Why can cyan look brighter than red on an RGB monitor?
Asked 4/14/2020
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On an RGB display, cyan is RGB (0,255,255) while red is (255,0,0). Since cyan uses two subpixels at full output and red uses one, does that mean cyan is physically brighter on the screen? I’m confused because HSV/HSL seem to describe both as fully bright or saturated colors, which sounds like they should be equally bright. How do RGB channel values, HSL/HSV, and perceived brightness relate when comparing colors like cyan and red on a monitor?
Originally by user91535. Source · Licensed CC BY-SA 4.0
user91535
6y ago
2 Answers
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The RGB values for cyan are 0,255,255 RGB (8bit), and red is 255,0,0. In RGB, if any of the max values are the same then the brightness is the same; and the other two values determine the hue. E.g. 255,0,0 (red) is the same brightness as 0,255,0 (green) and 0,255,255 (cyan).
HSL/HSB/HSV are also not good for direct comparison as the saturation and brightness values are also relative to the hue and not to each other.
I.e. any value of 255 or 100% means the color is as bright as it can be, but it does not mean the color is lighter/brighter than another color.
If you want to compare them for luminance, it's probably easiest to use the Lab color space; where red has a luminance value of 54% and cyan has a luminance value of 91%. I.e. cyan is a lighter/brighter color.
You will only get the maximum brightness from the monitor when all RGB pixels/sub pixels are at max value; i.e. displaying a white screen. When displaying a cyan screen it is less bright, but brighter than a red or black screen.
And similarly, you cannot assume that any two color pixels will generate the same luminance because they are weighted differently; perceptually, yellow/green is the most important. E.g. red (255,0,0) has an L value of 54, whereas green (0,255,0) has an L value of 88. And yellow (255,255,0) has an L value of 98 compared to an L value of 91 for cyan (0,255,255).
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
6y ago
0
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They’re describing different things.
RGB values tell you channel intensities, not perceived brightness. Cyan (0,255,255) drives green and blue fully; red (255,0,0) drives only red fully. On a real display, that can mean different emitted luminance, but not in a simple “100 nits per subpixel” way.
HSL/HSV also don’t represent perceptual brightness well. In HSV, both red and cyan can have V=1 because at least one channel is at maximum. In HSL, pure cyan is H=0.5, S=1, L=0.5, not L=1. So those models are not good for comparing how bright two hues appear.
For perceived brightness, use luminance/luma-style models, which weight green much more strongly than blue and red. A common formula is:
Y' = 0.2126R + 0.7152G + 0.0722B
By that kind of weighting, cyan is much brighter than red because it includes full green. So the short answer is: cyan can indeed appear brighter than red, and the apparent contradiction comes from using color models that encode hue/value, not human-perceived brightness.
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