[FFmpeg-devel] [PATCH] swscale_unscaled: fix DITHER_COPY macro, use it only for dst_depth == 8

Mateusz mateuszb at poczta.onet.pl
Sat Sep 23 20:18:21 EEST 2017

W dniu 2017-09-23 o 17:01, Michael Niedermayer pisze:
> On Fri, Sep 22, 2017 at 02:10:01AM +0200, Mateusz wrote:
>> To reduce bit depth in planar YUV or gray pixel formats ffmpeg uses DITHER_COPY macro.
>> Now it makes images greener and with visible dither pattern.
>> In my opinion there is no point to use dither tables for destination bit depth >= 9,
>> we can use simple down-shift which is neutral in full and limited range -- result images
>> are with the same brightness and with the same colors.
> Theres no reason why dither should mess up the average color tone.

In theory -- yes, I agree.
In reality -- current version of DITHER_COPY mess up the average color tone.
It's one of the reasons why I sending these patches.

> And if the user asks for >= 9 bit depth and has >= 10 bit input the
> user likely wants to get the best quality.
> Thats more so in a world where computers get faster every few years,
> this isnt 1995 where shaving off a add or a multiply per pixel was
> actually making a difference in being able to play something in
> realtime
> More so coverting between bit depths might be memory speed limited and
> not limited by arithmetic computations once its done in SIMD

Yes, I agree. Now I can't write patches in NASM syntax, but I started reading and learning.
I hope I'll back in a few months...


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