Visual Acuity and Color Confusion

I’ve been working on a project at work that involves visual acuity. We’re basically asking the question, “How easy is it to notice the difference between two parts of an image with the assistance of color?” As part of the process, we take two grayscale images and make a false-color composite from them. Similar regions of the image are gray (which includes black and white), while the differences show up as one hue or another.

There is an almost infinite number of colors we could use for the false coloring by spinning the color wheel, but it’s easiest to pick a primary color (red, green, or blue) and use its complementary color for the opposite. Some colors are harder to distinguish against bright backgrounds; think about picking out a bright yellow object against a bright white background. While other colors are harder to pick out against dark backgrounds; blue on black, for example. Is there an optimal choice that’s also inexpensive to compute?

Let’s find out. Here’s what happens when you perform the “What’s different?” test using the three obvious choices of red/cyan, green/magenta, and blue/yellow:

(Click any image to see it larger and/or full-size.)

The blue/yellow version is not very good because it suffers from the problems listed above, while the green/magenta and red/cyan false colorings seem to produce fairly equivalent results. What to do? The ever-practical Loren suggested that we see what happens if we ask people with color-blindness color confusion. Turns out, we know a couple of such folks, but they were in meetings. What to do?

Oh, that’s right!! A while back, I wrote some MATLAB code that simulates the two most common forms of anomalous color vision. Let’s run that and see what happens. (The speckling in the images is an artifact of the conversion and doesn’t represent how they actually look to people with protanopia or deuteranopia.)

The results are a no-brainer! (Sorry about the pun.) While the red/cyan and green/magenta images are very similar for the majority of us with normal color vision, the red/cyan images become very difficult to use for people with any kind of anomalous color vision. Even though the green/magenta starts to look a lot like the blue/yellow row, which has its own issues of color discrimination, it makes a very good compromise. Green/magenta it is.

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