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To: tiff@olympiakos.com
Subject: Photoshop layers, Lab colorspace
From: "Stephan Assmus" <stippi@yellowbites.com>
Date: Mon, 12 May 2003 19:12:10 +0200 CEST
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Hello all,

I'm new on the TIFF list. I don't know if any FAQs exist, and am just 
hoping that I don't bore anyone with my questions. I have written a 
graphics application for the BeOS, which is not yet published. TIFF 
would support saving any information I would like to remember in my 
image files. However, the main reason I'm interested in TIFF is that 
Photoshop can now save layer information in TIFF files. Unfortunately, 
I have not figured out, how it does that. My first guess, after reading 
a bit through the libtiff documentation, is that it maybe uses multiple 
TIFF directories, one for each layer. This seems not the case though. 
Is there any information available on how Photoshop saves layers in 
TIFF files?

My second question is about the Lab colorspace. I have read much about 
it, but am having difficulties implementing the correct conversion 
routines for RGB to Lab and back. I have used the matrix constants and 
algorithms described in the Color Space Conversions by Adrian Ford and 
Alan Roberts, however, converting some RGB color to Lab and back again 
produces some slightly more red result. And my routines are slow. 
Reading about in the TIFF spec, I was surprised to find some paragraphs 
on Lab, and this got my curious again, since it says that conversion 
can be fast using LUTs. Can someone point me to some resources that 
would be helpful? Or even provide code?

Best regards,
-Stephan

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