Extracting Spot Color equivalents from TIFF

Question

I'm trying to get the Spot color information from a TIFF file, it normally shows up under 'channels' in Photoshop. Each extra channel would have a name, which is usually a Pantone swatch name, and a CMYK equivalent.

So far, I'm getting the TIFFTAG_PHOTOSHOP with libtiff, and stepping through the blocks within. I'm finding the IRB WORD 0x03EE, which gives me the channel names, and IRB WORD 0x03EF which gives me their color equivalents...

BUT the color equivalents are in CIELab format (Luminance, and two axis of color space data) so I'm trying to use littleCMS to convert just a few TIFF packed Lab color to CMYK.

My question: Is there an easier way? The CMYK is just an approximation of the Pantone, so if there was a quick rough translation from Lab to CMYK, I would use it.

Answer 1

When I worked in printing and needed to get a spot color closest to some CMYK value, I would simply highlight the color in question with the sample tool, and open my spot color library. Usually the closest Pantone color would be already selected, based on it's CMYK approximation. Depending on the print shop, you may be able to give them the CMYK formula for the color and they can custom mix the spot color for you.

I should add that I wasn't sure if you were trying to do this programatically or not (sounds like programatically). I am not familiar with any data inside the TIFF that may point to a particular spot color. My above reference is just if you are using Adobe Photoshop and viewing the spot color libraries after selecting the CMYK (or RGB) color.

Answer 2

The answer was to use the photoshop docs to parse out the binary photoshop block in the tiff file and grab the fields I needed with bit manipulation.

littleCMS did the job of LAB -> CMYK just right.