Modifying GstVideoMeta format on a buffer in-place

[Ryan Talbot]

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://bugzilla.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.

Thanks for any insight!
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