compositor: Always draw if all input buffers are GAP

Otherwise output buffers will contain random scene
(previously rendered or arbitrary initial values) since we don't
draw onto output buffer at all.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1498>

diff --git a/subprojects/gst-plugins-base/gst/compositor/compositor.c b/subprojects/gst-plugins-base/gst/compositor/compositor.c
index 9938154..dc5d15f 100644 (file)
--- a/subprojects/gst-plugins-base/gst/compositor/compositor.c
+++ b/subprojects/gst-plugins-base/gst/compositor/compositor.c
@@ -1383,6 +1383,11 @@ gst_compositor_aggregate_frames (GstVideoAggregator * vagg, GstBuffer * outbuf)
       n_pads++;
   }
 
+  /* If no prepared frame, we should draw background unconditionally in order
+   * to clear output buffer */
+  if (n_pads == 0)
+    draw_background = TRUE;
+
   pads_info = g_newa (struct CompositePadInfo, n_pads);
   n_pads = 0;