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December 2006
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2006.12.05 22:56 "Re: Grayscale, or is it?", by Glenn Widener
> Anyone has any advice on this matter? Some, briefly. > The TIFF spec, in "Section 4: Grayscale images" does not contain a good > definition of grayscale. However, on page 73, "Comments for TIFF > writers", in a discussion of gamma, it says "Be aware that the > PhotometricInterpretation value of 0 or 1 (grayscale) implies linear > data because no gamma is specified. The PhotometricInterpretation value > of 2 (RGB data) specifies the NTSC gamma of 2.2 as a default." We (SwiftView) used to obey the spec regarding a default gamma of 2.2. But we found no one else that does, so we can't either, and we changed the default to a straight linear device RGB. "No one else" includes at least.Microsoft Office Document Imaging and Paint Shop Pro 6. > Now, what is linear data? Linear to what? What is grayscale, for that > matter? Linear means literally read, write and send the values to your display unchanged - "linear device values". If you don't know what the correction is, or haven't been told, don't do any. > 1) Greyscale is R=G=B. I think this interpretation is consistent with > most common practice. It is also consistent with the LibTiff RGBA > interface implementation, in that this just sets R, G, and B, to the > greyscale read from the TIFF. It is however a clear violation of the > above quoted remark on gamma. The standard conversion from RGB to gray, documented in the textbooks, is grey level = .33R+.5G+.17B. So a unitary conversion back to RGB is fine. Off the cuff, I would convert greyscale to RGB, apply whatever color correction you wish, then convert back. But then I'm not a color space expert. Converting gray levels 1:1 to the intensity component of a color space that has an intensity component makes perfect sense to me.