Re: maclife.de
Re: maclife.de
- Subject: Re: maclife.de
- From: Graeme Gill <email@hidden>
- Date: Fri, 22 Aug 2008 10:17:34 +1000
Uli Zappe wrote:
It's not the white point I referred to, but the expansion of luminosity
in the perceptual intent.
There may not be a perceptual intent table in an input profile, since
it is not required, was not valid before ICC V2.4, and can't be supported
by matrix/shaper based profiles, and there is the (usual) quandary about
what gamut mapping to use (ie. the destination gamut is possibly unknown).
As I understand it, luminosity is usually expanded in the perceptual
intent, while in the colorimetric, it's not.
Let's say you have a luminosity of 5 in the Dmin patch and 90 in the
Dmax patch of your target, and for the simplicity of the argument let's
Hmm. Don't you mean Dmax and Dmin (ie. doesn't "D" stand for "Density",
so maximum density should be L* = 5, and minimum density should
be L* = 90 ?)
say you map 1:1 to RGB and ignore gamma. So Dmin would correspond to
0.05 x 255 = RGB 13, and Dmax to 0.9 x 255 = RGB 230. This is where the
colorimetric intent would leave it at, whereas the perceptual intent
would expand luminosity to make the image look brighter (at least that's
how I understand it).
Absolute colorimetric will leave it at RGB 13 -> L* 5, and RGB 230 -> L* 90.
Relative colorimetric will map the device white to PCS white, so
RGB 13 -> (Approx) L* 6, RGB 230 -> L* 100
The only way a perceptual intent could make it brighter than relative
colorimetric is to fiddle with the mid tone values. The white can't
get any whiter than PCS white.
Obviously, you could do this full scale (map RGB 13 to RGB 0 and RGB 230
to RGB 255), or only partly, leaving a certain headroom for images with
Dmin or Dmax beyond the target patches (e.g. map RGB 13 to RGB 5 and RGB
230 to RGB 250) and avoiding that the images look "too bright". That's
what I referred to with "somewhere in between".
Certainly a perceptual table could do this. Do any of the profiling
packages actually create a A2B0 (perceptual) table for an input profile though ?
Graeme Gill.
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