YUV - RGB Conversion Algorithms in C

In order to understand this article you should probably read this article first:

 Color Space Structures.

If you are curious about the theory behind the HSI and RGB color spaces, you can read this articles:

YUV color space

RGB color space

1.RGB to YUV Conversion

This function will create a YUV model from a RGB model. The algorithm behind the conversion is described by the formulas below:

RGB to YUV algorithm

The meaning of the variables:
R,G,B - the components of the RGB model (red, green, blue)
Y,U,V - the components of YIQ model (luma, chrominance components)

The components of the RGB model (r,g,b) and luma should have values in the range [0,1], while the U component should have values in the range [-0.436 ,0.436] and the V component should have values in the range [-0.615, 0.615].

The prototype of the function is:

/* Description:
 *  Creates a YuvColor structure from RGB components
 * Parameters:
 *  r,g,b - the components of an RGB model expressed
 *          as real numbers
 * Returns:
 *  A pointer to the YuvColor is the parameters are
 * correct. Otherwise returns NULL.
 */
YuvColor* Yuv_CreateFromRgbF(double r, double g, double f);

The function can be implemented as:

YuvColor* Yuv_CreateFromRgbF(double r, double g, double b)
{
    YuvColor* color = NULL;
    if (RgbF_IsValid(r, g, b) == true)
    {
        color = Yuv_Create(0.0, 0.0, 0.0);
        color->Y = 0.299 * r + 0.587 * b + 0.114 * b;
        color->U = 0.492 * (b - color->Y);
        color->V = 0.877 * (r - color->Y);
    }
    return color;
}

2.YUV to RGB Conversion

This function will create a RGB model from a YUV model. The algorithm behind the conversion is described by the formulas below:

YUV to RGB algorithm

The meaning of the variables:
R,G,B - the components of the RGB model (red, green, blue)
Y,U,V - the components of YIQ model (luma, chrominance components)

The components of the RGB model (r,g,b) and luma should have values in the range [0,1], while the U component should have values in the range [-0.436 ,0.436] and the V component should have values in the range [-0.615, 0.615].

The prototype of the function is:

/*
 * Description:
 *  Creates a RgbFColor structure from YUV components
 * Parameters:
 *  y,u,v - the components of an YUV model expressed
 *          as real numbers
 * Returns:
 *  A pointer to the RgbFColor is the parameters are
 * correct. Otherwise returns NULL.
 */
RgbFColor* RgbF_CreateFromYuv(double y, double u, double v);

The function can be implemented as:

RgbFColor* RgbF_CreateFromYuv(double y, double u, double v)
{
    RgbFColor *color = NULL;
    if (Yiq_IsValid(y, u, v) == true)
    {
        color = RgbF_Create(0.0, 0.0, 0.0);
        color->R = y + 1.140 * v;
        color->G = y - 0.395 * u - 0.581 * v;
        color->B = y + 2.032 * u;
    }
    return color;
}

3.Exemplu

#include<stdio.h>
#include"colorspace.h"

int main(int argc, char** argv)
{
    YuvColor* yuv = NULL;
    RgbFColor* rgbF = NULL;
    RgbIColor* rgbI = NULL;

    /*YUV to RGB*/
    yuv = Yuv_Create(0.4, 0.1, -0.11);
    rgbF = RgbF_CreateFromYuv(yuv->Y, yuv->U, yuv->V);
    rgbI = RgbI_CreateFromRealForm(rgbF->R, rgbF->G, rgbF->B);
    printf("\nYIQ : %f %f %f", yuv->Y, yuv->U, yuv->V);
    printf("\nRGBf : %f %f %f", rgbF->R, rgbF->G, rgbF->B);
    printf("\nRGBi : %d %d %d", rgbI->R, rgbI->G, rgbI->B);

    /*Frees the resources*/
    free(yuv);
    free(rgbF);
    free(rgbI);

    /*RGB to YUV*/
    rgbI = RgbI_Create(108U, 198U, 78U);
    rgbF = RgbF_CreateFromIntegerForm(rgbI->R, rgbI->G, rgbI->B);
    yuv = Yuv_CreateFromRgbF(rgbF->R, rgbF->G, rgbF->B);
    printf("\nYIQ : %f %f %f", yuv->Y, yuv->U, yuv->V);
    printf("\nRGBf : %f %f %f", rgbF->R, rgbF->G, rgbF->B);
    printf("\nRGBi : %d %d %d", rgbI->R, rgbI->G, rgbI->B);
    return 0;
}
/*
 YIQ : 0.400000 0.100000 -0.110000
 RGBf : 0.274600 0.424410 0.603200
 RGBi : 70 108 154
 YIQ : 0.341059 -0.017307 0.072327
 RGBf : 0.423529 0.776471 0.305882
 RGBi : 108 198 78
 */

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