[Bug 785013] New: video-converter: conversion fails for YUV420 -> RGB with bitdepths > 8

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=785013

            Bug ID: 785013
           Summary: video-converter: conversion fails for YUV420 -> RGB
                    with bitdepths > 8
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: All
            Status: NEW
          Severity: major
          Priority: Normal
         Component: gst-plugins-base
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: k.philipp at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 355737
  --> https://bugzilla.gnome.org/attachment.cgi?id=355737&action=edit
Bad frame from gstreamer

YUV -> RGB conversion seems to fail when the YUV bitdepth is not 8, at least
for YUV420. To test:

Create demo video:
$ ffmpeg -y -f lavfi -i smptehdbars -framerate 23.976 -s 400x300 -t 00:00:10
-pix_fmt yuv420p12 -codec:v hevc /tmp/h265.mkv

Extract frames with GStreamer:
$ gst-launch-1.0 -v filesrc location=/tmp/h265.mkv ! decodebin ! videoconvert !
video/x-raw, format=RGB ! pngenc ! multifilesink location="/tmp/frame%d.png"

You will see that the extracted frames have stripes and broken colors. Good and
bad picture is attached for reference.

This affects totem-video-thumbnailer and causes bad thumbnails for all such
videos.

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