What's wrong with this RGB to XYZ color space conversion algorithm?

Question

My goal is to convert an RGB pixel into CIELab color space for some special calculations only possible in CIELab. For this, I must convert RGB to XYZ first, which is the rea

Answer 1

I just use your code to convert from RGB (to XYZ) to La*b*, and I just found that the XYZ values should go between 0 a 1 before you try to convert them to La*b*

var_R = var_R * 100;
var_G = var_G * 100;
var_B = var_B * 100;

So the previous code must be erased in order to get correct La*b* values.

Answer 2

The right values of this matrix is different slightly,the accurate one from "RGB/XYZ Matrices" in http://www.brucelindbloom.com

sX = sRed * 0.4124564 + sGreen * 0.3575761 + sBlue * 0.1804375
sY = sRed * 0.2126729 + sGreen * 0.7151522 + sBlue * 0.072175
sZ = sRed * 0.0193339 + sGreen * 0.119192 + sBlue * 0.9503041

Answer 3

I believe here is your problem, this is truncating to an integer:

CGFloat var_R = (*inR / 255); //R from 0 to 255
CGFloat var_G = (*inG / 255); //G from 0 to 255
CGFloat var_B = (*inB / 255); //B from 0 to 255

Try this:

CGFloat var_R = (*inR / 255.0f); //R from 0 to 255
CGFloat var_G = (*inG / 255.0f); //G from 0 to 255
CGFloat var_B = (*inB / 255.0f); //B from 0 to 255

I haven't checked the rest of the code for other problems.

Answer 4

As they said:

var_R = ( R / 255 )  ->  var_R = ( R / 255.0 ) or var_R = ( R * 0.003922 ) 
var_G = ( G / 255 )  ->  var_G = ( G / 255.0 ) or var_G = ( G * 0.003922 )
var_B = ( B / 255 )  ->  var_B = ( B / 255.0 ) or var_B = ( B * 0.003922 )

This is because of implicit conversion. Despite the fact that var_R, var_G and var_B variables are float type, the / operator sees two integers R, G, B and 255. it has to divide and returns an integer.

In order to get a float type value you can do a CAST or convert at least one of the variables into float type adding of a decimal point as follows:

var_B = ( B / 255.0f)

Another example of RGB2LAB and LAB2RGB conversion (bear in mind that is CIE Lab D65):

void RGB2LAB(uint8_t R, uint8_t G, uint8_t B, float *l, float *a, float *b) {
    float RGB[3], XYZ[3];

    RGB[0] = R * 0.003922;
    RGB[1] = G * 0.003922;
    RGB[2] = B * 0.003922;

    RGB[0] = (RGB[0] > 0.04045) ? pow(((RGB[0] + 0.055)/1.055), 2.4) : RGB[0] / 12.92;
    RGB[1] = (RGB[1] > 0.04045) ? pow(((RGB[1] + 0.055)/1.055), 2.4) : RGB[1] / 12.92;
    RGB[2] = (RGB[2] > 0.04045) ? pow(((RGB[2] + 0.055)/1.055), 2.4) : RGB[2] / 12.92;

    XYZ[0] = 0.412424  * RGB[0] + 0.357579 * RGB[1] + 0.180464  * RGB[2];
    XYZ[1] = 0.212656  * RGB[0] + 0.715158 * RGB[1] + 0.0721856 * RGB[2];
    XYZ[2] = 0.0193324 * RGB[0] + 0.119193 * RGB[1] + 0.950444  * RGB[2];

    *l = 116 * ( ( XYZ[1] / 1.000000) > 0.008856 ? pow(XYZ[1] / 1.000000, 0.333333) : 7.787 * XYZ[1] / 1.000000 + 0.137931) - 16;
    *a = 500 * ( ((XYZ[0] / 0.950467) > 0.008856 ? pow(XYZ[0] / 0.950467, 0.333333) : 7.787 * XYZ[0] / 0.950467 + 0.137931) - ((XYZ[1] / 1.000000) > 0.008856 ? pow(XYZ[1] / 1.000000, 0.333333) : 7.787 * XYZ[1] / 1.000000 + 0.137931) );
    *b = 200 * ( ((XYZ[1] / 1.000000) > 0.008856 ? pow(XYZ[1] / 1.000000, 0.333333) : 7.787 * XYZ[1] / 1.000000 + 0.137931) - ((XYZ[2] / 1.088969) > 0.008856 ? pow(XYZ[2] / 1.088969, 0.333333) : 7.787 * XYZ[2] / 1.088969 + 0.137931) );
}

void LAB2RGB(float L, float A, float B, uint8_t *r, uint8_t *g, uint8_t *b) {
    float XYZ[3], RGB[3];

    XYZ[1] = (L + 16 ) / 116;
    XYZ[0] = A / 500 + XYZ[1];
    XYZ[2] = XYZ[1] - B / 200;

    XYZ[1] = (XYZ[1]*XYZ[1]*XYZ[1] > 0.008856) ? XYZ[1]*XYZ[1]*XYZ[1] : (XYZ[1] - (16 / 116)) / 7.787;
    XYZ[0] = (XYZ[0]*XYZ[0]*XYZ[0] > 0.008856) ? XYZ[0]*XYZ[0]*XYZ[0] : (XYZ[0] - (16 / 116)) / 7.787;
    XYZ[2] = (XYZ[2]*XYZ[2]*XYZ[2] > 0.008856) ? XYZ[2]*XYZ[2]*XYZ[2] : (XYZ[2] - (16 / 116)) / 7.787;

    RGB[0] = 0.950467 * XYZ[0] *  3.2406 + 1.000000 * XYZ[1] * -1.5372 + 1.088969 * XYZ[2] * -0.4986;
    RGB[1] = 0.950467 * XYZ[0] * -0.9689 + 1.000000 * XYZ[1] *  1.8758 + 1.088969 * XYZ[2] *  0.0415;
    RGB[2] = 0.950467 * XYZ[0] *  0.0557 + 1.000000 * XYZ[1] * -0.2040 + 1.088969 * XYZ[2] *  1.0570;

    *r = (255 * ( (RGB[0] > 0.0031308) ? 1.055 * (pow(RGB[0], (1/2.4)) - 0.055) : RGB[0] * 12.92 ));
    *g = (255 * ( (RGB[1] > 0.0031308) ? 1.055 * (pow(RGB[1], (1/2.4)) - 0.055) : RGB[1] * 12.92 ));
    *b = (255 * ( (RGB[2] > 0.0031308) ? 1.055 * (pow(RGB[2], (1/2.4)) - 0.055) : RGB[2] * 12.92 ));
}

Answer 5

*inR/255 is an integer division. 1/255 is zero. Write *inR/255.0 instead.

Answer 6

#include <stdio.h>
#include <math.h>

float ref_X = 95.047;
float ref_Y = 100.0;
float ref_Z = 108.883;


void convertRGBtoXYZ(int inR, int inG, int inB, float * outX, float * outY, float * outZ) {


    float var_R = (inR / 255.0f); //R from 0 to 255
    float var_G = (inG / 255.0f); //G from 0 to 255
    float var_B = (inB / 255.0f); //B from 0 to 255

    if (var_R > 0.04045f)
        var_R = powf(( (var_R + 0.055f) / 1.055f), 2.4f);
    else 
        var_R = var_R / 12.92f;

    if (var_G > 0.04045)
        var_G = powf(( (var_G + 0.055f) / 1.055f), 2.4f);
    else
        var_G = var_G / 12.92f;

    if (var_B > 0.04045f)
        var_B = powf(( (var_B + 0.055f) / 1.055f), 2.4f);
    else
        var_B = var_B / 12.92f;

    var_R = var_R * 100;
    var_G = var_G * 100;
    var_B = var_B * 100;

    //Observer. = 2°, Illuminant = D65
    *outX = var_R * 0.4124f + var_G * 0.3576f + var_B * 0.1805f;
    *outY = var_R * 0.2126f + var_G * 0.7152f + var_B * 0.0722f;
    *outZ = var_R * 0.0193f + var_G * 0.1192f + var_B * 0.9505f;
}

void convertXYZtoLab(float inX, float inY, float inZ, float * outL, float * outa, float * outb) {

    float var_X = (inX / ref_X); //ref_X = 95.047
    float var_Y = (inY / ref_Y); //ref_Y = 100.0
    float var_Z = (inZ / ref_Z); //ref_Z = 108.883

    if ( var_X > 0.008856 ) 
        var_X = powf(var_X , ( 1.0f/3 )); 
    else 
        var_X = ( 7.787 * var_X ) + ( 16.0f/116 );

    if ( var_Y > 0.008856 )
        var_Y = powf(var_Y , ( 1.0f/3 )); 
    else
        var_Y = ( 7.787 * var_Y ) + ( 16.0f/116 );

    if ( var_Z > 0.008856 )
        var_Z = powf(var_Z , ( 1.0f/3 )); 
    else 
        var_Z = ( 7.787 * var_Z ) + ( 16.0f/116 );

    *outL = ( 116 * var_Y ) - 16;
    *outa = 500 * ( var_X - var_Y );
    *outb = 200 * ( var_Y - var_Z );
}

void convertLabtoXYZ( float inL, float ina, float  inb, float * outX, float * outY, float * outZ) {

    float var_Y = ( inL + 16 ) / 116;
    float var_X = (ina/500) + var_Y;
    float var_Z = var_Y - (inb/200);

    if ( powf(var_Y,3.f) > 0.008856 ) 
        var_Y = powf(var_Y,3.f);
    else
        var_Y = ( var_Y - (16/116) ) / 7.787;

    if ( powf(var_X,3.f) > 0.008856 ) 
        var_X = powf(var_X,3.f);
    else 
        var_X = ( var_X - (16/116) ) / 7.787;

    if ( powf(var_Z,3.f) > 0.008856 )
        var_Z = powf(var_Z,3.f);
    else
        var_Z = ( var_Z - (16/116) ) / 7.787;

    *outX = ref_X * var_X;     //ref_X =  95.047     Observer= 2°, Illuminant= D65
    *outY = ref_Y * var_Y;     //ref_Y = 100.000
    *outZ = ref_Z * var_Z;     //ref_Z = 108.883
}

void convertXYZtoRGB(float inX, float inY, float inZ, int * outR, int * outG, int * outB) {


    float var_X = inX/100;
    float var_Y = inY/100;
    float var_Z = inZ/100;

    float var_R = var_X *  3.2406 + (var_Y * -1.5372) + var_Z * (-0.4986);
    float var_G = var_X * (-0.9689) + var_Y *  1.8758 + var_Z *  0.0415;
    float var_B = var_X *  0.0557 + var_Y * (-0.2040) + var_Z *  1.0570;

    if ( var_R > 0.0031308 )
        var_R = 1.055 * powf(var_R, ( 1.0f / 2.4 ) )  - 0.055;
    else 
        var_R = 12.92 * var_R;

    if ( var_G > 0.0031308 ) 
        var_G = 1.055 * powf(var_G, ( 1.0f / 2.4 ) ) - 0.055;
    else 
        var_G = 12.92 * var_G;

    if ( var_B > 0.0031308 )
        var_B = 1.055 * powf(var_B, ( 1.0f / 2.4 ) ) - 0.055;
    else
        var_B = 12.92 * var_B;

    *outR = (int)(var_R * 255);
    *outG = (int)(var_G * 255);
    *outB = (int)(var_B * 255);


}

float Lab_color_difference( float inL1, float ina1, float  inb1, float inL2, float ina2, float  inb2){
    return( sqrt( powf(inL1 - inL2, 2.f) + powf(ina1 - ina2, 2.f) + powf(inb1 - inb2, 2.f) ) );
}

float RGB_color_Lab_difference( int R1, int G1, int B1, int R2, int G2, int B2){
    float x1=0,y1=0,z1=0;
    float x2=0,y2=0,z2=0;
    float l1=0,a1=0,b1=0;
    float l2=0,a2=0,b2=0;

    convertRGBtoXYZ(R1, G1, B1, &x1, &x1, &z1);
    convertRGBtoXYZ(R2, G2, B2, &x2, &x2, &z2);

    convertXYZtoLab(x1, y1, z1, &l1, &a1, &b1);
    convertXYZtoLab(x2, y2, z2, &l2, &a2, &b2); 

    return( Lab_color_difference(l1 ,a1 ,b1 ,l2 ,a2 ,b2) );
}


void main(int argc, char const *argv[])
{
    int R1,G1,B1,R2,G2,B2;
    float x=0.f,y=0.f,z=0.f;
    float l=0.f,a=0.f,b=0.f;

    R1 = 200;
    G1 = 2;
    B1 = 50; 

    R2 = 200;
    G2 = 2;
    B2 = 70; 

    printf("LAB DISTANCE = %lf \n", RGB_color_Lab_difference(R1,G1,B1,R2,G2,B2) );

    /*convertRGBtoXYZ(R, G, B, &x, &y, &z);
    printf("R =%d,G =%d,B =%d,x =%lf,y =%lf,z =%lf\n",R,G,B,x,y,z );
    convertXYZtoLab(x, y, z, &l, &a, &b);
    printf("x =%lf,y =%lf,z =%lf, l =%lf,a =%lf,b =%lf\n",x,y,z, l,a,b );
    convertLabtoXYZ( l, a, b ,&x, &y, &z);
    printf("x =%lf,y =%lf,z =%lf, l =%lf,a =%lf,b =%lf\n",x,y,z, l,a,b );
    convertXYZtoRGB( x, y, z,&R, &G, &B);
    printf("R =%d,G =%d,B =%d,x =%lf,y =%lf,z =%lf\n",R,G,B,x,y,z );*/

}

Color conversions and differences in C https://github.com/gi0rikas/Color-conversions Lab distance ~= 2.3 corresponds to JND(Just noticeable difference)