[FFmpeg-devel] [PATCH] lavc/vaapi_encode_h264: add support for maxframesize

[Fu, Linjie]

> -----Original Message-----
> From: mypopy at gmail.com [mailto:mypopy at gmail.com]
> Sent: Tuesday, April 23, 2019 13:42
> To: FFmpeg development discussions and patches <ffmpeg-
> devel at ffmpeg.org>
> Cc: Fu, Linjie <linjie.fu at intel.com>
> Subject: Re: [FFmpeg-devel] [PATCH] lavc/vaapi_encode_h264: add support
> for maxframesize
> 
> On Tue, Apr 23, 2019 at 11:29 AM Linjie Fu <linjie.fu at intel.com> wrote:
> >
> > Add support for max frame size:
> >     - max_frame_size (bytes) to indicate the allowed max frame size.
> >     - pass_num to indicate number of passes.
> >     - delta_qp to indicate adjust qp value.
> >
> > Currently only AVC encoder can support this settings in multiple pass case.
> > If the frame size exceeds, the encoder will do more pak passes to adjust
> the
> > QP value to control the frame size.
> >
> > Set Default num_passes to 4 (1~4), set delta_qp[4] = {1, 1, 1, 1}, use
> > new_qp for encoder if frame size exceeds the limitation:
> >     new_qp = base_qp + delta_qp[0] + delta_qp[1] + ...
> >
> > ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -f rawvideo \
> >         -v verbose -s:v 352x288 -i ./input.yuv -vf format=nv12,hwupload \
> >         -c:v h264_vaapi -profile:v main -g 30 -bf 3 -max_frame_size 40000 \
> >         -pass_num 2 -delta_qp 2 -vframes 100 -y ./max_frame_size.h264
> >
> Some question list as follow：
> 
> 1. Can I change delta_qp per pass, e,g, 4 pass 1, 2, 3, 4, use
> delta_qp[4] like: 1, 2, 4, 8?

Yes, it's available. 
Actually I considered this whether to expose the detailed settings for the user,
And end up with current solution:
	- setting max_frame_size only
		will use default 4 passes with delta_qp = {1, 1, 1, 1}
	- setting max_frame_size with detailed pass_num and delta_qp
		Can be customized by user.
Not splitting delta_qp setting for each pass is trying to make it easier for users.

> 2. So let's think about the limiting case, if we setting
> max_frame_size = 1 for 1080P, what's the action for this driver?

Maybe the commit message is a little bit confusing,
driver will adjust add delta_qp to original qp to decrease the frame size for each pass:
	new_qp = base_qp + delta_qp[0]
If the frame size is still too big, will try 2 passes
	new_qp = base_qp + delta_qp[0] + delta_qp[1]
If the frame size is still too big  after 4 passes qp modification, driver will use max qp allowed by the sum of delta_qp;
	new_qp = base_qp + delta_qp[0] + delta_qp[1] + delta_qp[2] + delta_qp[3] 

> 3. Maybe we can hide the pass_num and delta_qp, only export the
> max_frame_size for this case?  I don't think export the driver QP
> adjustment detail  logic to user space is good idea, user will
> confused about to how to set/adjust pass_num/delta_qp per pass.

Similar with 1, I think a  default path with a valuable set of parameters(Needs advice), together with a customizing method is good.

> 4. Missing docs

Will refine the commit message, and a doc if still needed.

> 5. What's the relationship about other bit rate control like VBR or MBBRC ?

IMHO, max frame size will modify the qp value set by bit rate control methods, and trying to meet the size limitation for each frame.

> 6. Only 264 encoder support this feature? What platform have you tested?

Yes, only AVC encoder can be supported by iHD driver, and tested in KBL.
 

Thanks for the comment.
Linjie