CHROMiX

Plotting Lab Images

I seem to be unable to plot LAB tif images in Colorthink. If I convert them to RGB they plot fine but I don’t want to limit the values to a RGB working space. Any ideas.

At 5:22 AM -0800 1/23/07, rick_lucas wrote:

ColorThink Pro should plot Lab TIFF images OK, but 2.2 doesn’t understand them… Also, Pro will only open 8 bit Lab TIFF images at this time. 16 bit Lab TIFF images are not well defined and so didn’t make it into Pro yet…

Regards,

Steve


o Steve Upton CHROMiX www.chromix.com
o (hueman) 866.CHROMiX


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Rick,

I have the same interest, so I would like to hitch a ride on your thread. I have been able to plot Lab TIFF images and text tables with CT 2.2. The TIFF images must be 8-bit.

I created a Photoshop script to generate Lab images or text tables populated with a range of Lab values. The text table is modeled after the ones provided in the CT Stunt Files folder. My current sample varies L from 0 to 100 in 8 steps and the ab values from 128 to +127 in 32 steps. This provides enough detail to see the shape without overloading CT memory and performance.

But the grapher seems to constrain the Lab image plot (LabGamut.tif) to the sRGB gamut. This doesnt help me at all.

It does graph the text table more reasonably. File, Open, LabGamut.txt. Showing Lab coordinates, you get a perfect cube limited only by the Lab values. Showing xyY coordinates you get an imperfect trapezoid. Most of the ab points where L is less than 30 are way outside of the xy horseshoe. FYI, the test data has no chromacity at L=0 and this point shows where I expect to see it. But I cant explain everything that I see in the graph.

I would like to be able to show the XYZ (xyY) gamut of the CIE standard observer as a 3D plot. And then, combine this with a 3D plot of the Lab gamut. Then I would like to be able to add any RGB profile.

The instructional value is to show users how Photoshop adjustment layers and such can introduce gamut problems in an otherwise valid photographic image.

The script is not ready for prime time, but I would be happy to email it (as is) to anyone who is interested. Or I can send my current test image and test tables.

[rags - at - compuserve.com]

At 10:18 AM -0800 1/29/07, rags wrote:

you are taking advantage of a side-effect of CT 2.2 opening files… more below…

good. that’s the way it should work…

no, I can see how it might not.

What happens in ColorThink 2.x is that files that are not recognized within CT code are opened using Quicktime and automatically converted by Quicktime before being handed to CT. As CT 2.x does not support (as in recognize and open directly) Lab images, Quicktime is gettin involved and opening the Lab image. It then converts the image to RGB and hands it to CT which plots it as an RGB file (and with the RGB-constrained gamut as well)

I’ll have to take a look at that graph to see what might be going on…

understood. You should certainly do this with text file data rather than image data in CT 2.x

Regards,

Steve


o Steve Upton CHROMiX www.chromix.com
o (hueman) 866.CHROMiX


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