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# Building Pseudo Color Tables

[Laboratory]

In this lesson, we will see how one can build a pseudo-color table to be applied in an image.

The image selected for this experiment is a MRI of a human head. The image original pixels range from 1024 to 1262. One could normalize the image to fit in a byte pixel, but the disadvantage would be that the original pixels values would be lost.

Shown below is the original grey-level image and the result of the application of a pseudo-color table. The purpose of this experiment is to demonstrate how to build a pseudo-color table suitable for this image. Note that the gray-level image has been displayed with the histogram stretch technique.

a)original; b)pseudo-colored
a) | b)

First, we can use sinusoid to create separate Red, Green and Blue columns and put together in a table. The size of the table is given by the difference between the highest and lowest pixel values in the image. In this case, the range is 238, i.e. 1262 minus 1024. We selected the phases of the sinusoids in such a way that the blue shades appear on the beginning and the red shades appear at the end of the table. The table columns are plotted in the graphic below.

color table plot

In order to apply the table to the image, it is necessary to shift the table so that the indexes of the table would match the original pixel values. To accomplish this, we add 1024 black color entries at the beginning of the table. This table is displayed below.

color table plot

Finally, the color table is applied to the image, given the result shown below. At right of the pseudo-colored image, it is shown the color-table represented in the color wheel. In the color wheel the color indexes increases in the anti-clockwise direction.

a)pseudo-colored; b)color table visualization
a) | b)

- Lab: Building Pseudo Color Tables
- Other Examples