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TIFF and LibTiff Mail List Archive

Thread

2000.10.19 19:27 "reading LAB files", by Michael O'Rourke
2000.10.19 21:11 "Re: reading LAB files", by Joris Van Damme
2000.10.20 08:11 "RE: reading LAB files", by Mart? Mar?ay7WE=
2000.10.20 18:20 "Re: reading LAB files", by Joris Van Damme
2000.10.20 19:02 "RE: reading LAB files", by Mart? Mar?ay7WE=
2000.10.20 22:25 "RE: reading LAB files", by Bill Radcliffe

2000.10.20 08:11 "RE: reading LAB files", by Mart? Mar?ay7WE=

Hi,

I seem to be having trouble reading LAB files.

I have an LAB file where Photshop show the first pixel as (55,22,-10).

When I read it into an unsigned char array with ReadScanline, I get (140, 22, 146) which maps to (-116, 22, -10) when typecast to signed chars.

So, what's up with the -116? Does libtiff do something special to LAB files? I grep'd the code, but didn't seem to find anything.

Lab TIFF are encoded using

        L   : 0...FF -> 0..100
        a,b:  0..FF  -> -128...127

So, you need to divide L by 2.55 and substract 128 to a,b parts to get float values. Also, TIFF lab does use D65 as white point (as opposed to D50, widely used on prepress). Note that this encoding is very usefull for simpler operations, and converting to float is very time consuming. If you want the float values to pass to XYZ and then to RGB by float formulae, you shurely will find this is a neverending process.

Marti Maria
The little cms project
http://www.abaforum.es/martim/lcms.htm
mailto:martim@abaforum.es