[FFmpeg-devel] [PATCH] avcodec/libx264: update notes to explain the scale chosen for ROI encoding

[Guo, Yejun]

> -----Original Message-----
> From: ffmpeg-devel [mailto:ffmpeg-devel-bounces at ffmpeg.org] On Behalf
> Of Mark Thompson
> Sent: Monday, February 04, 2019 5:34 PM
> To: ffmpeg-devel at ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH] avcodec/libx264: update notes to
> explain the scale chosen for ROI encoding
> 
> On 29/01/2019 10:14, Guo, Yejun wrote:
> > Signed-off-by: Guo, Yejun <yejun.guo at intel.com>
> > ---
> >  libavcodec/libx264.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
> > index a3493f3..8c96728 100644
> > --- a/libavcodec/libx264.c
> > +++ b/libavcodec/libx264.c
> > @@ -383,7 +383,9 @@ static int X264_frame(AVCodecContext *ctx,
> AVPacket *pkt, const AVFrame *frame,
> >                          qoffset = roi->qoffset.num * 1.0f / roi->qoffset.den;
> >                          qoffset = av_clipf(qoffset, -1.0f, 1.0f);
> >
> > -                        // 25 is a number that I think it is a possible proper scale value.
> > +                        /* qp range of x264 is from 0 to 51, just choose 25 as the scale
> value,
> > +                         * so the range of final qoffset is [-25.0, 25.0].
> > +                         */
> >                          qoffset = qoffset * 25;
> >
> >                          for (int y = starty; y < endy; y++) {
> >
> 
> I'm not understanding where this number has come from at all.  Is 25 purely

From H264 spec: SliceQPY is in the range of −QpBdOffsetY to +51, inclusive. And lots of documents/papers says the qp range of H264 is [0, 51].
So, I chose 25 (equal with 51/2) roughly to make qpoffset to be [-25.0, 25.0]. 

> arbitrary?  Why does the effect vary by bit depth?

imo, 51 has nothing to do with bit depth, not quite understand your concern.

> 
> From the docs explaining the best/worst behaviour on the range, I would
> expect this to map 0 to 0 and 1 to 3 + 6 * bit_depth (positive or negative).

for example if bit_depth=8, do you mean map -1.0 to -51, and map 1.0 to 51?
for example if bit_depth=10, do you mean map -1.0 to -63, and map 1.0 to 63?

If so, I'm not sure if it is better or not.

Imho, there is still a long way from qpoffset to the final quantization value, I would think a rough qpoffset (with range [-25, 25]) is enough. Anyway, I'm open to use whatever better method.

> 
> - Mark
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