[fallback] Create intermediate surface with same content.

The goal is to create a similar surface with an identical format to
maximise performance in the subsequent blit, e.g. the xlib backend could
make the similar surface with an identical depth and so use the core
protocol, or the image surface could indeed make an identical copy so
that pixman only has to do a fast memcpy. As there is no direct method
to specify such a clone, we ask the backend for a similar surface of
identical content, and trust that the semantics are clear enough for the
intent to obvious.

1 files changed, 7 insertions, 5 deletions

diff --git a/src/cairo-surface-fallback.c b/src/cairo-surface-fallback.c
index 068f06dc..f1453b8a 100644
--- a/src/cairo-surface-fallback.c
+++ b/src/cairo-surface-fallback.c
@@ -227,17 +227,19 @@ _clip_and_composite_combine (cairo_clip_t                  *clip,
     cairo_status_t status;
 
     /* We'd be better off here creating a surface identical in format
-     * to dst, but we have no way of getting that information.
-     * A CAIRO_CONTENT_CLONE or something might be useful.
+     * to dst, but we have no way of getting that information. Instead
+     * we ask the backend to create a similar surface of identical content,
+     * in the belief that the backend will do something useful - like use
+     * an identical format. For example, the xlib backend will endeavor to
+     * use a compatible depth to enable core protocol routines.
      */
     intermediate =
-	_cairo_surface_create_similar_scratch (dst,
-					       CAIRO_CONTENT_COLOR_ALPHA,
+	_cairo_surface_create_similar_scratch (dst, dst->content,
 					       extents->width,
 					       extents->height);
     if (intermediate == NULL) {
 	intermediate =
-	    _cairo_image_surface_create_with_content (CAIRO_CONTENT_COLOR_ALPHA,
+	    _cairo_image_surface_create_with_content (dst->content,
 						      extents->width,
 						      extents->width);
     }