opencv - How to calculate the distance between ofColors in openframeworks

Question

What I was thinking to do is to convert ofColor to l*a*b color space and measure the euclidean distance. But I don't know how should I do it in openframeworks?

Answer 1

I'm not very experienced with c++ but I ported this snippet over:

//ported from http://cookbooks.adobe.com/post_Useful_color_equations__RGB_to_LAB_converter-14227.html
struct Color{
    float R,G,B,X,Y,Z,L,a,b;
};

#define REF_X 95.047; // Observer= 2°, Illuminant= D65
#define REF_Y 100.000;
#define REF_Z 108.883;

Color rgb2xyz(int R,int G,int B){
    float r = R / 255.0;
    float g = G / 255.0;
    float b = B / 255.0;

    if (r > 0.04045){ r = pow((r + 0.055) / 1.055, 2.4); }
    else { r = r / 12.92; }
    if ( g > 0.04045){ g = pow((g + 0.055) / 1.055, 2.4); }
    else { g = g / 12.92; }
    if (b > 0.04045){ b = pow((b + 0.055) / 1.055, 2.4); }
    else {  b = b / 12.92; }

    r = r * 100;
    g = g * 100;
    b = b * 100;
    //Observer. = 2°, Illuminant = D65
    Color xyz;
    xyz.X = r * 0.4124 + g * 0.3576 + b * 0.1805;
    xyz.Y = r * 0.2126 + g * 0.7152 + b * 0.0722;
    xyz.Z = r * 0.0193 + g * 0.1192 + b * 0.9505;
    return xyz;
}
Color xyz2lab(float X,float Y, float Z){
    float x = X / REF_X;
    float y = Y / REF_X;
    float z = Z / REF_X;

    if ( x > 0.008856 ) { x = pow( x , .3333333333f ); }
    else { x = ( 7.787 * x ) + ( 16/116.0 ); }
    if ( y > 0.008856 ) { y = pow( y , .3333333333f ); }
    else { y = ( 7.787 * y ) + ( 16/116.0 ); }
    if ( z > 0.008856 ) { z = pow( z , .3333333333f ); }
    else { z = ( 7.787 * z ) + ( 16/116.0 ); }

    Color lab;
    lab.L = ( 116 * y ) - 16;
    lab.a = 500 * ( x - y );
    lab.b = 200 * ( y - z );
    return lab;
}
Color lab2xyz(float l, float a, float b){
    float y = (l + 16) / 116;
    float x = a / 500 + y;
    float z = y - b / 200;

    if ( pow( y , 3 ) > 0.008856 ) { y = pow( y , 3 ); }
    else { y = ( y - 16 / 116 ) / 7.787; }
    if ( pow( x , 3 ) > 0.008856 ) { x = pow( x , 3 ); }
    else { x = ( x - 16 / 116 ) / 7.787; }
    if ( pow( z , 3 ) > 0.008856 ) { z = pow( z , 3 ); }
    else { z = ( z - 16 / 116 ) / 7.787; }

    Color xyz;
    xyz.X = x * REF_X;
    xyz.Y = y * REF_Y;
    xyz.Z = z * REF_Z;
    return xyz;
}
Color xyz2rgb(float X,float Y,float Z){
    //X from 0 to  95.047      (Observer = 2°, Illuminant = D65)
    //Y from 0 to 100.000
    //Z from 0 to 108.883
    X = ofClamp(X, 0, 95.047);

    float x = X * .01;
    float y = Y * .01;
    float z = Z * .01;

    float r = x * 3.2406 + y * -1.5372 + z * -0.4986;
    float g = x * -0.9689 + y * 1.8758 + z * 0.0415;
    float b = x * 0.0557 + y * -0.2040 + z * 1.0570;

    if ( r > 0.0031308 ) { r = 1.055 * pow( r , ( 1 / 2.4f ) ) - 0.055; }
    else { r = 12.92 * r; }
    if ( g > 0.0031308 ) { g = 1.055 * pow( g , ( 1 / 2.4f ) ) - 0.055; }
    else { g = 12.92 * g; }
    if ( b > 0.0031308 ) { b = 1.055 * pow( b , ( 1 / 2.4f ) ) - 0.055; }
    else { b = 12.92 * b; }

    Color rgb;
    rgb.R = round( r * 255 );
    rgb.G = round( g * 255 );
    rgb.B = round( b * 255 );
    return rgb;
}
Color rgb2lab(int R,int G,int B){
    Color xyz = rgb2xyz(R, G, B);
    return xyz2lab(xyz.X, xyz.Y, xyz.Z);
}
Color lab2rgb(int L,int a,int b){
    Color xyz = lab2xyz(L, a, b);
    return xyz2rgb(xyz.X, xyz.Y, xyz.Z);
}

Measuring the distance would be something as trivial as:

float distLab(Color c1,Color c2){
    float dL = c1.L - c2.L;
    float da = c1.a - c2.a;
    float db = c1.b - c2.b;
    return sqrt(dL*dL + da*da + db*db);
}

or ofVec3f(c1.L,c1.a,c1.b).distance(ofVec3f(c2.L,c2.a,c2.b));

Also see this answer for an openframeworks basic example.