{"id":65,"date":"2006-06-19T16:53:04","date_gmt":"2006-06-19T20:53:04","guid":{"rendered":"https:\/\/blogs.mathworks.com\/steve\/?p=65"},"modified":"2019-10-22T12:30:29","modified_gmt":"2019-10-22T16:30:29","slug":"hue-shifts-near-the-l0-axis","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/steve\/2006\/06\/19\/hue-shifts-near-the-l0-axis\/","title":{"rendered":"Hue shifts near the L*=0 axis"},"content":{"rendered":"
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At the European Colour in Graphics, Imaging, and Vision conference today, I took a tutorial called \"Transforms for Colour and Spectral Reproduction.\" Mitch Rosen of the Munsell Color Science\r\n Laboratory, Rochester Institute of Technology, taught the class.\r\n <\/p>\r\n

Mitch's topics included how to solve inverse problems in order to construct multidimensional lookup tables that transform\r\n colors from one space to another. One comment caught my ear - small approximation errors in these lookup tables can be relatively\r\n worse for colors that are neutral (gray) or near-neutral.\r\n <\/p>\r\n

Let's explore this idea using the L*a*b* color space. L* is lightness, while a* and b* represent red-green and yellow-blue\r\n color differences, respectively. Colors for which a* and b* equal zero are neutral, or gray.\r\n <\/p>\r\n

First let's look at a pair of reds that have the same luminance, and are separated by a distance of 20 in the a*-b* plane.<\/p>

L = 85;\r\na = 70;\r\nb = 0;\r\n\r\nlab1 = [L a b];\r\nlab2 = [L a+20 b];<\/pre>
Now convert these colors to sRGB space and display each as a single-pixel image.<\/p>
cform = makecform('lab2srgb'<\/span>);\r\nrgb1 = applycform(lab1, cform);\r\nrgb2 = applycform(lab2, cform);\r\n\r\nsubplot(1,2,1)\r\nimshow(reshape(rgb1,1,1,3))\r\ntitle('L*a*b* = [85 70 0]'<\/span>)\r\n\r\nsubplot(1,2,2)\r\nimshow(reshape(rgb2,1,1,3))\r\ntitle('L*a*b* = [85 90 0]'<\/span>)\r\n\r\nset(gcf, 'Color'<\/span>, 'w'<\/span>)<\/pre> 
These colors are visibly different, but they have the same basic hue.<\/p>\r\n   
Now let's try the same thing with colors that are the same distance apart, but which are located close to the neutral axis.<\/p>
lab3 = [85 10 0];\r\nlab4 = [85 -10 0];\r\n\r\nrgb3 = applycform(lab3, cform);\r\nrgb4 = applycform(lab4, cform);\r\n\r\nsubplot(1,2,1)\r\nimshow(reshape(rgb3,1,1,3))\r\ntitle('L*a*b* = [85 10 0]'<\/span>)\r\n\r\nsubplot(1,2,2)\r\nimshow(reshape(rgb4,1,1,3))\r\ntitle('L*a*b* = [85 -10 0]'<\/span>)\r\n\r\nset(gcf, 'Color'<\/span>, 'w'<\/span>)<\/pre> 
These colors have a distinctively different hue - one is pink and the other is green.  The moral of the story is that \"small\"\r\n      changes in a*-b* are more likely to produce dramatic hue shifts for colors close to the L*=0 axis.\r\n   <\/p>\r\n   
Conference quote of the day: \"In color science, if you want a real controversy, start a terminology discussion.\" - Jack Holm, Hewlett-Packard<\/i><\/p>\r\n