On Sat, Sep 26, 2020 at 10:01:30PM -0700, Mark Reid wrote: > On Mon, Sep 14, 2020 at 6:31 PM Mark Reid <mindm...@gmail.com> wrote: > > > > > > > On Mon, Sep 14, 2020 at 2:44 PM Michael Niedermayer <mich...@niedermayer.cc> > > wrote: > > > >> On Sun, Sep 13, 2020 at 04:04:42PM -0700, Mark Reid wrote: > >> > 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. > >> > >> You quite possibly are correct, can you test that this actually works > >> out. The test sample only covers 1 color (black) > >> a testsample covering a wide range of the color cube would be more > >> convincing that this change is needed and sufficient to fix this. > >> > >> thx > >> > > > > I wrote a small python script to compare the raw gbrpf32le images if that > > works? I attached it and also a more colorful test pattern. > > > > it runs these two commands and compares the 2 raw float images > > ffmpeg -y -i test_pattern.exr -f rawvideo original.gbrpf32le > > ffmpeg -y -i test_pattern.exr -vf > > format=pix_fmts=rgb48le,format=pix_fmts=gbrpf32le -f rawvideo > > converted.gbrpf32le > > > > python gbrpf32le_diff.py test_pattern.exr > > > > without patch: > > avg error: 237.445495855 > > min error: 0.0 > > max error: 468.399102688 > > > > with patch: > > avg error: 15.9312244829 > > min error: 0.0 > > max error: 69.467689991 > > > > > > These are floating points scaled to 16-bit values. > > Even with my patch, I'm kinda disturbed how much error there is. > > > > ping > I re-wrote the python script as a c swscale test, if that helps > replicate my results. attached is patch for that. > it generates an image of random float values and does the > conversion/comparison . > > before patch: > gbrpf32le -> yuva444p16le -> gbrpf32le > avg diff: 0.003852 > min diff: 0.000000 > max diff: 0.006638 > > after patch: > gbrpf32le -> yuva444p16le -> gbrpf32le > avg diff: 0.000125 > min diff: 0.000000 > max diff: 0.000501
is it better for all middle formats ? Iam asking as it seems this should be rather easy to test with your code But from what i see above, obviously this is an improvment and should be applied thx [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Awnsering whenever a program halts or runs forever is On a turing machine, in general impossible (turings halting problem). On any real computer, always possible as a real computer has a finite number of states N, and will either halt in less than N cycles or never halt.
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