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greyscale versus greyfail

September 7, 2009 Leave a comment

If you’ve ever wondered why desaturate gives disappointing greyscale results in Photoshop or the Gimp, here’s a (slightly) scientific demo of why.

The diagram below is the sRGB gamut, viewed from above. In reality the gamut a twisted blob in 3d space, but this is a ‘plan’ – you’re looking down on it, so you’re seeing every possible RGB hue at its highest luminosity level. You’re seeing the bright sunlit version, rather than the shadowy underbelly. Darker versions of the same hue are hidden behind the pixels you see. (This was generated with a C# program I wrote a while back.)

rgb-gamut

This triangle is called the Maxwell Triangle; the primary colours (R, G and B) are the vertices of the triangle, and every hue is a weighted average of those three primaries in different proportions. The secondary colours (Cyan, Magenta, Yellow) are mixtures of two of the primaries, and appear on the edges of the triangle.

Draw an imaginary line between each primary and the opposite secondary; where the 3 lines cross, you have the White Point – this is the axis (disappearing into your screen) of neutral tones between white and black.

Look what happens if you apply Grayscale to it

rgb-gamut-after-greyscale

Notice how Blue is darkest, Red is dark, and that White and Yellow are close together in brightness.

This is good; the greyscale algorithm takes into account human sensitivity to colour, and the influence of colour on tone. Pure yellow is lighter than pure Blue, as it is in real life.

Contrast this with the effect of applying Desaturate on the same gamut image.

rgb-gamut-after-desaturatio

Notice how white maps to white; as you’d expect. Now, the fully saturated hues (those lying on the boundary of the triangle) are all mid-grey.

Yellow is now the same tone as pure blue.

Greyscale takes into account the true tonal values of colours.

Desaturate is a simple average of the tonal values of each channel.

Desaturate? Don’t bother.

You can get a CC-NC-BY version of the gamut image at higher resolution on my Flickr stream here.

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