[x265] [PATCH 2 of 2] rc: tune initial predictor values for better frame size predictions in vbv lookahead

[Steve Borho]

On 04/08, aarthi at multicorewareinc.com wrote:
> # HG changeset patch
> # User Aarthi Thirumalai
> # Date 1426487738 -19800
> #      Mon Mar 16 12:05:38 2015 +0530
> # Node ID b4646bdaf2202748d3a3b159b9bc8e3cfcd272de
> # Parent  6f159ec858f4eec484d722916c41911e3e4540f9
> rc: tune initial predictor values for better frame size predictions in vbv lookahead
> 
> an overall improvement in ssim of around .05-.1 db can be seen.
> Also, improves visual quality at the start of the encode.
> 
> SteamLocomotiveTrain_2560x1600_60_10bit_crop.yuv --bitrate 9000 --vbv-bufsize 9000 --strict-cbr
> 
>        Bitrate	 Y PSNR	 U PSNR	 V PSNR	 Global PSNR	 SSIM	 SSIM (dB)
> before  9168.43 39.689  45.096 45.152	  41.048	 0.93176  11.66
> after   9106.16  39.864 45.169	45.199	  41.194	 0.93518  11.883

do these pair of commits belong on stable?

> diff -r 6f159ec858f4 -r b4646bdaf220 source/encoder/ratecontrol.cpp
> --- a/source/encoder/ratecontrol.cpp	Tue Mar 31 22:16:21 2015 +0530
> +++ b/source/encoder/ratecontrol.cpp	Mon Mar 16 12:05:38 2015 +0530
> @@ -372,12 +372,17 @@
>      /* Frame Predictors and Row predictors used in vbv */
>      for (int i = 0; i < 4; i++)
>      {
> -        m_pred[i].coeff = 1.5;
> +        m_pred[i].coeff = 1.0;
>          m_pred[i].count = 1.0;
>          m_pred[i].decay = 0.5;
>          m_pred[i].offset = 0.0;
>      }
> -    m_pred[0].coeff = 1.0;
> +    m_pred[0].coeff = m_pred[3].coeff = 0.75;
> +    if (m_param->rc.qCompress >= 0.8) // when tuned for grain 
> +    {
> +        m_pred[1].coeff = 0.75;
> +        m_pred[0].coeff = m_pred[3].coeff = 0.50;
> +    }
>      if (!m_statFileOut && (m_param->rc.bStatWrite || m_param->rc.bStatRead))
>      {
>          /* If the user hasn't defined the stat filename, use the default value */
> @@ -1797,8 +1802,11 @@
>          double pbits = predictSize(&m_pred[m_predType], q, (double)m_currentSatd);
>          if (pbits > rce->frameSizeMaximum)
>              q *= pbits / rce->frameSizeMaximum;
> -
> -        if (!m_isCbr || (m_isAbr && m_currentSatd >= rce->movingAvgSum && q <= q0 / 2))
> +        /* To detect frames that are more complex in SATD costs compared to prev window, yet 
> +         * lookahead vbv reduces its qscale by half its value. Be on safer side and avoid drastic 
> +         * qscale reductions for frames high in complexity */
> +        bool mispredCheck = rce->movingAvgSum && m_currentSatd >= rce->movingAvgSum && q <= q0 / 2;
> +        if (!m_isCbr || (m_isAbr && mispredCheck))
>              q = X265_MAX(q0, q);
>  
>          if (m_rateFactorMaxIncrement)
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-- 
Steve Borho