Re: [Lcms-user] alpha blending in LAB space

[Kai-Uwe B. <ku...@gm...>]

Possibly reducing the number of blending colour spaces is a solution. 
Blending of mixed colour space layers is not a consistent thing anyway.

You could read the pdf spec how it was solved there. Chapter 7.2.3 
in the pdf reference 1.6 for instance.

regards
Kai-Uwe Behrmann
                                + development for color management 
                                + imaging / panoramas
                                + email: ku...@gm...
                                + http://www.behrmann.name


Am 05.12.06, 14:18 +0200 schrieb Yaron Tadmor:

> Hi Kai,
> 
> First, Thanx for the help.
> 
> 
> I'll try to explain better what I do.
> I have several layers 1 to n, each with its own profile (let's call it
> profile-i). I'm blending them all together (similar to layers in
> PhotoShop) on a single canvas which has its own profile as well (let's
> call it profile-c). I use profile-c as the output profile for exported
> images from my software (the output of the application). Now we also
> have the screen profile I'm showing the user the data on (let's call it
> profile-s).
> 
> What I first did was this: For display, I convert all layers to
> profile-s, so I have n conversions profile-0->profile-s to
> profile-n->profile-s. Then I blend layers in profile-s. For output I do
> the same with the destination profile being profile-c. The problem here
> is that the blending output is different.
> That is, the result of the blending depends on the profile I'm blending
> in.
> 
> I figured out the proper thing to do, is to always convert layers to
> profile-c, blend, and then if I'm displaying convert the result to
> profile-s for display.
> 
> I hope up to here everything is clear.
> 
> For this, blending can be done with openGL even if the format is 8-bit.
> The problem is that if profile-c is CMYK, I need to do 2 passes with
> openGL, since openGL doesn't support 4 color channels. So that's the
> problem.
> 
> I thought of blending in LAB PCS to avoid blending in CMYK and doing 2
> passes. However my concern was exactly the one you pointed out. That LAB
> is a wide space, and I might lose precision and get color stepping. So I
> guess my worries were justified. :-) and :-(.
> 
> So I guess my question, is if there's another thing to do what I want to
> do? I'm using openGL since I use 3D graphics to create the layers. They
> can be turned and manipulated in 3D space. openGL gives me a robust and
> easy to use rendering engine which also does depth testing on hardware.
> 
> I don't think openGL poses any performance problems or bottlenecks. As
> long as I really give the GPU RGB (or CMYK) data, it does everything
> fast and accurate. Perhaps giving it LAB data would be asking to much of
> low-precision GPU's, and that's exactly my question.
> 
> Yaron
>