Created
February 21, 2014 06:05
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RGB to HSI and HSI to RGB color space conversion
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| inline void HSI2RGB(double h, double s, double i, double* r, double* g, double* b) | |
| { | |
| double x = i * (1 - s); | |
| if(h < 2 * M_PI / 3) | |
| { | |
| double y = i * (1 + (s * cos(h)) / (cos(M_PI / 3 - h))); | |
| double z = 3 * i - (x + y); | |
| *b = x; *r = y; *g = z; | |
| } | |
| else if(h < 4 * M_PI / 3) | |
| { | |
| double y = i * (1 + (s * cos(h - 2 * M_PI / 3)) / (cos(M_PI / 3 - (h - 2 * M_PI / 3)))); | |
| double z = 3 * i - (x + y); | |
| *r = x; *g = y; *b = z; | |
| } | |
| else | |
| { | |
| double y = i * (1 + (s * cos(h - 4 * M_PI / 3)) / (cos(M_PI / 3 - (h - 4 * M_PI / 3)))); | |
| double z = 3 * i - (x + y); | |
| *r = z; *g = x; *b = y; | |
| } | |
| } | |
| inline void RGB2HSI(double r, double g, double b, double* h, double* s, double* i) | |
| { | |
| *i = (r + g + b) / 3.0; | |
| double rn = r / (r + g + b); | |
| double gn = g / (r + g + b); | |
| double bn = b / (r + g + b); | |
| *h = acos((0.5 * ((rn - gn) + (rn - bn))) / (sqrt((rn - gn) * (rn - gn) + (rn - bn) * (gn - bn)))); | |
| if(b > g) | |
| { | |
| *h = 2 * M_PI - *h; | |
| } | |
| *s = 1 - 3 * min(rn, min(gn, bn)); | |
| } |
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HSI to RGB conversion works incorrectly for black, white and grayscale. For example, black color is
(1, 1, 1). In HSI is(-nan, 0, 1). Inverse conversion HSI to RGB gives(-nan, 0, 1). Need to check h value on NaN.https://gist.github.com/KernelA/b462a49200c26028344783edaa639b2d