[Bug 792435] v4l2src selecting suboptimal colorspace

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=792435

Nicolas Dufresne (stormer) <nicolas at ndufresne.ca> changed:

           What    |Removed                     |Added
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                 CC|                            |nicolas at ndufresne.ca

--- Comment #1 from Nicolas Dufresne (stormer) <nicolas at ndufresne.ca> ---
Looking into this, I don't think it's a regression of the new negotiation code.
But please, give it a try on 1.12 or less to confirm.

Currently, the format is totally ignored when picking up the "best" option.
Though, the list we iterate over has been sorted with a made up rank for each
color format, where YUY2 gets 1010 as a rank, while NV12 gets 50. That may
explain why YUY2 is picked.

Just to recap the current state:
  - GstV4L2Object create a sorted formats list, with YUY2 first
  - GstV4l2Object create 1 caps structure per format (in the sorted order)
  - GstV4l2Src does gst_caps_intersect_full (peercaps, thiscaps, _FIRST);
  - GstV4l2Src iterate over the structure (and fixate) to find a format

What I observe is that the result of the intersection maintains the
GstV4l2Object chosen order, loosing the "preferred" format that was prepended
by videoconvert. So the chosen format shall be optional in regard to the size
(with videoscale) but nothing is done for videoconvert. Maybe we need to add a
format preference to the preferred structure, and sort the fixated structure
with matching format first.

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