Color-adjusted Greyscale Image filter

This filter allows you to convert an image to greyscale in a color-aware fashion, preserving the contrast between different colors. To apply the filter to your own images, click here.

The algorithm used is a simplification of the one described in this paper. The basic idea is to try and preserve the visual difference between as many pairs of colors in the image as possible during the greyscale conversion. So if you have two colors, for example a bright red and a bright green, that both end up looking the same shade of grey, one or both of them are darkened or lightened so that you can tell them apart. For the color image, the difference between two colors is measured as the distance between them in CIELab colorspace, as a fraction of the maximum difference between any two colors in the image. For the greyscale image, the difference is measured as the difference in brightness between the two shades of grey, as a fraction of the total range from black to white. reHue.net implements the algorithm like this:


The results are somewhat similar to what can be achieved using the Black and White Adjustment Layer in Photoshop CS3, but it is a completely automatic process. The downside is that the results can sometimes be aesthetically unappealing compared to a hand-tuned solution.

A related filter is the Brightness-adjusted Daltonized Image filter, which adjusts the brightness of different colors within an image according to this algorithm, but does not convert the image to greyscale.

In the images below, compare the difference in contrast between the greens and reds in the gamma-corrected greyscale image with those in the color-adjusted greyscale image:

Original Image:



Gamma-corrected Greyscale Image filter applied:



Color-adjusted Greyscale Image filter applied:



References:

Detail Preserving Reproduction of Color Images for Monochromats and Dichromats Karl Rasche, Robert Geist, and James Westall, 2005.


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