#3801: SWSCALE: Incorrect Values (+-1) in Full Range Conversion RGB to YCbCr
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Reporter: troy_s | Owner: (none)
Type: defect | Status: reopened
Priority: normal | Component:
| undetermined
Version: unspecified | Resolution:
Keywords: range | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Comment (by Balling):
Right, so test-full-444-out.png has 1.0 gAMA png chunk. How beautiful. You
know that Photoshop, Paint and GIMP until my fixes and FFmpeg do not
support gAMA or cHRM chunks? While Chrome, Mozilla and Windows photo
viewer do support it? Let me guess, considering your ramp from 0 to 255
uses 186 as Cb=Cr with applying gAMA, gAMA of 1.0 is actually NOT CORRECT,
because without applying it (and falling back to sRGB) it will be 128 for
Cb=Cr??? WTF. Are you serious right now? Okay, whatever. P.S. test-full-
diff.png​ also has 1.0 gAMA.
That means you do understand PNG spec or gAMA too. Wow, just wow.
exiftool test-full-444-out.png
[Color Type : RGB
Compression : Deflate/Inflate
Filter : Adaptive
Interlace : Noninterlaced
Gamma : 1
Background Color : 255 255 255
Pixels Per Unit X : 72
Pixels Per Unit Y : 72
Pixel Units : Unknown]
So I need to remove gAMA with pngcrush -m 3 -rem gAMA:
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Ticket URL: <https://trac.ffmpeg.org/ticket/3801#comment:7>
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