compositor: make GL_EXT_read_format_bgra optional

Some GL implementations do not provide GL_EXT_read_format_bgra
extension.

Set a glReadPixels format based on whether the extensions is supported
or not, and use that format in all backends.

Add RGBA->BGRA swapping copy to screenshooter to keep the shm buffer
data format as BGRA.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/compositor-drm.c

@@ -1052,8 +1052,9 @@ drm_output_read_pixels(struct weston_output *output_base, void *data)
 	eglMakeCurrent(compositor->base.display, output->egl_surface,
 			output->egl_surface, compositor->base.context);
 
-	glReadPixels(0, 0, output_base->current->width, output_base->current->height,
-				GL_BGRA_EXT, GL_UNSIGNED_BYTE, data);
+	glReadPixels(0, 0, output_base->current->width,
+		     output_base->current->height,
+		     compositor->base.read_format, GL_UNSIGNED_BYTE, data);
 }
 
 static int

src/compositor-wayland.c

@@ -387,8 +387,9 @@ wayland_output_read_pixels(struct weston_output *output_base, void *data)
 	eglMakeCurrent(compositor->base.display, output->egl_surface,
 			output->egl_surface, compositor->base.context);
 
-	glReadPixels(0, 0, output_base->current->width, output_base->current->height,
-				GL_BGRA_EXT, GL_UNSIGNED_BYTE, data);
+	glReadPixels(0, 0, output_base->current->width,
+		     output_base->current->height,
+		     compositor->base.read_format, GL_UNSIGNED_BYTE, data);
 }
 
 static int

src/compositor-x11.c

@@ -354,8 +354,9 @@ x11_output_read_pixels(struct weston_output *output_base, void *data)
 	eglMakeCurrent(compositor->base.display, output->egl_surface,
 			output->egl_surface, compositor->base.context);
 
-	glReadPixels(0, 0, output_base->current->width, output_base->current->height,
-				GL_BGRA_EXT, GL_UNSIGNED_BYTE, data);
+	glReadPixels(0, 0, output_base->current->width,
+		     output_base->current->height,
+		     compositor->base.read_format, GL_UNSIGNED_BYTE, data);
 }
 
 static int

src/compositor.c

@@ -2391,10 +2391,10 @@ weston_compositor_init(struct weston_compositor *ec, struct wl_display *display)
 		return -1;
 	}
 
-	if (!strstr(extensions, "GL_EXT_read_format_bgra")) {
-		fprintf(stderr, "GL_EXT_read_format_bgra not available\n");
-		return -1;
-	}
+	if (strstr(extensions, "GL_EXT_read_format_bgra"))
+		ec->read_format = GL_BGRA_EXT;
+	else
+		ec->read_format = GL_RGBA;
 
 	if (strstr(extensions, "GL_EXT_unpack_subimage"))
 		ec->has_unpack_subimage = 1;

src/compositor.h

@@ -223,6 +223,7 @@ struct weston_compositor {
 	PFNEGLDESTROYIMAGEKHRPROC destroy_image;
 
 	int has_unpack_subimage;
+	GLenum read_format;
 
 	PFNEGLBINDWAYLANDDISPLAYWL bind_display;
 	PFNEGLUNBINDWAYLANDDISPLAYWL unbind_display;

src/screenshooter.c

@@ -50,6 +50,37 @@ copy_bgra_yflip(uint8_t *dst, uint8_t *src, int height,
 	}
 }
 
+static void
+copy_row_swap_RB(void *vdst, void *vsrc, int bytes)
+{
+	uint32_t *dst = vdst;
+	uint32_t *src = vsrc;
+	uint32_t *end = dst + bytes / 4;
+
+	while (dst < end) {
+		uint32_t v = *src++;
+		/*                    A R G B */
+		uint32_t tmp = v & 0xff00ff00;
+		tmp |= (v >> 16) & 0x000000ff;
+		tmp |= (v << 16) & 0x00ff0000;
+		*dst++ = tmp;
+	}
+}
+
+static void
+copy_rgba_yflip(uint8_t *dst, uint8_t *src, int height,
+		int dst_stride, int src_stride)
+{
+	uint8_t *end;
+
+	end = dst + height * dst_stride;
+	while (dst < end) {
+		copy_row_swap_RB(dst, src, src_stride);
+		dst += dst_stride;
+		src -= src_stride;
+	}
+}
+
 static void
 screenshooter_shoot(struct wl_client *client,
 		    struct wl_resource *resource,
@@ -82,8 +113,19 @@ screenshooter_shoot(struct wl_client *client,
 	d = wl_shm_buffer_get_data(buffer) + output->y * buffer_stride +
 							output->x * 4;
 	s = tmp + output_stride * (output->current->height - 1);
+
+	switch (output->compositor->read_format) {
+	case GL_BGRA_EXT:
 		copy_bgra_yflip(d, s, output->current->height,
 				buffer_stride, output_stride);
+		break;
+	case GL_RGBA:
+		copy_rgba_yflip(d, s, output->current->height,
+				buffer_stride, output_stride);
+		break;
+	default:
+		break;
+	}
 
 	free(tmp);
 }