[FFmpeg-trac] #10428(documentation:new): HSV filters are not actually doing HSV

[FFmpeg]

#10428: HSV filters are not actually doing HSV
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             Reporter:  Mingye Wang  |                    Owner:  (none)
                 Type:  defect       |                   Status:  new
             Priority:  minor        |                Component:
                                     |  documentation
              Version:  git-master   |               Resolution:
             Keywords:  terminology  |               Blocked By:
  colorspace                         |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Description changed by Mingye Wang:

Old description:

> Summary of the bug: The `hsvhold` and `hsvkey` filters claim to be doing
> HSV, but a quick examination of the source code `do_hsvkey_pixel()`
> reveals that they do not actually use the famous
> [https://en.wikipedia.org/wiki/HSL_and_HSV#Basic_principle hexcone
> geometry] based on RGB, but instead piggybacks on YCC. Some clarification
> is needed.
>
> How to reproduce: Navigate to https://ffmpeg.org/ffmpeg-
> filters.html#hsvhold.
>
> (Not using the real HSV is actually a good thing. At least this "V" we
> use is based on video luma, making it a much better representation of how
> bright something looks like than the real thing that weighs RGB equally.
> However, people could have incorrect expectations about what "hsv" value
> their key color is.)
>
> Oh and also every filter that deals with hue works on YCC, because of
> course, that's what's in the video stream! A disclaimer in `filters.texi`
> should minimize the number of files to change.

New description:

 Summary of the bug: The `hsvhold` and `hsvkey` filters claim to be doing
 HSV, but a quick examination of the source code `do_hsvkey_pixel()`
 reveals that they do not actually use the famous
 [https://en.wikipedia.org/wiki/HSL_and_HSV#Basic_principle hexcone
 geometry] based on RGB, but instead piggybacks on YCC. Some clarification
 is needed.

 How to reproduce: Navigate to https://ffmpeg.org/ffmpeg-
 filters.html#hsvhold.

 (Not using the real HSV is actually a good thing. At least this "V" we use
 is based on video luma, making it a much better representation of how
 bright something looks like than the real thing that weighs RGB equally.
 However, people could have incorrect expectations about what "hsv" value
 their key color is.)

 This also applies to filters like `hue` and `huesaturation` which does the
 rotation in polarized YCC space. A disclaimer in `filters.texi` should
 minimize the number of files to change.

 Funny enough, there really isn't a name applied to this polar YCC / "Y
 with UV in polar coordinates" thing. Papers and patents has used this idea
 since the time of color TV vectorscopes, but nobody has bothered to name
 it.

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Ticket URL: <https://trac.ffmpeg.org/ticket/10428#comment:2>
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