I am doing a bit of research for a small project I intend to make and that included determining the brightness of a pixel. First idea that came to my mind was to just compute brightness = (R + G + B)/3 but then I stumbled upon this answer which had very different and specific equations. Why wouldn't averaging RGB values work?
Why doesn't computing the average of RGB produce brightness of a pixel?
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1 Answers
Our eyes are more sensitive to green light, so 255 in the green channel leads to a larger perception of brightness than 255 in the red or blue channel.
The International Commission on Illumination (CIE) created two standard color spaces in 1931 (CIE 1931 XYZ and CIE 1931 RGB color space), derived from perceptual experiments performed during the 1920's. These experiments and color spaces are the basis of all colorimetry today*.
In the CIE 1931 RGB color space, the luminance equation Y, is defined as:
Y = ( 0.17697 R + 0.81240 G + 0.01063 B ) / 0.17697
However, the definition of Y from RGB depends on the primaries used for R, G and B (e.g. the three pure colors emitted by an LCD monitor) and the color of the white produced by their addition. For example, for a standard D65 white point and the primaries from Rec. 709 and sRGB, the conversion from RGB to Y is as follows:
Y = 0.2126 R + 0.7152 G + 0.0722 B
This equation weights green about ten times more strongly than blue. This matches what is known about color perception, and explains for example why, on a computer screen, green text on a black background is quite OK to read, whereas blue text on the same background is much harder to read.
If you want to learn more about different color spaces, and how to convert between them, you should read Poynton's Color FAQ.
*A lot has been learned about color perception since, and new color spaces such as CIELAB and CIELUV have been published, but CIE XYZ is still the foundation for them.
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