Modifying GstVideoMeta format on a buffer in-place

[Ryan Talbot]

Hi Nicolas,

Thanks for the response.

>This element should be ported to the BaseOpenCVFilter class (like all
>the others) and should use the BaseTransform mechanism indeed. Doing
>in-place color conversion isn't common though. We usually concern into
>a new buffer.

OK, that's good to know.  I did not take a close look at the other filters.  I was doing the transformation in place since the plugin already does an RGB->GRAY->RGB colorspace conversion in place, but it seems this is not the best strategy.  I will see about trying to port it to the BaseOpenCVFilter class.

________________________________________
From: gstreamer-devel [gstreamer-devel-bounces at lists.freedesktop.org] on behalf of Nicolas Dufresne [nicolas at ndufresne.ca]
Sent: Sunday, February 19, 2017 2:58 PM
To: Discussion of the development of and with GStreamer
Subject: Re: Modifying GstVideoMeta format on a buffer in-place

Le vendredi 17 février 2017 à 10:37 +0000, Ryan Talbot a écrit :
> I'm modifying the OpenCV disparity plugin to get at the raw 16-bit
> signed disparity values, if the downstream caps request "video/x-
> raw,format=GRAY16_LE".  After some headache, I managed to get the
> caps negotiation to work so that either RGB or GRAY16_LE could be
> obtained from the source pad.  I was still running into trouble
> trying to use the buffers in pipelines, though, most notably when I
> tried to use videoconvert on them to prep them for encoding or
> display, I would get a message like this with every video frame:
>
> ** (gst-launch-1.0:9670): CRITICAL **: gst_video_frame_map_id:
> assertion 'info->finfo->format == meta->format' failed
> WARNING: from element
> /GstPipeline:pipeline0/GstVideoConvert:videoconvert0: Internal
> GStreamer error: code not implemented.  Please file a bug at http://b
> ugzilla.gnome.org/enter_bug.cgi?product=GStreamer.
> Additional debug info:
> gstvideofilter.c(292): gst_video_filter_transform ():
> /GstPipeline:pipeline0/GstVideoConvert:videoconvert0:
> invalid video buffer received
>
> I didn't realize that just getting the caps set wasn't enough, and
> that there was essentially another copy of some of the caps data in
> meta data embedded in the buffer.
>
> What is the correct way to modify this meta data?  As it is, I'm
> basically doing:
>
> GstVideoMeta *meta = gst_buffer_get_video_meta (buffer);
> meta->format = GST_VIDEO_FORMAT_GRAY16_LE;
> ret = gst_pad_push (srcpad, buffer);
>
> ...that works, pipelines are happy, but it seems like a hack.  Is
> there a more preferred way of doing this?  It seems like maybe this
> plugin should derive from GST_TYPE_VIDEO_FILTER instead of
> GST_TYPE_ELEMENT, so that GstBaseTransform::transform_meta could be
> hooked into, although I'm not sure I have time to rewrite the plugin
> to that extent at the moment.

This element should be ported to the BaseOpenCVFilter class (like all
the others) and should use the BaseTransform mechanism indeed. Doing
in-place color conversion isn't common though. We usually concern into
a new buffer.

Nicolas

>
> Thanks for any insight!
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