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

[Michiel Konstapel]

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