Why does RGB2HSV return an incorrect result for double images in MATLAB 7.7(R2008b)?

When I used rgb2hsv on a double image, I expected return numbers of less than or equal to 1.0.

For example, the following code snippet results in [0.0370 0.5385 117.0000] whereas I expected [0.0370 0.5385 0.4588].

Reproduction steps:

   x=[117,68,54]

class(x)

rgb2hsv(x)

Answer 1

[MathWorks Support Team]

This is an expected behavior in MATLAB 7.7 (R2008b) in the way the function RGB2HSV handles double images.

The reason is as follows:

When working with images, MATLAB assumes that if the datatype of the image is double then the range of input values are from 0 to 1. So in the case where the input x is double, one should scale the inital vector [117,68,54] by 255 to get the correct answers.

x=[117,68,54]/255

rgb2hsv(x)

For more information on 8-bit and 16-bit image handling in MATLAB, please refer the following:

 <http://www.mathworks.com/access/helpdesk/help/techdoc/creating_plots/f2-1667.html>