[FFmpeg-trac] #11182(swscale:closed): yuvtestsrc and yuv444p->rgba->yuv444p conversion result in huge difference

[FFmpeg]

#11182: yuvtestsrc and yuv444p->rgba->yuv444p conversion result in huge difference
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             Reporter:  Andrew-R     |                    Owner:  (none)
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:  swscale
              Version:  unspecified  |               Resolution:  invalid
             Keywords:  colorspace   |               Blocked By:
  color_primaries                    |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by Andrew-R):

 ah, found this technical reference:

 https://techpubs.jurassic.nl/library/manuals/3000/007-3513-001/sgi_html/ch08.html

 ```
 Constant Hue

 In addition to the standard color-space conversion model, the OCTANE
 Digital Video color-space feature provides a constant-hue algorithm. This
 algorithm allows illegal YUV values to survive a YUV-to-RGB-to-YUV
 conversion. In normal conversion, YUV values that cannot be represented in
 the RGB color space are clamped or otherwise forced into the legal RGB
 range. **Because the YUV (YCrCb) color space is a superset of the RGB
 color space, illegal RGB values can be generated when YUV is converted to
 RGB.** If the constant-hue block is disabled, then the illegal RGB values
 are clipped by the output LUT. The lost (clipped) information can result
 in significantly degraded quality when the image is subsequently
 transformed back to YUV for video output.

 The constant-hue algorithm saves the normally lost information in a
 correction factor that can be stored in the alpha channel. To restore the
 original YUV image, this correction factor must be saved with the pixel
 data.

 If the constant-hue algorithm is enabled, the illegal RGB values are
 converted into legal R'G'B' values. A constant-hue factor, used to restore
 R'G'B' to the original YUV values, can optionally be stored in the alpha
 channel. If the constant-hue factor is not saved, then the R'G'B' image
 appears as if it were range-compressed. The VL_MGV_CSC_ALPHA_CORRECTION
 control determines whether the alpha channel is replaced by the constant-
 hue factors, or if the alpha from the color-space converter's input is
 retained.

 Note that because the correction factor computed by the algorithm is
 directly related to the pixel value, the correction factor is invalidated
 if the pixel value is recalculated (for example, during compositing).
 ```

 Interesting technology, but probably not something ffmpeg can do (or
 expose to external use). Honestly, until now I was under impression RGB
 was superset of YUV, not other way around ... I was wrong!
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11182#comment:30>
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