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The application of perceptual weighting factors to RGB channels in color calculations, accounting for the fact that human vision perceives green as much brighter than blue.
Not all RGB channels contribute equally to perceived brightness. The human eye has far more green-sensitive cone cells than red or blue, meaning that changes in the green channel have a much larger impact on how bright or dark a color appears. Luminance weighting applies correction factors to account for this asymmetry.
The standard luminance coefficients for sRGB (from the HDTV specification) are:
Green contributes over 71% of perceived brightness, red about 21%, and blue just 7%. This is why pure blue (#0000FF) appears much darker than pure green (#00FF00) despite both having a single channel at maximum.
A common improvement over naive Euclidean RGB distance is to weight the channels by their luminance contribution. Compuphase's "redmean" approximation is a popular cheap approach:
function weightedColorDistance(
r1: number, g1: number, b1: number,
r2: number, g2: number, b2: number,
): number {
const rMean = (r1 + r2) / 2
const dR = r1 - r2
const dG = g1 - g2
const dB = b1 - b2
return Math.sqrt(
(2 + rMean / 256) * dR * dR +
4 * dG * dG +
(2 + (255 - rMean) / 256) * dB * dB
)
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