Confused about GST_PAD_SET_PROXY_CAPS and negotiation

[Tim Müller]

On Fri, 2015-01-23 at 17:46 +0100, Guillaume POIRIER wrote:

Hi Guillaume,

> I'm trying to implement a video filter in GStreamer that would take
> RGB as input (sink) and would output ARGB64 (src).
> I naively thought that since my src and sink only support a fixed
> format, I didn't need to perform any negotiation... is that right?

That's not quite right. Your input and output formats may be fixed, but
other things such as width, height, framerate, pixel aspect ratio etc.
are not fixed (presumably).


> Since my code didn't do what I wanted, I tried to add negotiation by
> calling gst_event_new_caps() + gst_pad_send_event() on my sink and src
> pads... but this didn't seem to work (gst_pad_send_event() returned
> FALSE each time). How come? Who should have handled these events? Not
> me I presume?

What class does your element derive from? GstElement? GstBaseTransform?
GstVideoFilter? Something else? (Should probably be GstVideoFilter)

The simplest implementation possible would be: when you receive a
GST_EVENT_CAPS on your sink pad, you parse the caps, copy it (to make it
writable), change the format from RGB to ARGB64, and then push a new
caps event with the new caps out on your src pad. This ignores all other
aspects of negotiation (such as queries, and buffer pools etc.). Just
conceptually it can work like this.

For more proper negotiation you would need to proxy things (caps) from
downstream to upstream, whilst only changing the format.

For example:

 ... ! videoconvert ! videoscale ! yourfilter !
video/x-raw,format=ARGB64,width=1920,height=1080 ! ...

in this case the upstream elements need to know that you will only
accept width=1920,height=1080 (since you can't scale), so you would have
to return video/x-raw,format=RGB,width=1920,height=1080 to a caps query
from upstream here.


> Then, just calling GST_PAD_SET_PROXY_CAPS() on each of my pads did
> allow me to get a pipeline that seemed to have negotiated a format,
> ... but the verbose output of gst-launch-1.0 does not show that my src
> pad has been seen as ARGB64. When I look at the documentation of
> GST_PAD_SET_PROXY_CAPS() states 'Set this if the element always
> outputs data in the exact same format as it receives as input."

This won't work in your case, since your output caps are different than
your input caps.


> My question is: what is the simplest way to have a filter take fixed
> format in input, what do I need to do negotiation-wise (if any)?

Derive from GstVideoFilter, and override
GstBaseTransformClass::transform_caps() in your class_init function.

In your transform_caps function you check the direction of the
transform, and depending on it, you copy the caps (to make them
writable), and loop over all caps structures, and for each set the
"format" field to "RGB" or "ARGB64" (depending on the direction), but
leave all other fields untouched.

The base class should then handle everything else for you.

 Cheers 
  -Tim

> Here is my pipeline:
> gst-launch-1.0 --gst-debug=3 -vv
> --gst-plugin-path=/local/poirierg/gst-template/gst-plugin/src/my_build/
> filesrc location=/local/poirierg/mpc_spaceship.png ! pngdec !
> videoconvert ! myfilter ! videoconvert ! imagefreeze ! autovideosink
> 
> 
> I hope you guys can help me, be it by a simple RTFM with the right
> page pointed to :-)
> 
> Regards,
> 
> Guillaume
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> gstreamer-devel at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/gstreamer-devel

-- 
Tim Müller, Centricular Ltd - http://www.centricular.com