RTSP stream, effect of max-lateness + qos

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    On Friday, May 12, 2023, 08:45:28 a.m. EDT, Guillermo Rodriguez Garcia via gstreamer-devel <gstreamer-devel at lists.freedesktop.org> wrote:  
 
 Hi all,
I have an embedded device showing RTSP / H.264 video streams from IP cameras.
The pipeline is more or less like this:
gst-launch-1.0 rtspsrc XXX ! rtph264depay ! h264parse ! avdec_h264 ! waylandsink 
In some cases the resolution of the video stream is too high for the device to decode comfortably (unfortunately this is not something I can control). In these cases, Gstreamer (waylandsink actually I guess) decides that the frames are arriving too late, and discards them. The result is that the video is frozen (or almost -- I see a frame being displayed every now and then).
I thought that displaying late frames was better than displaying a frozen video so I tried to add max-lateness=-1 to the sink. However this does not seem to change anything. Next I tried to set qos=false instead, this didn't help either.
However setting _both_ max-lateness=-1 and qos=false does work. WIth this I can see the video stream even if there is a very noticeable delay.
I am trying to undestand how this works and I have some questions; hopefully someone can help.
1. Why setting max-lateness=-1 alone is not enough? If I understand correctly this should make the sink consider that no frames are late, so it should not be sending qos events. Why is it necessary to combine max-lateness=-1 with qos=false ?
2. What is the difference in practice between max-lateness=-1 + qos=false and sync=false? Both seem to achieve a similar effect, but I have read that "setting sync=false is almost never the right solution for timing problems".
Thank you,
Guillermo Rodriguez Garcia
guille.rodriguez at gmail.com  
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