RGB空间转为HSI空间

理论部分转自 https://blog.csdn.net/sinat_34953360/article/details/64445041

最近在做的眼底项目经常出现这个空间变换，就结合一些网站的和自己实践看看。

HSI颜色空间是从人的视觉系统出发，用色调（Hue）、色饱和度（Saturation或Chroma）和亮度（Intensity或Brightness）来描述色彩。HSI颜色空间可以用一个圆锥空间模型来描述。用这种描述HIS色彩空间的圆锥模型相当复杂，但确能把色调、亮度和色饱和度的变化情形表现得很清楚，如下图所示，半径表示饱和度，角度表示色调，高低表示亮度。
RGB 向HSI 模型的转换是由一个基于笛卡尔直角坐标系的单位立方体向基于圆柱极坐标的双锥体的转换。

RGB<->HSI

function hsi = rgb2hsi(rgb) 
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);

2.HSI 转 RGB
function rgb = hsi2rgb(hsi) 
% 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)); 
rgb = cat(3, R, G, B); 
rgb = max(min(rgb, 1), 0);

我本来以为在H通道血管会明显不一样的，结果H通道中，血管和视盘是一类（基本都是最黑的），渗出是另一类（偏白），背景是一类（第二黑= =）；S通道中，渗出和视盘是一类（偏白），血管和黄斑是一类（基本为黑的），背景是一类；I通道和灰度图基本一样。

RGB空间转为HSI空间

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