Blacks appear as 'fickery white' when compositing certain sized images

[Duncan Palmer]

Hi,

I have a pipeline I use to compose a video stream from multiple cameras.
I've found a problem whereby blacks appear as 'flickery white' in the
resulting image. This problem only appears when I blend using pixel formats
which contain an alpha channel, and using images below a certain height.

The following example feeds a 1280x577 stream to the compositor, and does
not exhibit the problem;

gst-launch-1.0 uridecodebin uri=file://`pwd`/b.1.mkv ! videobox  top=143  !
videorate  ! 'video/x-raw,framerate=25/1,format=AYUV' !   compositor  !
glimagesink

The following example feeds a 1280x576 stream to the compositor, and does
exhibit the problem (the '576' is a bit of a red flag perhaps?).

gst-launch-1.0 uridecodebin uri=file://`pwd`/b.1.mkv ! videobox  top=144  !
videorate  ! 'video/x-raw,framerate=25/1,format=AYUV' !   compositor  !
glimagesink

I'm still digging, but feel this could take some time. Can anyone offer
suggestions or point me in a direction for investigation? The problem
occurs with both the software and gl compositors.

Note: I force the used of an alpha pixel format in the pipeline above, to
produce a small example, but in practice I insert the alpha element. I'm
also showing only one video source; in practice I use more.

Cheers,
Dunk
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