diff --git a/src/backend/backend.c b/src/backend/backend.c
index 439cb037..27ca8d93 100644
--- a/src/backend/backend.c
+++ b/src/backend/backend.c
@@ -259,7 +259,8 @@ void paint_all_new(session_t *ps, struct managed_win *t, bool ignore_damage) {
 
 	if (ps->o.monitor_repaint) {
 		reg_damage = get_damage(ps, false);
-		ps->backend_data->ops->fill(ps->backend_data, 0.5, 0, 0, 0.5, &reg_damage);
+		ps->backend_data->ops->fill(ps->backend_data,
+		                            (struct color){0.5, 0, 0, 0.5}, &reg_damage);
 		pixman_region32_fini(&reg_damage);
 	}
 
diff --git a/src/backend/backend.h b/src/backend/backend.h
index 6d91e8ab..37c55d1b 100644
--- a/src/backend/backend.h
+++ b/src/backend/backend.h
@@ -9,6 +9,7 @@
 #include "driver.h"
 #include "kernel.h"
 #include "region.h"
+#include "types.h"
 #include "x.h"
 
 typedef struct session session_t;
@@ -104,8 +105,7 @@ struct backend_operations {
 	                const region_t *reg_paint, const region_t *reg_visible);
 
 	/// Fill rectangle of target, mostly for debug purposes, optional.
-	void (*fill)(backend_t *backend_data, double r, double g, double b, double a,
-	             const region_t *clip);
+	void (*fill)(backend_t *backend_data, struct color, const region_t *clip);
 
 	/// Blur a given region of the target.
 	bool (*blur)(backend_t *backend_data, double opacity, const region_t *reg_blur,
diff --git a/src/backend/gl/gl_common.c b/src/backend/gl/gl_common.c
index 604f7d7f..70ade2da 100644
--- a/src/backend/gl/gl_common.c
+++ b/src/backend/gl/gl_common.c
@@ -367,7 +367,7 @@ bool gl_blur(backend_t *base, double opacity, const region_t *reg_blur,
 	auto coord = ccalloc(nrects * 16, GLfloat);
 	auto indices = ccalloc(nrects * 6, GLuint);
 	x_rect_to_coords(nrects, rects, extent->x1, extent->y2, gd->width, gd->height,
-			gd->height, false, coord, indices);
+	                 gd->height, false, coord, indices);
 
 	GLuint vao;
 	glGenVertexArrays(1, &vao);
@@ -575,7 +575,10 @@ static const char fill_vert[] = GLSL(330,
 );
 // clang-format on
 
-void gl_fill(backend_t *base, double r, double g, double b, double a, const region_t *clip) {
+/// Fill a given region in bound framebuffer.
+/// @param[in] y_inverted whether the y coordinates in `clip` should be inverted
+static void
+_gl_fill(backend_t *base, struct color c, const region_t *clip, int height, bool y_inverted) {
 	int nrects;
 	const rect_t *rect = pixman_region32_rectangles((region_t *)clip, &nrects);
 	struct gl_data *gd = (void *)base;
@@ -587,7 +590,8 @@ void gl_fill(backend_t *base, double r, double g, double b, double a, const regi
 	GLuint bo[2];
 	glGenBuffers(2, bo);
 	glUseProgram(gd->fill_shader.prog);
-	glUniform4f(gd->fill_shader.color_loc, (GLfloat)r, (GLfloat)g, (GLfloat)b, (GLfloat)a);
+	glUniform4f(gd->fill_shader.color_loc, (GLfloat)c.red, (GLfloat)c.green,
+	            (GLfloat)c.blue, (GLfloat)c.alpha);
 	glEnableVertexAttribArray((GLuint)gd->fill_shader.in_coord_loc);
 	glBindBuffer(GL_ARRAY_BUFFER, bo[0]);
 	glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, bo[1]);
@@ -595,11 +599,11 @@ void gl_fill(backend_t *base, double r, double g, double b, double a, const regi
 	auto coord = ccalloc(nrects * 8, GLint);
 	auto indices = ccalloc(nrects * 6, GLuint);
 	for (int i = 0; i < nrects; i++) {
+		GLint y1 = y_inverted ? height - rect[i].y2 : rect[i].y1,
+		      y2 = y_inverted ? height - rect[i].y1 : rect[i].y2;
 		memcpy(&coord[i * 8],
-		       (GLint[][2]){{rect[i].x1, gd->height - rect[i].y2},
-		                    {rect[i].x2, gd->height - rect[i].y2},
-		                    {rect[i].x2, gd->height - rect[i].y1},
-		                    {rect[i].x1, gd->height - rect[i].y1}},
+		       (GLint[][2]){
+		           {rect[i].x1, y1}, {rect[i].x2, y1}, {rect[i].x2, y2}, {rect[i].x1, y2}},
 		       sizeof(GLint[2]) * 4);
 		indices[i * 6 + 0] = (GLuint)i * 4 + 0;
 		indices[i * 6 + 1] = (GLuint)i * 4 + 1;
@@ -624,6 +628,11 @@ void gl_fill(backend_t *base, double r, double g, double b, double a, const regi
 	glDeleteBuffers(2, bo);
 }
 
+void gl_fill(backend_t *base, struct color c, const region_t *clip) {
+	struct gl_data *gd = (void *)base;
+	return _gl_fill(base, c, clip, gd->height, true);
+}
+
 void gl_release_image(backend_t *base, void *image_data) {
 	struct gl_image *wd = image_data;
 	struct gl_data *gl = (void *)base;
@@ -967,7 +976,20 @@ static inline void gl_image_decouple(backend_t *base, struct gl_image *img) {
 	img->opacity = 1;
 }
 
-static void gl_image_apply_alpha(struct gl_image *img, const region_t *reg_op, double alpha) {
+static void gl_image_apply_alpha(backend_t *base, struct gl_image *img,
+                                 const region_t *reg_op, double alpha) {
+	glBlendFunc(GL_ONE, GL_CONSTANT_COLOR);
+	glBlendColor(1, 1, 1, (GLclampf)alpha);
+	GLuint fbo;
+	glGenFramebuffers(1, &fbo);
+	glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo);
+	glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
+	                       img->inner->texture, 0);
+	glDrawBuffer(GL_COLOR_ATTACHMENT0);
+	_gl_fill(base, (struct color){0, 0, 0, 0}, reg_op, 0, false);
+	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
+	glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
+	glDeleteFramebuffers(1, &fbo);
 }
 
 /// stub for backend_operations::image_op
@@ -983,8 +1005,8 @@ bool gl_image_op(backend_t *base, enum image_operations op, void *image_data,
 	case IMAGE_OP_APPLY_ALPHA_ALL: tex->opacity *= *(double *)arg; break;
 	case IMAGE_OP_APPLY_ALPHA:
 		gl_image_decouple(base, tex);
-		gl_image_apply_alpha(tex, reg_op, *(double *)arg);
-		log_warn("IMAGE_OP_APPLY_ALPHA not implemented yet");
+		assert(tex->inner->refcount == 1);
+		gl_image_apply_alpha(base, tex, reg_op, *(double *)arg);
 		break;
 	case IMAGE_OP_RESIZE_TILE:
 		// texture is already set to repeat, so nothing else we need to do
diff --git a/src/backend/gl/gl_common.h b/src/backend/gl/gl_common.h
index efc81d58..06221bb1 100644
--- a/src/backend/gl/gl_common.h
+++ b/src/backend/gl/gl_common.h
@@ -132,7 +132,7 @@ bool gl_blur(backend_t *base, double opacity, const region_t *reg_blur,
              const region_t *reg_visible);
 
 bool gl_is_image_transparent(backend_t *base, void *image_data);
-void gl_fill(backend_t *base, double r, double g, double b, double a, const region_t *clip);
+void gl_fill(backend_t *base, struct color, const region_t *clip);
 
 static inline void gl_delete_texture(GLuint texture) {
 	glDeleteTextures(1, &texture);
diff --git a/src/backend/xrender/xrender.c b/src/backend/xrender/xrender.c
index 18ae257d..63e2a841 100644
--- a/src/backend/xrender/xrender.c
+++ b/src/backend/xrender/xrender.c
@@ -107,17 +107,17 @@ static void compose(backend_t *base, void *img_data, int dst_x, int dst_y,
 }
 
 static void
-fill(backend_t *base, double r, double g, double b, double a, const region_t *clip) {
+fill(backend_t *base, struct color c, const region_t *clip) {
 	struct _xrender_data *xd = (void *)base;
 	const rect_t *extent = pixman_region32_extents((region_t *)clip);
 	x_set_picture_clip_region(base->c, xd->back[xd->curr_back], 0, 0, clip);
 	// color is in X fixed point representation
 	xcb_render_fill_rectangles(
 	    base->c, XCB_RENDER_PICT_OP_OVER, xd->back[xd->curr_back],
-	    (xcb_render_color_t){.red = (uint16_t)(r * 0xffff),
-	                         .green = (uint16_t)(g * 0xffff),
-	                         .blue = (uint16_t)(b * 0xffff),
-	                         .alpha = (uint16_t)(a * 0xffff)},
+	    (xcb_render_color_t){.red = (uint16_t)(c.red * 0xffff),
+	                         .green = (uint16_t)(c.green * 0xffff),
+	                         .blue = (uint16_t)(c.blue * 0xffff),
+	                         .alpha = (uint16_t)(c.alpha * 0xffff)},
 	    1,
 	    (xcb_rectangle_t[]){{.x = to_i16_checked(extent->x1),
 	                         .y = to_i16_checked(extent->y1),
diff --git a/src/types.h b/src/types.h
index 99807d0a..c8d747b6 100644
--- a/src/types.h
+++ b/src/types.h
@@ -22,6 +22,10 @@ typedef struct {
 	int right;
 } margin_t;
 
+struct color {
+	double red, green, blue, alpha;
+};
+
 typedef uint32_t opacity_t;
 
 #define MARGIN_INIT                                                                      \