Re: Who's right?
Re: Who's right?
- Subject: Re: Who's right?
- From: Graeme Gill <email@hidden>
- Date: Mon, 07 Jun 2004 13:31:11 +1000
- Organization: Color Technology Solutions
Chris Murphy wrote:
Supposedly Adobe is sharing their black point compensation method with
the ICC so that it can be adopted outside of Adobe applications. This is
something Apple needs to wrap into the Apple CMM ASAP and document to
developers. Black point compensation support is necessary.
I don't quite follow you. Black point compensation seems to be a workaround
for particular usage's and workflows using ICC profiles. In other contexts
it doesn't make any sense. It's hard to tell exactly (because there
doesn't seem to a technical description available of what Adobe black point
compensation is), but from what I gather it's a partial workaround for
the problem ICC profiles have with embedding a perceptual/saturation gamut
mapping in the destination profile, and expecting that profiles will normally
be linked by simply concatenating them.
The consequence is that the gamut mapping is often generic, and the most obvious
consequent limitation is that the luminance range doesn't get specifically mapped
from source to destination. The nature of the ICC data being relative to the white
point means that the white end is mapped acceptably, but the black point doesn't
get mapped in any specific way. I would guess that black point compensation
enables a rescaling of the luminance range, to better map the luminance dimension
of the source and destination gamuts.
Note that there are many ways of remapping luminance ranges (it's a sub set
of gamut mapping, so many approaches are possible). A simple approach is to
linearly map the luminance range. Another might be to do it non-linearly,
compressing highlights and shadows more than the middle range. It may
well be premature to standardize such a thing (at least as a "one and only"
way of doing things).
I've personally never felt the need for "black point compensation", simply because if
I'm interested in a quality result for a perceptual reproduction, I'll make sure
that a proper gamut mapping from source to destination is used, rather than trying
to use the (usually) wrong one in the B2A table of the ICC profile.
Graeme Gill.
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