void RGBtoCMYK(float color[4])
{
	if (color[0]==0 && color[1]==0 && color[2]==0)
	{
		color[0] = 0;
		color[1] = 0;
		color[2] = 0;
		color[3] = 1.0;
	}
	else if (color[0]==1.0 && color[1]==1.0 && color[2]==1.0)
	{
		color[0] = 0;
		color[1] = 0;
		color[2] = 0;
		color[3] = 0;
	}
	else
	{
		color[0] = 1.0 - color[0];
		color[1] = 1.0 - color[1];
		color[2] = 1.0 - color[2];
 		
		float minK = MIN(color[0], MIN(color[1], color[2]));
		color[0] = (color[0] - minK) / (1.0 - minK);
		color[1] = (color[1] - minK) / (1.0 - minK);
		color[2] = (color[2] - minK) / (1.0 - minK);
		color[3] = minK;
	}
}

void CMYKtoRGB(float color[4])
{
	color[0] = 1.0 - (color[0] * (1.0 - color[3]) + color[3]);
	color[1] = 1.0 - (color[1] * (1.0 - color[3]) + color[3]);
	color[2] = 1.0 - (color[2] * (1.0 - color[3]) + color[3]);
	color[3] = 1.0;
}

RGB/CMYK Conversion

C++