Am 15.02.11, 12:59 +0100 schrieb Mark:
> I'm trying to find the ranges of a* and b* so I can normalize Lab values to 
> 0.0...1.0.
> For this I sliced several RGB color spaces (srgb, adobe rgb, prophoto, wide 
> gamut) and printed out the resulting Lab values.
>
> For "wide gamut" I am seeing a* values as high as -211 and b* of +166.
>
> Digging into the LCMS source code I saw the "integer encodable" ranges for a* 
> b* are defined as
> #define MIN_ENCODEABLE_ab4  (-128.0)
> #define MAX_ENCODEABLE_ab4  (127.0)

Thats the typical value range.

> Does that mean that  "wide gamut" contains RGB values that are outside of the 
> max encodable ICC a*b* ranges (for integer encoding) ?

Thats normal up to virtual colours which are not physically reproducible.

> So, now I am a bit lost how to best convert a* and b* to the ranges 0...1.0

Given that the range of 0...1.0 is tailored to RGB, you might ask yourself 
what to do with pure chroma channels in *a and *b. Present in a UI? pass 
through RGB workflows with accoring tools?

> I will not be using color spaces largen than adobe rgb (which fits fine 
> inside MIN_ENCODEABLE_ab4 and MAX_ENCODEABLE_ab4.

That value use just the limit of the integer representations. For floats 
such a limitation does not exist. The gamut can well exceed that range and 
colours can fit into that range, which will physically not exist.

> So I'm thinking of just normalizing using these limits and clamping as 
> necessary.

Why clamping? The whole point of floating point processing is not to 
clamp, except maybe for final display and print.

kind regards
Kai-Uwe Behrmann
-- 
developing for colour management 
www.behrmann.name + www.oyranos.org


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