Lightroom, Core Image, Quartz Composer, and Channel Mixing

Lightroom has a greyscale mixer, similar to the channel mixer in Photoshop and the GIMP, and adds HSL Tuning with a similar user interface, but instead of the RGB values I’m used to, it also provides access to CMY. I’ve had a lot of fun converting some images from my archive in to black and white, and radically altering the color balance on others.

It made me realize that channel mixing, levels, and curves are probably the most common photoshop or gimp tool for me, with layers and masks coming in as distant runners-up, though that may be more due to my lack of skill with the tools than with their usefullness.

It also got me thinking, why aren’t there free image editors that aren’t the GIMP offering cool tools like that? Not that there’s anything WRONG with the GIMP, it’s just not nearly as pleasant to use as a typical Macintosh application. We’ve got Core Image and Quartz Composer on the Mac; how hard could it really be to do up a six-color mixer like Lightroom’s?

Scott and I are going to find out. After work today I started browsing around for answers and didn’t find any, but I did find an Apple example on RGB value transformation. I looked up how to compute CMY values from RGB, and in about half an hour, thanks to key value coding, I had added CMY selectors to the RGB Value Transformation example.

The images didn’t look right, and when Scott looked at it, he realized the CMY values were not masks like we wanted. The shortfall was I wasn’t finding the black point for each pixel, so my CMY values were actually intermediate values; other colors were showing up in my “mask”.

I’d never used Quartz Composer before, but with Scott’s help, we had duplicated the results of the RGB Value Transformation example in about half an hour, this time calculating CMY per pixel. The results look a lot closer to what I would expect.

I intend to learn how to access Core Image kernels from Objective C, so I can make a real app out of this once we’ve figured out the method. That way I should be able to add sliders to control the recombination of the channels/colors.