W dniu 2017-09-05 o 23:37, Michael Niedermayer pisze:
> On Tue, Sep 05, 2017 at 04:42:06PM +0200, Mateusz wrote:
>> W dniu 2017-09-05 o 15:40, Michael Niedermayer pisze:
>>> On Mon, Sep 04, 2017 at 09:33:34AM +0200, Mateusz wrote:
>>>> If ffmpeg reduces bit-depth to 8-bit or more, it uses DITHER_COPY macro.
>>>> The problem is DITHER_COPY macro make images darker (on all planes).
>>>>
>>>> In x265 project there is issue #365 which is caused by this DITHER_COPY
>>>> bug.
>>>> I think it is time to fix -- there are more and more high bit-depth
>>>> sources.
>>>>
>>>> Please review.
>>>>
>>>> libswscale/swscale_unscaled.c | 44
>>>> +++++++++++++------------------------------
>>>> 1 file changed, 13 insertions(+), 31 deletions(-)
>>>
>>> please provide a git compatible patch with with commit message as produced
>>> by git format-patch or git send-email
>>>
>>> this mail is not accepted as is by git
>>> Applying: libswscale/swscale_unscaled: fix DITHER_COPY macro
>>> error: corrupt patch at line 6
>>> error: could not build fake ancestor
>>> Patch failed at 0001 libswscale/swscale_unscaled: fix DITHER_COPY macro
>>>
>>> [...]
>>
>> I've attached the result of git format-patch command.
>>
>> Sorry for 1 private e-mail (I clicked wrong button).
>>
>> Mateusz
>
>> swscale_unscaled.c | 44 +++++++++++++-------------------------------
>> 1 file changed, 13 insertions(+), 31 deletions(-)
>> 9973b13b3f74319abe9c97302ee87b2b3468b3b6
>> 0001-fix-DITHER_COPY-macro-to-avoid-make-images-darker.patch
>> From 9b96d612fef46ccec7e148cd7f8e8666b4e7a4cd Mon Sep 17 00:00:00 2001
>> From: Mateusz <mateu...@poczta.onet.pl>
>> Date: Tue, 5 Sep 2017 16:09:28 +0200
>> Subject: [PATCH] fix DITHER_COPY macro to avoid make images darker
>>
>> ---
>> libswscale/swscale_unscaled.c | 44
>> +++++++++++++------------------------------
>> 1 file changed, 13 insertions(+), 31 deletions(-)
>>
>> diff --git a/libswscale/swscale_unscaled.c b/libswscale/swscale_unscaled.c
>> index ef36aec..3b7a5a9 100644
>> --- a/libswscale/swscale_unscaled.c
>> +++ b/libswscale/swscale_unscaled.c
>> @@ -110,24 +110,6 @@ DECLARE_ALIGNED(8, static const uint8_t,
>> dithers)[8][8][8]={
>> { 112, 16,104, 8,118, 22,110, 14,},
>> }};
>>
>> -static const uint16_t dither_scale[15][16]={
>> -{ 2, 3, 3, 5, 5, 5, 5, 5, 5, 5, 5, 5,
>> 5, 5, 5, 5,},
>> -{ 2, 3, 7, 7, 13, 13, 25, 25, 25, 25, 25, 25,
>> 25, 25, 25, 25,},
>> -{ 3, 3, 4, 15, 15, 29, 57, 57, 57, 113, 113, 113,
>> 113, 113, 113, 113,},
>> -{ 3, 4, 4, 5, 31, 31, 61, 121, 241, 241, 241, 241,
>> 481, 481, 481, 481,},
>> -{ 3, 4, 5, 5, 6, 63, 63, 125, 249, 497, 993, 993,
>> 993, 993, 993, 1985,},
>> -{ 3, 5, 6, 6, 6, 7, 127, 127, 253, 505, 1009, 2017,
>> 4033, 4033, 4033, 4033,},
>> -{ 3, 5, 6, 7, 7, 7, 8, 255, 255, 509, 1017, 2033,
>> 4065, 8129,16257,16257,},
>> -{ 3, 5, 6, 8, 8, 8, 8, 9, 511, 511, 1021, 2041,
>> 4081, 8161,16321,32641,},
>> -{ 3, 5, 7, 8, 9, 9, 9, 9, 10, 1023, 1023, 2045,
>> 4089, 8177,16353,32705,},
>> -{ 3, 5, 7, 8, 10, 10, 10, 10, 10, 11, 2047, 2047,
>> 4093, 8185,16369,32737,},
>> -{ 3, 5, 7, 8, 10, 11, 11, 11, 11, 11, 12, 4095,
>> 4095, 8189,16377,32753,},
>> -{ 3, 5, 7, 9, 10, 12, 12, 12, 12, 12, 12, 13,
>> 8191, 8191,16381,32761,},
>> -{ 3, 5, 7, 9, 10, 12, 13, 13, 13, 13, 13, 13,
>> 14,16383,16383,32765,},
>> -{ 3, 5, 7, 9, 10, 12, 14, 14, 14, 14, 14, 14,
>> 14, 15,32767,32767,},
>> -{ 3, 5, 7, 9, 11, 12, 14, 15, 15, 15, 15, 15,
>> 15, 15, 16,65535,},
>> -};
>> -
>>
>> static void fillPlane(uint8_t *plane, int stride, int width, int height,
>> int y,
>> uint8_t val)
>> @@ -1502,22 +1484,22 @@ static int packedCopyWrapper(SwsContext *c, const
>> uint8_t *src[],
>> }
>>
>> #define DITHER_COPY(dst, dstStride, src, srcStride, bswap, dbswap)\
>> - uint16_t scale= dither_scale[dst_depth-1][src_depth-1];\
>> - int shift= src_depth-dst_depth +
>> dither_scale[src_depth-2][dst_depth-1];\
>> + unsigned shift= src_depth-dst_depth, tmp;\
>> for (i = 0; i < height; i++) {\
>> - const uint8_t *dither= dithers[src_depth-9][i&7];\
>> + const uint8_t *dither= dithers[shift-1][i&7];\
>> for (j = 0; j < length-7; j+=8){\
>> - dst[j+0] = dbswap((bswap(src[j+0]) + dither[0])*scale>>shift);\
>> - dst[j+1] = dbswap((bswap(src[j+1]) + dither[1])*scale>>shift);\
>> - dst[j+2] = dbswap((bswap(src[j+2]) + dither[2])*scale>>shift);\
>> - dst[j+3] = dbswap((bswap(src[j+3]) + dither[3])*scale>>shift);\
>> - dst[j+4] = dbswap((bswap(src[j+4]) + dither[4])*scale>>shift);\
>> - dst[j+5] = dbswap((bswap(src[j+5]) + dither[5])*scale>>shift);\
>> - dst[j+6] = dbswap((bswap(src[j+6]) + dither[6])*scale>>shift);\
>> - dst[j+7] = dbswap((bswap(src[j+7]) + dither[7])*scale>>shift);\
>> + tmp = (bswap(src[j+0]) + dither[0])>>shift; dst[j+0] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+1]) + dither[1])>>shift; dst[j+1] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+2]) + dither[2])>>shift; dst[j+2] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+3]) + dither[3])>>shift; dst[j+3] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+4]) + dither[4])>>shift; dst[j+4] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+5]) + dither[5])>>shift; dst[j+5] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+6]) + dither[6])>>shift; dst[j+6] =
>> dbswap(tmp - (tmp>>dst_depth));\
>> + tmp = (bswap(src[j+7]) + dither[7])>>shift; dst[j+7] =
>> dbswap(tmp - (tmp>>dst_depth));\
>
> This does not look correct
>
> flat black should be mapped to flat black (ok)
> flat white should be mapped to flat white (ok)
> black+1 should not be mapped to flat black but to a dithered pattern mixing
> black with the next brighter color (ok)
> white-1 should not be mapped to flat white but to a dithered pattern mixing
> white with the next darker color (not ok)
>
> example if you take 9 bit to 7 bit
> 511 + {0..3} = {511..514}
> {511..514} >> 2 = {127,128}
> {127,128} - ({127,128} >> 7) = 127
>
> 510 + {0..3} = {510..513}
> {510..513} >> 2 = {127,128}
> {127,128} - ({127,128} >> 7) = 127
>
> Thus multiple of the brigher colors all get mapped to the same output,
> that way the brightest shades become indistingishable and this is not
> correct.
>
> above is based on code review not testing so i might have missed
> something
>
> [...]
Yes, you are right.
There is one more rule, that is important and (partially) in contradiction to
white-1 to not flat white rule:
if you convert 7-bit source to 9-bit and next resulting 9-bit to 7-bit, it
should be the same picture as original.
if we convert 7 bit white (or max value at plane) == 127, we have 127 << 2 ==
508
now 508 goes always to 127
If you want 510 goes to 127 or 126, 508 will be not always converted to 127, so
if you make multiple 7bit -> 9bit -> 7bit conversions, you will have darker
result (or greener).
My point is: if we upscale bit-depth by simple shift (127 to 508), this patch
is OK.
If we upscale bit-depth different for luma and chroma, we should downscale
bit-depth different for luma and chroma.
Now I don't know what ffmpeg is doing when upscaling bit-depth -- I will check
tomorrow.
Mateusz
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