[x265] [PATCH] ratecontrol: prevent one layer of non-zero I-slice qp clipping [CHANGES OUTPUT]

[Santhoshini Sekar]

On Thu, Apr 10, 2014 at 12:57 PM, <deepthi at multicorewareinc.com> wrote:

> # HG changeset patch
> # User Deepthi Nandakumar <deepthi at multicorewareinc.com>
> # Date 1397114813 -19800
> # Node ID 1c0f1a4ce209badcca1272f87c3917ec69ee1cc7
> # Parent  83ccf2f1453ffd91c9fd8b84b72e3d3daacba6c6
> ratecontrol: prevent one layer of non-zero I-slice qp clipping [CHANGES
> OUTPUT].
>
> This seems to have been an error importing RC code from x264. It could be
> responsible
> for error reports on I-slices (after POC 0) having much higher QP/low
> visual quality.
> Needs review.
>
> diff -r 83ccf2f1453f -r 1c0f1a4ce209 source/encoder/ratecontrol.cpp
> --- a/source/encoder/ratecontrol.cpp    Thu Apr 10 11:17:46 2014 +0530
> +++ b/source/encoder/ratecontrol.cpp    Thu Apr 10 12:56:53 2014 +0530
> @@ -554,33 +554,34 @@
>              q = x265_qp2qScale(accumPQp / accumPNorm);
>              q /= fabs(param->rc.ipFactor);
>          }
> -
> -        if (param->rc.rateControlMode != X265_RC_CRF)
> +        else if(framesDone > 0)
>          {
> -            double lqmin = 0, lqmax = 0;
> -            if (totalBits == 0 && !isVbv)
> +            if (param->rc.rateControlMode != X265_RC_CRF)
>              {
> -                lqmin = x265_qp2qScale(ABR_INIT_QP_MIN) / lstep;
> -                lqmax = x265_qp2qScale(ABR_INIT_QP_MAX) * lstep;
> -                q = Clip3(lqmin, lqmax, q);
> -            }
> -            else if (totalBits > 0 || (isVbv && framesDone > 0))
> -            {
> -                lqmin = lastQScaleFor[sliceType] / lstep;
> -                lqmax = lastQScaleFor[sliceType] * lstep;
> -                if (overflow > 1.1 && framesDone > 3)
> -                    lqmax *= lstep;
> -                else if (overflow < 0.9)
> -                    lqmin /= lstep;
> -                q = Clip3(lqmin, lqmax, q);
> +                double lqmin = 0, lqmax = 0;
> +                if (totalBits == 0 && !isVbv)
> +                {
> +                    lqmin = x265_qp2qScale(ABR_INIT_QP_MIN) / lstep;
> +                    lqmax = x265_qp2qScale(ABR_INIT_QP_MAX) * lstep;
> +                    q = Clip3(lqmin, lqmax, q);
> +                }
>
The above clipping logic is not present in x264. This was consciously
introduced only in x265 to clip I slice's qp along with N concurrent
frames' qp when frame parallelism is enabled. If I slice's clipping is
removed then we need to remove the above piece of code as well.  Otherwise
the frames following the I frame might have qp that gets clipped - lower
than the I frame's qp.

> +                else if (totalBits > 0 || (isVbv && framesDone > 0))
> +                {
> +                    lqmin = lastQScaleFor[sliceType] / lstep;
> +                    lqmax = lastQScaleFor[sliceType] * lstep;
> +                    if (overflow > 1.1 && framesDone > 3)
> +                        lqmax *= lstep;
> +                    else if (overflow < 0.9)
> +                        lqmin /= lstep;
> +                    q = Clip3(lqmin, lqmax, q);
> +                }
>              }
>          }
> -        else
> +        else if (qCompress != 1 && param->rc.rateControlMode ==
> X265_RC_CRF)
>          {
> -            if (qCompress != 1 && framesDone == 0)
> -                q = x265_qp2qScale(ABR_INIT_QP) /
> fabs(param->rc.ipFactor);
> +            q = x265_qp2qScale(ABR_INIT_QP) / fabs(param->rc.ipFactor);
>          }
> -
> +
>          q = Clip3(MIN_QPSCALE, MAX_MAX_QPSCALE, q);
>          qpNoVbv = x265_qScale2qp(q);
>
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>
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