Calling set_property for each buffer (video frame) from probe or signal handler?

Matthew Waters ystreet00 at
Thu Nov 12 06:06:56 UTC 2020

There is a samples-selected signal on glvideomixerelement (in 1.18+) 
where you can get a callback at the correct place to set the necessary 
pad information after selection but before composition which should work 
for this case.

See e.g. 


On 12/11/20 2:42 am, Michiel Konstapel wrote:
> Hi all,
> What I am trying to achieve, is to do "digital pan/zoom" on live video 
> frames using a glvideomixer. For each frame, I want to set xpos, ypos, 
> width, and height on the glvideomixer sink (input) pad - ideally, 
> right before each buffer is processed. However, when I do this in a 
> buffer probe callback on the sink pad, I occasionally get incorrect 
> frames, where some of the properties have updated and others have not. 
> For example, the xpos might have been updated while the ypos, height 
> and width are still the old values, resulting in a distorted frame. 
> Note that I am using the Python bindings, so I don't think I have 
> access to a `set_property` that can set multiple properties in one 
> call, as would be possible from C.
> My workaround so far is to set the properties from a probe on the 
> *source* pad instead. That way, the property changes have one frame 
> duration (40 ms, at 25 fps) to be applied. That appears to be working, 
> and I can live with the extra one frame delay, but I am still curious 
> why this is required. I would expect the buffer not to be delivered to 
> the input pad until the probe callback has returned, and the 
> set_property calls to have completed.
> Is this an issue specifically with glvideomixer? Are set_property 
> calls allowed from a probe callback? If not, what about a signal 
> handler? Would it be better to insert an identity element before the 
> mixer and call set_property from its handoff signal?
> Alternatively, I could do the set_property calls from the main thread, 
> but then I don't see any way to synchronize them against the 
> glvideomixer's redraws; there's no way to do locking to ensure all 
> four properties are updated at once, is there?
> Kind regards,
> Michiel
> _______________________________________________
> gstreamer-devel mailing list
> gstreamer-devel at

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