2018.05.09 14:51 "Re: [Tiff] Question about transfer function", by Larry Gritz
On May 9, 2018, at 6:05 AM, Bob Friesenhahn <firstname.lastname@example.org> wrote:
In the real world of semi-random sources and software, it is most likely that a grayscale image has a gamma of approximately 2.2. Lots of software uses Rec.601 or Rec.709 Luma calculations to produce a gray image from RGB, or might even just select one of the RGB channels (e.g. green) and use it as the gray channel. GraphicsMagick still uses Rec.601 Luma transforms by default although it is likely that Rec.709 is a better choice.
Lots of "grayscale" (that is, single channel) images that I see don't represent color at all -- they might be bump amount, or shininess, or some kind of matte or mask. In my world these are usually linear.
TIFF 6.0 is very old. Since then ICC color profiles and other means have emerged to provide colorspace information.
I see ICC profiles occasionally from images that come out of PhotoShop and in some camera-related images, but virtually nothing else. I've never seen a renderer (for example) output an ICC profile to indicate that it represented a linear image, which it usually is.
It's unfortunate that TIFF didn't originally require a tag that would simply (without an enormous lookup table or ICC profile) let you know if it was one of the common cases of linear response or sRGB's transfer function.
OK, so it looks like I shouldn't take that passage in the TIFF spec very seriously, because people are just shoving whatever random stuff into TIFF files without any consistent ways to say what it is.