Re: [FFmpeg-devel] [PATCH] avcodec/libx264: update notes to explain the scale chosen for ROI encoding
> -Original Message-
> From: ffmpeg-devel [mailto:ffmpeg-devel-boun...@ffmpeg.org] On Behalf
> Of Mark Thompson
> Sent: Monday, February 04, 2019 5:34 PM
> To: ffmpeg-devel@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
> > ---
> > 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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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
> ---
> 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
arbitrary? Why does the effect vary by bit depth?
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).
- Mark
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[FFmpeg-devel] [PATCH] avcodec/libx264: update notes to explain the scale chosen for ROI encoding
Signed-off-by: Guo, Yejun
---
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++) {
--
2.7.4
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