Re: [Lcms-user] LUTs in littlecms

[Guy K. K. <g....@ma...>]

That's an interesting one.

On Sat, 18 Apr 2009 21:19:08 Ignacio Ruiz-de-Conejo wrote:
> And I was wrong when I said number of nodes is not important, for I
> think a CLUT with 65^3 nodes will reduce the space between nodes and the
> bad/interpolated values, compared to a 17^3 or even a 33^3 one.
>
> Still, the same question apply: can anybody suggest a different approach
> to build such a profile?

Actually, as I understand it, you're trying to use the ICC profile's LUT for 
something it hasn't been designed for. The LUT has been intended to provide 
support nodes for a multi-dimensional interpolation. Your values are usually 
unlikely to be located precisely on the nodes of the LUT, so the interpolation 
takes the surrounding neighbour nodes into consideration and computes a smooth 
transitional point location for the intermediate space.

In your case, if a point is located in the "dead-man's strip" between common 
areas of your segmentation, then you're going to get blurring or anti-aliasing 
effects, that are probably not what you want (as you're looking for sharp 
segmentation boundaries). You're working not with a smooth profile, but with a 
function that is multiply non-continuous (multiply not differentiable).

I guess I can see only two possibilities for this: Use a "fully" occupied LUT 
for your sRGB colour space (256^3 nodes) so you won't run into interpolation 
on 8 bit sRGB images, or use an altogether different approach (non-ICC or non-
colour engine based).

HTH,

Guy


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Guy K. Kloss
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