RE: [PATCH] v4l: add V4L2_CID_MPEG_VIDEO_FORCE_FRAME_TYPE.

Hi,

From: Nicolas Dufresne [mailto:nicolas.dufresne@collabora.com]
Sent: Thursday, January 14, 2016 8:02 PM
To: Kamil Debski; 'Wu-Cheng Li'; pawel@osciak.com;
mchehab@osg.samsung.com; hverkuil@xs4all.nl; crope@iki.fi;
standby24x7@gmail.com; ricardo.ribalda@gmail.com; ao2@ao2.it;
bparrot@ti.com; 'Andrzej Hajda'
Cc: linux-media@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
api@vger.kernel.org
Subject: Re: [PATCH] v4l: add
V4L2_CID_MPEG_VIDEO_FORCE_FRAME_TYPE.

Le jeudi 14 janvier 2016 à 18:21 +0100, Kamil Debski a écrit :

quoted

I had a look into the documentation of MFC. It is possible to force
two types of a frame to be coded.
The one is a keyframe, the other is a not coded frame. As I understand
this is a type of frame that means that image did not change from
previous frame. I am sure I seen it in an MPEG4 stream in a movie
trailer. The initial board with the age rating is displayed for a
couple of seconds and remains static. Thus there is one I-frame and a
number of non-coded frames.

That is the reason why the control was implemented in MFC as a menu
and not a button. Thus the question remains - is it better to leave it
as a menu, or should there be two (maybe more in the future) buttons?

Then I believe we need both. Because with the menu, setting I-Frame, I
would expect to only receive keyframes from now-on. While the useful
feature we are looking for here, is to get the next buffer (or nearby) to be a
keyframe. It's the difference between creating an I-Frame only stream and
requesting a key-frame manually for recovery (RTP use case).
In this end, we should probably take that time to review the features we
have as we need:

I think we had a discussion about this long, long time ago. Should it be
deterministic which frame Is encoded as a key frame? Should it be the
next queued frame, or the next processed frame? How to achieve this?
I vaguely remember that we discussed per buffer controls on the mailing
list, but I am not sure where the discussion went from there.

- A way to trigger a key frame to be produce
- A way to produce a I-Frame only stream

This control can be used to do this:
- V4L2_CID_MPEG_VIDEO_GOP_SIZE (It is not well documented as I can see ;) )
	+ If set to 0 the encoder produces a stream with P only frames
	+ if set to 1 the encoder produces a stream with I only frames
	+ other values indicate the GOP size (I-frame interval)

- A way to set the key-frame distance (in frames) even though this could
possibly be emulated using the trigger.

As described above V4L2_CID_MPEG_VIDEO_GOP_SIZE can be used to achieve this.

cheers,
Nicolas

 
Best wishes,
-- 
Kamil Debski
Samsung R&D Institute Poland