RGB2HSI__matlab

function hsi = rgb2hsi(rgb)

%RGB2HSI Converts an RGB image to HSI.

%   HSI = RGB2HSI(RGB) converts an RGB image to HSI. The input image

%   is assumed to be of size M-by-N-by-3, where the third dimension

%   accounts for three image planes: red, green, and blue, in that

%   order. If all RGB component images are equal, the HSI conversion

%   is undefined. The input image can be of class double (with values

%   in the range [0, 1]), uint8, or uint16. 

%

%   The output image, HSI, is of class double, where:

%     hsi(:, :, 1) = hue image normalized to the range [0, 1] by

%                    dividing all angle values by 2*pi. 

%     hsi(:, :, 2) = saturation image, in the range [0, 1].

%     hsi(:, :, 3) = intensity image, in the range [0, 1].

%   Copyright 2002-2004 R. C. Gonzalez, R. E. Woods, & S. L. Eddins

%   Digital Image Processing Using MATLAB, Prentice-Hall, 2004

%   $Revision: 1.4 $  $Date: 2003/09/29 15:21:54 $

% Extract the individual component immages.

rgb = im2double(rgb);

r = rgb(:, :, 1);

g = rgb(:, :, 2);

b = rgb(:, :, 3);

% Implement the conversion equations.

num = 0.5*((r - g) + (r - b));

den = sqrt((r - g).^2 + (r - b).*(g - b));

theta = acos(num./(den + eps));

H = theta;

H(b > g) = 2*pi - H(b > g);

H = H/(2*pi);

num = min(min(r, g), b);

den = r + g + b;

den(den == 0) = eps;

S = 1 - 3.* num./den;

H(S == 0) = 0;

I = (r + g + b)/3;

% Combine all three results into an hsi image.

hsi = cat(3, H, S, I);