W dniu 2017-03-16 o 19:17, Michael Niedermayer pisze: > On Wed, Mar 15, 2017 at 10:52:29PM +0100, Mateusz Brzostek wrote: >> Hello! >> >> There are 3 problems with DITHER_COPY macro in libswscale/swscale_unscaled.c: >> 1) there is overflow in dithering from 12-bit to 10-bit (output value > >> 1023); >> 2) for limit range the lower limit is not respected, for example from 10-bit >> to 8-bit value 64 is converted to 15; >> 3) for many iteration of downscale/upscale of the same image the 200th >> iteration is significantly darker. >> >> The first bug is due to wrong dithers table (now it is OK only for 8-bit >> destination), fix is: >> - const uint8_t *dither= dithers[src_depth-9][i&7];\ >> + const uint8_t *dither= dithers[src_depth-dst_depth-1][i&7];\ >> >> For bugs 2) and 3) it is needed formula that do not make images darker (in >> attachment). So please review. > does your code maintain white and black levels ? > with 4 bits white is 15, with 7 bits white is 127 for example > white should stay white > black should stay black > in both directions === short version ===
Yes, white goes to white, black goes to black. === long version === This DITHER_COPY macro is for case when destination bit-depth >= 8 and source bit-depth > destination bit-depth and the picture type is the same (for example yuv444p12 to yuv444p10). It is replacement for down-shift. For destination bit-depth < 8 it is needed real dither (not simplified like this DITHER_COPY). The formula before the patch was: dst = (src + dither) * scale >> shift where: dst - destination, src - source, dither - constant values from table 8x8, scale and shift are chosen to fulfill formula (in mathematics/floating point arithmetic): (max_value_src + max_value_dither) * scale / (2^shift) < max_value_dst + 1 This is due to lack of clip function (for speed reason). If you consider (scale / (2^shift)) number for example for conversion 10-bit to 8-bit, we have (scale / (2^shift)) < 1/4 because (1023 + 3) * 1/4 = 256.5 and 256.5 >= 255 + 1 (with 1/4 there will be overflow: 1023 go to 256 for dither > 0). So for src > 0 with 2 least significant bits = 0 and dither = 0 we have dst <= (src >> 2) - 1 which is the darker destination effect (it is unavoidable in this model). In my patch the number (scale / (2^shift)) == 1/4 for 10-bit to 8-bit example. This fix the darker destination effect but create possible overflow problem. In my patch the possible overflow go to sign bit of int32_t variable and is clipped to 0x7FFFFFFF. In my tests the new DITHER_COPY is a little bit slower than old but is much faster than real dither in x265. Mateusz _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
