hsi2rgb.m

Image Processing Toolbox

function rgb = hsi2rgb(hsi)
%HSI2RGB Converts an HSI image to RGB.
%   RGB = HSI2RGB(HSI) converts an HSI image to RGB, where HSI is
%   assumed to be of class double with:  
%     hsi(:, :, 1) = hue image, assumed to be in the range
%                    [0, 1] by having been divided by 2*pi.
%     hsi(:, :, 2) = saturation image, in the range [0, 1].
%     hsi(:, :, 3) = intensity image, in the range [0, 1].
%
%   The components of the output image are:
%     rgb(:, :, 1) = red.
%     rgb(:, :, 2) = green.
%     rgb(:, :, 3) = blue.

%   Copyright 2002-2004 R. C. Gonzalez, R. E. Woods, & S. L. Eddins
%   Digital Image Processing Using MATLAB, Prentice-Hall, 2004
%   $Revision: 1.5 $  $Date: 2003/10/13 01:01:06 $

% Extract the individual HSI component images.
H = hsi(:, :, 1) * 2 * pi;
S = hsi(:, :, 2);
I = hsi(:, :, 3);

% Implement the conversion equations.
R = zeros(size(hsi, 1), size(hsi, 2));
G = zeros(size(hsi, 1), size(hsi, 2));
B = zeros(size(hsi, 1), size(hsi, 2));

% RG sector (0 <= H < 2*pi/3).
idx = find( (0 <= H) & (H < 2*pi/3));
B(idx) = I(idx) .* (1 - S(idx));
R(idx) = I(idx) .* (1 + S(idx) .* cos(H(idx)) ./ ...
                                          cos(pi/3 - H(idx)));
G(idx) = 3*I(idx) - (R(idx) + B(idx));

% BG sector (2*pi/3 <= H < 4*pi/3).
idx = find( (2*pi/3 <= H) & (H < 4*pi/3) );
R(idx) = I(idx) .* (1 - S(idx));
G(idx) = I(idx) .* (1 + S(idx) .* cos(H(idx) - 2*pi/3) ./ ...
                    cos(pi - H(idx)));
B(idx) = 3*I(idx) - (R(idx) + G(idx));

% BR sector.
idx = find( (4*pi/3 <= H) & (H <= 2*pi));
G(idx) = I(idx) .* (1 - S(idx));
B(idx) = I(idx) .* (1 + S(idx) .* cos(H(idx) - 4*pi/3) ./ ...
                                           cos(5*pi/3 - H(idx)));
R(idx) = 3*I(idx) - (G(idx) + B(idx));

% Combine all three results into an RGB image.  Clip to [0, 1] to
% compensate for floating-point arithmetic rounding effects.
rgb = cat(3, R, G, B);
rgb = max(min(rgb, 1), 0);