Placeholder for thoughts about Rubik’s Cube colourspace reduction for an art project.

= Basics =

[[Image(cube.png,align=right)]]

I use a very simple palette: #fff #f00 !#0f0 !#00f #f80 #ff0

[[Image(rubik.png)]]

I have seen #FFFFFF, !#000E98, #FF0000, !#25C200, #F6FF00 #FF9C00 being used, too.

What’s difficult in this specific colour reduction exercise is that the convex hull of all available colours does not fill the entire RGB cube. The most important missing colour is black, but also cyan and magenta are missing. And yet we have orange, which is totally useless since we already have red and yellow: what did the Rubik’s Cube creators have in mind? Do they despise art so much?

= Tests with existing software =

Original image (left) and gamma-corrected image (linear intensity, will output correctly if a gamma = 2.2 correction is applied):

[[Image(lena.png)]] [[Image(lena-g.png)]]

== libpipi ==

[[Image(lena-pipi.png)]] [[Image(lena-gpipi.png)]]

== Photoshop CS2 ==

Results using Photoshop’s Floyd-Steinberg dithering:

[[Image(lena-cs2fs.png)]] [[Image(lena-gcs2fs.png)]]

Photoshop’s Floyd-Steinberg with 75% error distribution:

[[Image(lena-cs2fs2.png)]] [[Image(lena-gcs2fs2.png)]]

== !ImageMagick ==

This is !ImageMagick’s Floyd-Steinberg algorithm. However, the results are so blurry that I suspect it to actually be their ridiculous Hilbert curve dithering implementation.

[[Image(lena-im.png)]] [[Image(lena-gim.png)]]

== The Gimp ==

The Gimp fails pathetically. Results using its Floyd-Steinberg dithering algorithm:

[[Image(lena-gimpfs.png)]] [[Image(lena-gimpgfs.png)]]

Gimp’s Floyd-Steinberg with "reduced color bleeding" (probably a F-S variant that propagates less than 100% of the error):

[[Image(lena-gimpfs2.png)]] [[Image(lena-gimpgfs2.png)]]

== Links ==

 * [http://www.squidoo.com/rubikcubism RubikCubism]
 * [http://diwww.epfl.ch/w3lsp/publications/colour/cgrtfpwci.html Color Gamut Reduction Techniques for Printing with Custom Inks]
 * [http://diwww.epfl.ch/w3lsp/publications/colour/vcstfpwci.html Visually-based color space tetrahedrizations for printing with custom inks]