/pass.c

@@ -134,6 +134,8 @@ static void legacy_add_rect(struct wlr_render_pass *wlr_pass,
 
 	pixman_region32_t clip;
 	get_clip_region(pass, options->clip, &clip);
+	pixman_region32_intersect_rect(&clip, &clip,
+		options->box.x, options->box.y, options->box.width, options->box.height);
 
 	float color[4] = {
 		options->color.r,
@@ -146,7 +148,14 @@ static void legacy_add_rect(struct wlr_render_pass *wlr_pass,
 	const pixman_box32_t *rects = pixman_region32_rectangles(&clip, &rects_len);
 	for (int i = 0; i < rects_len; i++) {
 		scissor(pass->renderer, &rects[i]);
-		wlr_render_quad_with_matrix(pass->renderer, color, matrix);
+		switch (options->blend_mode) {
+		case WLR_RENDER_BLEND_MODE_PREMULTIPLIED:
+			wlr_render_quad_with_matrix(pass->renderer, color, matrix);
+			break;
+		case WLR_RENDER_BLEND_MODE_NONE:
+			wlr_renderer_clear(pass->renderer, color);
+			break;
+		}
 	}
 
 	wlr_renderer_scissor(pass->renderer, NULL);

render/pixman/pass.c

@@ -143,6 +143,17 @@ static void render_pass_add_rect(struct wlr_render_pass *wlr_pass,
 	struct wlr_pixman_buffer *buffer = pass->buffer;
 	struct wlr_box box = options->box;
 
+	pixman_op_t op = 0;
+	switch (options->blend_mode) {
+	case WLR_RENDER_BLEND_MODE_PREMULTIPLIED:
+		op = options->color.a == 1 ? PIXMAN_OP_SRC : PIXMAN_OP_OVER;
+		break;
+	case WLR_RENDER_BLEND_MODE_NONE:
+		op = PIXMAN_OP_SRC;
+		break;
+	}
+	assert(op != 0);
+
 	struct pixman_color color = {
 		.red = options->color.r * 0xFFFF,
 		.green = options->color.g * 0xFFFF,
@@ -153,7 +164,7 @@ static void render_pass_add_rect(struct wlr_render_pass *wlr_pass,
 	pixman_image_t *fill = pixman_image_create_solid_fill(&color);
 
 	pixman_image_set_clip_region32(buffer->image, (pixman_region32_t *)options->clip);
-	pixman_image_composite32(PIXMAN_OP_OVER, fill, NULL, buffer->image,
+	pixman_image_composite32(op, fill, NULL, buffer->image,
 		0, 0, 0, 0, box.x, box.y, box.width, box.height);
 	pixman_image_set_clip_region32(buffer->image, NULL);