I’m probably not the first person to use RawTherapee’s wonderful CIECAM02 module in this way. But it was a nice illustration of what color appearance models are all about. So I thought I’d share what happened when I photographed a small painting and realized the colors in the photograph didn’t match the colors in the painting.
A few days ago I painted a small “exercise in mixing foliage greens” (https://www.handprint.com/HP/WCL/tech34.html#foliage), in connection with topics discussed here:
To mix and paint the green colors, I used a set of 12 Mungyo water-soluble oil pastels. Similar to most such “sets of 12”, the Mungyo set provides 10 highly saturated colors plus black and white, so making believable foliage green colors did require substantial mixing.
I worked on the painting under the halogen track lighting that’s just to the right of my computer workstation. When I was done, I carried the painting upstairs to my little tabletop photography studio, where I set a custom in-camera white balance and then photographed the oil pastel painting. The lighting was mostly from a large window, augmented with an LED spot light reflected off the ceiling.
Back at the computer I opened the raw file in RawTherapee and exported a scene-referred rendition using the in-camera white balance - no use of the CIECAM02 module at this point - didn’t even cross my mind.
Then I opened the exported image with GIMP, did some perspective correction and such, and brought the actual painting back downstairs to compare the colors in the painting with the colors on the screen.
The colors on the screen didn’t look anything at all like the colors in the painting. Which upon reflection made perfect sense - I had already noticed that moving the actual painting from halogen light to window light was enough to push the greens very noticeably from warmer greens to cooler, bluer greens. I also had noticed while working on the painting that making the halogen lighting on the painting brighter made the painting itself look less saturated.
“Colors change when the lighting changes” being a major reason we have color appearance models , using RawTherapee’s wonderful CIECAM02 module seemed like just the thing to do to make the camera colors match the painting colors.
So I exported the perspective-corrected image from GIMP, opened it with RawTherapee, and put the actual painting back under the halogen lighting. Then I used RawTherapee’s CIECAM02 module to make the colors on the screen match the colors in the oil pastel, using the "Free temp+green + CAT02 + [output] option and modifying the Temperature, Lightness (J) and Saturation (S) sliders, plus setting all sliders for luminance to 18.
This isn’t saying I actually know what I’m doing when using RT’s CIECAM02 module! other than noticing that there were saturation and hue changes when viewing the actual painting colors under different lighting conditions, and so these would be the parameters in the RT CIECAM02 module to try adjusting.
For some reason I couldn’t get the cyan-blue colors in the upper right corner to match as closely as I’d have preferred. But the rest of the image is a pretty good match to the colors in the actual painting. Here’s the “before” and “after” versions of the photograph of the painting:
If anyone is curious, a larger version of the mixing exercise currently is at the top of this page (contents of the page change over time): https://ninedegreesbelow.com/galleries/pictures-in-progress.html
As a side note, originally I was trying to match colors while comparing colors with different “surround” colors:
- dark surround from RawTherapee’s default dark theme
- white surround around the actual painting (leaning it against white watercolor paper)
- image exported from RawTherapee after the CIECAM02 adjustment, and viewed in GIMP using a middle gray theme
But I couldn’t actually get the colors in the painting to match the colors as seen in RawTherapee or as reopened in GIMP until I selected RT’s gray theme and also put a more or less middle gray “surround” behind the actual painting.