Call glReadPixels right before swapping buffers

backend/drm/drm.c

@@ -634,14 +634,6 @@ static bool wlr_drm_connector_move_cursor(struct wlr_output *output,
 	return drm->iface->crtc_move_cursor(drm, conn->crtc, x, y);
 }
 
-static void wlr_drm_connector_read_pixels(struct wlr_output *output,
-		void *out_data) {
-	int width, height;
-	wlr_output_effective_resolution(output, &width, &height);
-	wlr_drm_connector_make_current(output);
-	glReadPixels(0, 0, width, height, GL_BGRA_EXT, GL_UNSIGNED_BYTE, out_data);
-}
-
 static void wlr_drm_connector_destroy(struct wlr_output *output) {
 	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
 	wlr_drm_connector_cleanup(conn);
@@ -660,7 +652,6 @@ static struct wlr_output_impl output_impl = {
 	.swap_buffers = wlr_drm_connector_swap_buffers,
 	.set_gamma = wlr_drm_connector_set_gamma,
 	.get_gamma_size = wlr_drm_connector_get_gamma_size,
-	.read_pixels = wlr_drm_connector_read_pixels,
 };
 
 static int retry_pageflip(void *data) {
@@ -845,7 +836,6 @@ static void page_flip_handler(int fd, unsigned seq,
 
 	if (drm->session->active) {
 		wl_signal_emit(&conn->output.events.frame, &conn->output);
-		wl_signal_emit(&conn->output.events.post_frame, &conn->output);
 	}
 }
 

examples/screenshot.c

@@ -204,7 +204,7 @@ write_png(int width, int height)
 	}
 
 	surface = cairo_image_surface_create_for_data(data,
-						      CAIRO_FORMAT_ARGB32,
+						      CAIRO_FORMAT_RGB24,
 						      width, height, buffer_stride);
 	cairo_surface_write_to_png(surface, "wayland-screenshot.png");
 	cairo_surface_destroy(surface);

include/wlr/interfaces/wlr_output.h

@@ -19,7 +19,6 @@ struct wlr_output_impl {
 	void (*set_gamma)(struct wlr_output *output,
 			uint16_t size, uint16_t *r, uint16_t *g, uint16_t *b);
 	uint16_t (*get_gamma_size)(struct wlr_output *output);
-	void (*read_pixels)(struct wlr_output *_output, void *out_data);
 };
 
 void wlr_output_init(struct wlr_output *output, const struct wlr_output_impl *impl);

include/wlr/types/wlr_output.h

@@ -37,7 +37,7 @@ struct wlr_output {
 
 	struct {
 		struct wl_signal frame;
-		struct wl_signal post_frame;
+		struct wl_signal swap_buffers;
 		struct wl_signal resolution;
 		struct wl_signal destroy;
 	} events;

types/wlr_output.c

@@ -8,6 +8,7 @@
 #include <wlr/util/list.h>
 #include <wlr/util/log.h>
 #include <GLES2/gl2.h>
+#include <GLES2/gl2ext.h>
 #include <wlr/render/matrix.h>
 #include <wlr/render/gles2.h>
 #include <wlr/render.h>
@@ -105,7 +106,7 @@ void wlr_output_init(struct wlr_output *output,
 	output->transform = WL_OUTPUT_TRANSFORM_NORMAL;
 	output->scale = 1;
 	wl_signal_init(&output->events.frame);
-	wl_signal_init(&output->events.post_frame);
+	wl_signal_init(&output->events.swap_buffers);
 	wl_signal_init(&output->events.resolution);
 	wl_signal_init(&output->events.destroy);
 }
@@ -232,6 +233,8 @@ void wlr_output_swap_buffers(struct wlr_output *output) {
 		wlr_render_with_matrix(output->cursor.renderer, output->cursor.texture, &matrix);
 	}
 
+	wl_signal_emit(&output->events.swap_buffers, &output);
+
 	output->impl->swap_buffers(output);
 }
 
@@ -250,7 +253,8 @@ uint16_t wlr_output_get_gamma_size(struct wlr_output *output) {
 }
 
 void wlr_output_read_pixels(struct wlr_output *output, void *out_data) {
-	if (output->impl->read_pixels) {
-		output->impl->read_pixels(output, out_data);
-	}
+	// TODO: is wlr_output_make_current required?
+	wlr_output_make_current(output);
+	glReadPixels(0, 0, output->width, output->height, GL_BGRA_EXT,
+		GL_UNSIGNED_BYTE, out_data);
 }

types/wlr_screenshooter.c

@@ -59,7 +59,7 @@ static void screenshooter_shoot(struct wl_client *client,
 	state->output = output;
 	state->screenshot = screenshot;
 	state->frame_listener.notify = output_frame_notify;
-	wl_signal_add(&output->events.post_frame, &state->frame_listener);
+	wl_signal_add(&output->events.swap_buffers, &state->frame_listener);
 }
 
 static struct orbital_screenshooter_interface screenshooter_impl = {