2006.12.05 15:52 "[Tiff] Grayscale, or is it?", by Joris Van Damme

2006.12.06 23:30 "Re: [Tiff] Re: Tiff Digest, Vol 31, Issue 3", by Joris Van Damme


If you colour model goes as far as to be based on actual physical properties of light, it's extremely complicated to convert RGB to "greyscale". You'll have to dig into spectra and whatnot, for all I know.

I can't agree with you on this. Taking the weighted sum of an RGB output in linear light is not an approximation, it's actually correct, equivalent to CIE Y value if the right weightings are used. The weightings simply depend on the primary locations and the white point.

We don't really dissagree. I personally use the second option I enumerated, i.e. converting RGB to CIE Y and back to greyscale. That seems to be exactly what you propose. Is this an approximation, or is it 'actually correct'? I prefer to avoid these terms. Some people are bound to say CIE XYZ sucks and CIE L*a*b* is about as different from human vision as you can get. They may be right, they may be wrong. In the absence of ways to deduce any of this, other then by experiment and averaging out the 'standard observer', the discussion is meaningless as far as I'm concerned. I'm not shy to use terms like 'right' and 'wrong' when it comes to the interpretation and implementation of a single spec that defines a file format, but there is no single spec that defines human vision. All I know about CIE XYZ and CIE L*a*b* is, I'm satisfied using it and building my model around it.

As to the first opion, spectra and whatnot, I must admit I don't know the first thing about that. Perhaps you can take 'actual physical properties of light' into account without spectra, perhaps that's exactly what we're doing with our conversion to CIE Y and back, I wouldn't know really.

And as to weighted sum without compensating for gamma first, thus making a linear calculation on non-linear data... Well, I was trying to be friendly on the masses that do this because 'it's a well-know textbook standard'. I didn't actually mean to say this is valid. Is why I added that small remark about us being free to have our personal preference. ;-)

Best regards,

Joris Van Damme
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