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Added C syntax highlighting

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edited Jun 12, 2020 at 14:34

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// reverses the rgb gamma
#define inverseGamma(t) (((t) <= 0.0404482362771076) ? ((t)/12.92) : pow(((t) + 0.055)/1.055, 2.4))

//CIE L*a*b* f function (used to convert XYZ to L*a*b*)  http://en.wikipedia.org/wiki/Lab_color_space
#define LABF(t) ((t >= 8.85645167903563082e-3) ? powf(t,0.333333333333333) : (841.0/108.0)*(t) + (4.0/29.0))


float
rgbToCIEL(PIXEL p)
{
   float y;
   float r=p.r/255.0;
   float g=p.g/255.0;
   float b=p.b/255.0;
   
   r=inverseGamma(r);
   g=inverseGamma(g);
   b=inverseGamma(b);
   
   //Observer = 2°, Illuminant = D65 
   y = 0.2125862307855955516*r + 0.7151703037034108499*g + 0.07220049864333622685*b;
   
   // At this point we've done RGBtoXYZ now do XYZ to Lab
   
   // y /= WHITEPOINT_Y; The white point for y in D65 is 1.0
    
    y = LABF(y);
    
   /* This is the "normal conversion which produces values scaled to 100
    Lab.L = 116.0*y - 16.0;
   */
   return(1.16*y - 0.16); // return values for 0.0 >=L <=1.0
}

// reverses the rgb gamma
#define inverseGamma(t) (((t) <= 0.0404482362771076) ? ((t)/12.92) : pow(((t) + 0.055)/1.055, 2.4))

//CIE L*a*b* f function (used to convert XYZ to L*a*b*)  http://en.wikipedia.org/wiki/Lab_color_space
#define LABF(t) ((t >= 8.85645167903563082e-3) ? powf(t,0.333333333333333) : (841.0/108.0)*(t) + (4.0/29.0))


float
rgbToCIEL(PIXEL p)
{
   float y;
   float r=p.r/255.0;
   float g=p.g/255.0;
   float b=p.b/255.0;
   
   r=inverseGamma(r);
   g=inverseGamma(g);
   b=inverseGamma(b);
   
   //Observer = 2°, Illuminant = D65 
   y = 0.2125862307855955516*r + 0.7151703037034108499*g + 0.07220049864333622685*b;
   
   // At this point we've done RGBtoXYZ now do XYZ to Lab
   
   // y /= WHITEPOINT_Y; The white point for y in D65 is 1.0
    
    y = LABF(y);
    
   /* This is the "normal conversion which produces values scaled to 100
    Lab.L = 116.0*y - 16.0;
   */
   return(1.16*y - 0.16); // return values for 0.0 >=L <=1.0
}

// reverses the rgb gamma
#define inverseGamma(t) (((t) <= 0.0404482362771076) ? ((t)/12.92) : pow(((t) + 0.055)/1.055, 2.4))

//CIE L*a*b* f function (used to convert XYZ to L*a*b*)  http://en.wikipedia.org/wiki/Lab_color_space
#define LABF(t) ((t >= 8.85645167903563082e-3) ? powf(t,0.333333333333333) : (841.0/108.0)*(t) + (4.0/29.0))


float
rgbToCIEL(PIXEL p)
{
   float y;
   float r=p.r/255.0;
   float g=p.g/255.0;
   float b=p.b/255.0;
   
   r=inverseGamma(r);
   g=inverseGamma(g);
   b=inverseGamma(b);
   
   //Observer = 2°, Illuminant = D65 
   y = 0.2125862307855955516*r + 0.7151703037034108499*g + 0.07220049864333622685*b;
   
   // At this point we've done RGBtoXYZ now do XYZ to Lab
   
   // y /= WHITEPOINT_Y; The white point for y in D65 is 1.0
    
    y = LABF(y);
    
   /* This is the "normal conversion which produces values scaled to 100
    Lab.L = 116.0*y - 16.0;
   */
   return(1.16*y - 0.16); // return values for 0.0 >=L <=1.0
}

// reverses the rgb gamma
#define inverseGamma(t) (((t) <= 0.0404482362771076) ? ((t)/12.92) : pow(((t) + 0.055)/1.055, 2.4))

//CIE L*a*b* f function (used to convert XYZ to L*a*b*)  http://en.wikipedia.org/wiki/Lab_color_space
#define LABF(t) ((t >= 8.85645167903563082e-3) ? powf(t,0.333333333333333) : (841.0/108.0)*(t) + (4.0/29.0))


float
rgbToCIEL(PIXEL p)
{
   float y;
   float r=p.r/255.0;
   float g=p.g/255.0;
   float b=p.b/255.0;
   
   r=inverseGamma(r);
   g=inverseGamma(g);
   b=inverseGamma(b);
   
   //Observer = 2°, Illuminant = D65 
   y = 0.2125862307855955516*r + 0.7151703037034108499*g + 0.07220049864333622685*b;
   
   // At this point we've done RGBtoXYZ now do XYZ to Lab
   
   // y /= WHITEPOINT_Y; The white point for y in D65 is 1.0
    
    y = LABF(y);
    
   /* This is the "normal conversion which produces values scaled to 100
    Lab.L = 116.0*y - 16.0;
   */
   return(1.16*y - 0.16); // return values for 0.0 >=L <=1.0
}

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answered Jun 3, 2016 at 21:53

EddingtonsMonkey

Here's a bit of C code that should properly calculate perceived luminance.

// reverses the rgb gamma
#define inverseGamma(t) (((t) <= 0.0404482362771076) ? ((t)/12.92) : pow(((t) + 0.055)/1.055, 2.4))

//CIE L*a*b* f function (used to convert XYZ to L*a*b*)  http://en.wikipedia.org/wiki/Lab_color_space
#define LABF(t) ((t >= 8.85645167903563082e-3) ? powf(t,0.333333333333333) : (841.0/108.0)*(t) + (4.0/29.0))


float
rgbToCIEL(PIXEL p)
{
   float y;
   float r=p.r/255.0;
   float g=p.g/255.0;
   float b=p.b/255.0;
   
   r=inverseGamma(r);
   g=inverseGamma(g);
   b=inverseGamma(b);
   
   //Observer = 2°, Illuminant = D65 
   y = 0.2125862307855955516*r + 0.7151703037034108499*g + 0.07220049864333622685*b;
   
   // At this point we've done RGBtoXYZ now do XYZ to Lab
   
   // y /= WHITEPOINT_Y; The white point for y in D65 is 1.0
    
    y = LABF(y);
    
   /* This is the "normal conversion which produces values scaled to 100
    Lab.L = 116.0*y - 16.0;
   */
   return(1.16*y - 0.16); // return values for 0.0 >=L <=1.0
}

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