Convert RGB to YCbCr - C code

Question

I need to convert RGB to YCbCr for my final project and I trying to do this way (I'm programming in C):

/* Autor: Vinicius Garcia
 * Data : 09.ago.2011
 * 
 * Função que converte um pixel RGB em YCbCr
 * 
 * param : int R valor do pixel no canal red
 * param : int G valor do pixel no canal green
 * param : int B valor do pixel no canal blue
 * return: int* vetor de inteiros com os valores H, S e V calculados - nesta ordem
 */
int* converter_RGB_para_YCbCr(int R, int G, int B){
    int* YCbCr = (int*) malloc(3 * sizeof(int));

    double delta = 128.0; //Constante necessaria para o calculo da conversão de cor
    double Y  = (0.299 * R + 0.587 * G + 0.114 * B);
    double Cb = ((B - Y) * 0.564 + delta);
    double Cr = ((R - Y) * 0.713 + delta);

    YCbCr[0] = (int) Y;
    YCbCr[1] = (int) Cb;
    YCbCr[2] = (int) Cr;
    return YCbCr;
}

But it doesn't work for me!

I was comparing with cvCvtColor (from OpenCv library) and the results doesn't match:

            R = 88,  G  = 76,   B = 78
cvCvtColor: Y = 80,  Cb = 127, Cr = 134
myfunction: Y = 382, Cb = 132, Cr = 132 (cr and cr are always equal!)

I really need help with this, I trying do this for a long time and I couldn't find any answer for my doubt.

---

Do you guys think I'm getting the RGB values wrong? I'm doing this way:

uchar B = CV_IMAGE_ELEM(img_RGB, uchar, linha, coluna * 3);
uchar G = CV_IMAGE_ELEM(img_RGB, uchar, linha, coluna * 3 + 1);
uchar R = CV_IMAGE_ELEM(img_RGB, uchar, linha, coluna * 3 + 2);

Then I'm calling my conversion function this way:

converter_RGB_para_YCbCr(R, G, B);

My function expects int R, int G, int B but I'm passing uchar B, uchar G, uchar R. its that ok?

Answer 1

int y  = (int)( 0.299   * R + 0.587   * G + 0.114   * B);
int cb = (int)(-0.16874 * R - 0.33126 * G + 0.50000 * B);
int cr = (int)( 0.50000 * R - 0.41869 * G - 0.08131 * B);

Answer 2

you should use this formula matlab uses it

Y = 0.257R´ + 0.504G´ + 0.098B´ + 16

Cb = -0.148R´ - 0.291G´ + 0.439B´ + 128

Cr = 0.439R´ - 0.368G´ - 0.071B´ + 128

and to rgb

R´ = 1.164(Y - 16) + 1.596(Cr - 128)

G´ = 1.164(Y - 16) - 0.813(Cr - 128) - 0.392(Cb - 128)

B´ = 1.164(Y - 16) + 2.017(Cb - 128)

I have tested them in a software im making they work ok