Colorblindness and Image Processing*

Photoreceptors

• Eyes have two kinds of photoreceptors, rods and cones
• Cones are what we care about! (We can ignore rods for this topic)
  • Cones are sensitive to different wavelengths of light that differentiate color

Cones

• Three major types of cones:
  • Cones sensitive to short wavelengths (blue)
  • Cones sensitive to medium wavelengths (green)
  • Cones sensitive to long wavelengths (red)

Colorblindness

• Color-impaired vision is the result of one type of cone malfunctioning
  • Causes can be either genetic or just physical damage sustained in the course of someone's life
• The technical terms are based on which kind of cone is malfunctioning:
  • protanopia for Long/Red
  • deuteranoipa for Medium/Green
  • tritanopia for Short/Blue
    • terms are for dichromacy, two working sets and one malfunctioning set

What Colorblind People Perceieve

• Protanopia and deuteranopia both get called "red-green" colorblindness
  • Wavelengths close
  • Though deuteranopia is the most common, it's the male pattern genetic colorblindness
• Tritanopia is "blue-yellow" colorblindness

Testing Vision

• Early tests made by Dr. Shinobu Ishihara, known as Ishihara Plates
  • Dots clustered together, forming numbers or letters or shapes with color contrast
• Top plate is a "control" plate visible to even people with colorblindness
  • Everyone should see "12"
• Bottom plate indistinguishable to people with at least deuteranopia, possibly also protanopia
  • ... can someone tell me?

Color Theories and Spaces

• Trichromatic Theory covers physical view
  • Color composed of three components
    • standard RGB space
    • LMS (description of wavelengths) space
    • CIE XYZ space
• Opponent Theory covers perception
  • Opposing colors on opposite axes
    • Light/intensity axis
    • Red/Green axis
    • Blue/Yellow axis
      • CIE Lab space

Image Processing

Okay that's cool and all, but we're smart, what can we actually do?

• In image processing, or anything that's a collection of pixels, we can add filters:
  • Simulate colorblind vision
  • Adjust images to 'fix' colorblind vision
  • Both at once!

Math!

• I find it easiest to think about color geometrically
  • Web colors like #FFFFFF to describe white or #FF0000 to describe red are examples of a color space called standard RGB, or sRGB
  • Think of it like a 3-dimensional space

More Color Spaces

• Opponent theory
  • CIE Lab space
  • Visualization of colorblindness

Color Space Transforms

Got a good handle on the concept of color spaces?

• Transformations exist to convert from one color space to another
  • That probably sounds more complicated than it is
  • To convert from sRGB space to LMS space (or between spaces in general), just multiply
    • Getting what you need to multiply by is the trickier part, but in practice, it really is just multiplying

Simulating Colorblindness

http://vision.psychol.cam.ac.uk/jdmollon/papers/Dichromatsimulation.pdf

• Above paper explains math on simulating colorblind vision
• Basic gist:
  • ​Given a pixel in sRGB,
  • Transform into LMS space
  • Cancel out a cone type's contribution based on desired simulation
    • Protanopia: Cancel out L
    • Deuteranopia: Cancel out M
    • Tritanopia: Cancel out S
  • Transform back into sRGB space for final output pixel

Simulated

Correcting for Colorblind Vision

We can go a step further!

• Colorblind people have a smaller range of color
• We can stretch out that range
• For an input pixel,
  • Simulate colorblindness, get simulated pixel in sRGB (as prior)
  • Subtract: original pixel - simulated pixel = offset
  • Multiply offset by error correction matrix*, multiply by input pixel of choice (original or simulated), get new corrected output pixel

Simulated + Corrected