[PATCH 1/2] compositor: Clear only the the damage that was actually repainted

[Ander Conselvan de Oliveira]

From: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira at intel.com>

Instead of clearing the whole output region after a repaint, clear
only the regions that were actually painted. This way, the damage
added when a surface moves from the primary plane to another one is
kept while this region is obscured by the opaque region. This allows
the contents below an overlaid surface to be culled, but to make this
work properly, it is also necessary to change the way previous damage
is drawn.

Consider the following scenario: a surface is moved to an overlay plane
leaving some damage in the primary plane. On the following frame, the
surface on the overlay moves, revealing part of the damaged region on
the primary plane. On the frame after that, the overlaid surface moves
back to its previous position obscuring the region of the primary plane
repainted before. At this point, the repainted region was added to the
output's previous damage so that it is draw to both buffers. But since
this region is now obscured, the redrawing is skipped. If the overlaid
surface moves again revealing this region, one of the buffers actually
contains the wrong content.

To fix this problem, this patch ensures that any previous damage that
would be lost is actually drawn, by subtracting it from the opaque
region.
---
 src/compositor.c |   26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/src/compositor.c b/src/compositor.c
index 7370435..26c57b9 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -928,6 +928,9 @@ weston_surface_draw(struct weston_surface *es, struct weston_output *output,
 	if (!pixman_region32_not_empty(&repaint))
 		goto out;
 
+	pixman_region32_subtract(&ec->primary_plane.damage,
+				 &ec->primary_plane.damage, &repaint);
+
 	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
 	if (es->blend || es->alpha < 1.0)
 		glEnable(GL_BLEND);
@@ -1095,7 +1098,8 @@ update_shm_texture(struct weston_surface *surface)
 
 static void
 surface_accumulate_damage(struct weston_surface *surface,
-			  pixman_region32_t *opaque)
+			  pixman_region32_t *opaque,
+			  pixman_region32_t *exclude)
 {
 	if (surface->buffer && wl_buffer_is_shm(surface->buffer))
 		update_shm_texture(surface);
@@ -1121,9 +1125,8 @@ surface_accumulate_damage(struct weston_surface *surface,
 	pixman_region32_union(&surface->plane->damage,
 			      &surface->plane->damage, &surface->damage);
 	empty_region(&surface->damage);
-	pixman_region32_copy(&surface->clip, opaque);
-	if (surface->plane == &surface->compositor->primary_plane)
-		pixman_region32_union(opaque, opaque, &surface->transform.opaque);
+	pixman_region32_subtract(&surface->clip, opaque, exclude);
+	pixman_region32_union(opaque, opaque, &surface->transform.opaque);
 }
 
 static void
@@ -1135,7 +1138,7 @@ weston_output_repaint(struct weston_output *output, uint32_t msecs)
 	struct weston_animation *animation, *next;
 	struct weston_frame_callback *cb, *cnext;
 	struct wl_list frame_callback_list;
-	pixman_region32_t opaque, output_damage;
+	pixman_region32_t opaque, output_damage, previous;
 	int32_t width, height;
 
 	weston_compositor_update_drag_surfaces(ec);
@@ -1170,8 +1173,16 @@ weston_output_repaint(struct weston_output *output, uint32_t msecs)
 
 	pixman_region32_init(&opaque);
 
+	/* Once we draw something on one of the back buffers it needs to be
+	 * drawn on the other one, otherwise we might endup with artifacts.
+	 * Make sure we draw any previous damage that would be lost otherwise
+	 */
+	pixman_region32_init(&previous);
+	pixman_region32_subtract(&previous, &output->previous_damage,
+				 &ec->primary_plane.damage);
+
 	wl_list_for_each(es, &ec->surface_list, link)
-		surface_accumulate_damage(es, &opaque);
+		surface_accumulate_damage(es, &opaque, &previous);
 
 	pixman_region32_init(&output_damage);
 	pixman_region32_union(&output_damage, &ec->primary_plane.damage,
@@ -1180,10 +1191,9 @@ weston_output_repaint(struct weston_output *output, uint32_t msecs)
 			     &ec->primary_plane.damage);
 	pixman_region32_intersect(&output_damage,
 				  &output_damage, &output->region);
-	pixman_region32_subtract(&ec->primary_plane.damage,
-				 &ec->primary_plane.damage, &output->region);
 
 	pixman_region32_fini(&opaque);
+	pixman_region32_fini(&previous);
 
 	if (output->dirty)
 		weston_output_update_matrix(output);
-- 
1.7.9.5