Re: Who's right?
Re: Who's right?
- Subject: Re: Who's right?
- From: Andre Schützenhofer <email@hidden>
- Date: Tue, 8 Jun 2004 14:23:28 +0200
Am 07.06.2004 um 05:31 schrieb Graeme Gill:
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).
Basically, the decision between linear mapping or non-linear mapping
and the way of doing it should depend on the gamuts involved in one /
this particular transformation. The current approach of BPC is to map
L* linearly - or, more precisely: After determining the Black Points,
they are scaled to zero and mapping happens linearly on L (White Point
is relative anyway), regardless of the real 'sizes' of Ls.
Consequentially, best results will be achieved if the determined Black
Points do not differ too much.
In other cases, e.g. if we map L*= 0 to L*= 17, linear compression of L
will be perceived in shadows more than in midtones, quaters or
highlights. (Virtually, there is no loss because of scaling the BP to
ideal black. So for the sense of the transformation, mapping happens on
the full dynamic range of L*.) The issue here is that this may happen
to colorspaces with Black Points far from absolute black.
So, one possible solution might be to examine the distance between
source and destination black point, which is basis for the 'strength'
of a non-linear L mapping. Method of mapping is result of the
correlation between the black points, determined by examination of how
to compress L* full-dynamic to limited dynamic. Clear defintion of
perception is the equal-distance of distinguishable gray-steps, which
might be standardizeable (similar to LAB itself). The strength
dynamically increases or decreases equal to the distance between black
points.
Regards, Andre Sch|tzenhofer
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