[FFmpeg-devel] [PATCH] librsvgdec: Fix frame clearing code

[Calvin Walton]

The existing code attempts to clear the frame by painting in OVER mode
with transparent black - which is, of course, a no-op. As a result if
you have many input frames (e.g. you're using a sequence of svg files),
you'll start to see new frames drawn over old frames as memory gets
re-used.

Fix that by clearing the frame using the SOURCE blend mode, which
will replace whatever is in the buffer with transparent black.

Referencing the cairo FAQ,
https://www.cairographics.org/FAQ/#clear_a_surface
---
 libavcodec/librsvgdec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

It would probably be nice to get this into 3.4 branch as well?

diff --git a/libavcodec/librsvgdec.c b/libavcodec/librsvgdec.c
index e57070f8e4..e16cb6247d 100644
--- a/libavcodec/librsvgdec.c
+++ b/libavcodec/librsvgdec.c
@@ -82,8 +82,11 @@ static int librsvg_decode_frame(AVCodecContext *avctx, void *data, int *got_fram
 
     crender = cairo_create(image);
 
-    cairo_set_source_rgba(crender, 0.0, 0.0, 0.0, 1.0f);
-    cairo_paint_with_alpha(crender, 0.0f);
+    cairo_save(crender);
+    cairo_set_source_rgba(crender, 0.0, 0.0, 0.0, 0.0f);
+    cairo_set_operator(crender, CAIRO_OPERATOR_SOURCE);
+    cairo_paint(crender);
+    cairo_restore(crender);
 
     cairo_scale(crender, dimensions.width / (double)unscaled_dimensions.width,
                 dimensions.height / (double)unscaled_dimensions.height);