klar-plugin-cvd-brettel

A klar plugin that measures whether two colors stay distinguishable for a person with color blindness — specifically full dichromacy, using the Brettel, Viénot & Mollon (1997) simulation.

It adds three contrast types:

| --type | Deficiency | |---|---| | cvd-deuteranopia | no functioning M (green) cones | | cvd-protanopia | no functioning L (red) cones | | cvd-tritanopia | no functioning S (blue) cones |

What it measures

Each type simulates both input colors as that dichromat would see them, then returns the CIEDE2000 (ΔE₀₀) between the simulated colors:

• high → the two colors stay clearly separated (good)
• low → they collapse together / become confusable (a real accessibility problem)

Higher-is-better, integer-rounded — same polarity and rounding as klar's built-in deltaE, so it composes with the rest of klar.

Install

Install alongside klar (klar discovers it automatically by the klar-plugin-* convention):

npm install -g klar-cli klar-plugin-cvd-brettel

Usage

# a red and a green that look very different to normal vision...
$ klar contrast "#d40000" "#00a000" --type deltaE -q
72
# ...nearly collapse for a deuteranope:
$ klar contrast "#d40000" "#00a000" --type cvd-deuteranopia -q
7
# but stay distinct under tritanopia (which spares the red–green axis):
$ klar contrast "#d40000" "#00a000" --type cvd-tritanopia -q
62

$ klar plugins list   # confirm the three types are registered

A common convention is to treat ΔE ≥ 11 as "still clearly distinct." That threshold is your policy to apply, though — the plugin returns the raw number.

Why Brettel (and why all three types here)

Brettel 1997 is the reference-accuracy model for full dichromacy and the only common method that handles tritanopia correctly (the dichromat gamut is two half-planes in LMS, which a single matrix can't represent). For anomalous trichromacy (partial color blindness — the more common form), see the companion klar-plugin-cvd-machado.

Attribution & license

• Plugin code: MIT.
• Simulation constants vendored from libDaltonLens (public domain), which provides the corrected Brettel/Viénot matrices.
• Brettel, H., Viénot, F., & Mollon, J. D. (1997). Computerized simulation of color appearance for dichromats. JOSA A, 14(10).