On Sun, Sep 13, 2020 at 8:55 AM Michael Niedermayer <mich...@niedermayer.cc> wrote:
> On Sat, Sep 12, 2020 at 02:07:14AM -0700, mindm...@gmail.com wrote: > > From: Mark Reid <mindm...@gmail.com> > > > > --- > > libswscale/input.c | 12 +++++------- > > tests/ref/fate/filter-pixfmts-scale | 8 ++++---- > > 2 files changed, 9 insertions(+), 11 deletions(-) > > Can you provide some tests that show that this is better ? > Iam asking that because some of the numbers in some of the code > (i dont remember which) where tuned to give more accurate overall results > > an example for tests would be converting from A->B->A then compare to the > orig > > thx > > Hopefully i can explain this clearly! It's easier to see the error if you run a black image through the old conversion. zero values don't get mapped to zero. (attached sample image) ffmpeg -i 4x4_zero.exr -pix_fmt rgb48le -f rawvideo 4x4_zero.rawvideo The image should be rgb 0, 0, 0 everywhere but instead it's 353, 0, 407 I think this is a error in fixed point rounding, the issue is basically boils down to 128 << 8 != 257 << 7 and 16 << 8 != 33 << 7 https://en.wikipedia.org/wiki/YUV#Studio_swing_for_BT.601 the 8 bit rgb to yuv formula is Y = ry*r + gy*g + by*b + 16 U = ru*r + gu*g + bu*b + 128 V = rv*r + gv*g + bv*b + 128 I think the studio swing offsets at the end are calculated wrong in the old code. (257 << (RGB2YUV_SHIFT + bpc - 9))) 257 is correct for 8 bit rounding but not for 16-bit. the 257 i believe is from (128 << 1) + 1 the +1 is for rounding for rounding 16-bit (128 << 9) + 1 = 0x10001 therefore I think the correct rounding any bit depth with the old formula would be (untested) (((128 << (bpc - 7)) + 1) << (RGB2YUV_SHIFT-1) ) I just simplified it to (0x10001 << (RGB2YUV_SHIFT - 1)) The rgb48ToUV and rgb48ToY funcs in input.c use the formula I'm using. > > [...] > -- > Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB > > Everything should be made as simple as possible, but not simpler. > -- Albert Einstein > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > https://ffmpeg.org/mailman/listinfo/ffmpeg-devel > > To unsubscribe, visit link above, or email > ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
4x4_zero.exr
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