test3.cpp

// Another test. RGB->YCoCg conversion. Think I discovered a problem with negative 
// values being returned on RGB->YCoCg->RGB.
#include <stdio.h>

struct IVect { int x, y, z; };

/*
 * [Y ] = [ 1/4 1/2  1/4][R] R, G, B = [0..255] => Y = [0..255], Co, Cg = [-128..127]
 * [Co]   [ 1/2  0  -1/2][G]
 * [Cg]   [-1/4 1/2 -1/4][B]
 */
IVect
RGB_to_YCoCg(const IVect& rgb)
{
	IVect ycocg;

//	int s = (rgb.x >> 2) + (rgb.z >> 2);
	int s = (rgb.x + rgb.z) >> 2;
	int t = rgb.y >> 1;
	ycocg.x = t + s;
	ycocg.z = t - s;
//	ycocg.y = (rgb.x >> 1) - (rgb.z >> 1);
	ycocg.y = (rgb.x - rgb.z) >> 1;

	return ycocg;
}


IVect
YCoCg_to_RGB(const IVect& ycocg)
{
	IVect rgb;

	int t = ycocg.x - ycocg.z;
	rgb.x = t + ycocg.y;
	rgb.y = ycocg.x + ycocg.z;
	rgb.z = t - ycocg.y;

	return rgb;
}


int
main()
{

	IVect colors[] =
	{
		{   0,   0,   0 },
		{ 255,   0,   0 },
		{   0, 255,   0 },
		{ 255, 255,   0 },
		{   0,   0, 255 },
		{ 255,   0, 255 },
		{   0, 255, 255 },
		{ 255, 255, 255 },
		{ 255, 127,   0 },
		{ 127, 255,   0 },
		{ 255,   0, 127 },
		{ 127,   0, 255 },
		{   0, 255, 127 },
		{   0, 127, 255 },
	};

	for (int i = 0; i < 14; ++i)
	{
		const IVect t1 = RGB_to_YCoCg(colors[i]);
		const IVect t2 = YCoCg_to_RGB(t1);
		printf("{R=%3d, G=%3d, B=%3d} => {Y=%3d, Co=%4d, Cg=%4d} => {R=%3d, G=%3d, B=%3d}\n",
			   colors[i].x, colors[i].y, colors[i].z, t1.x, t1.y, t1.z, t2.x, t2.y, t2.z);
	}

	return 0;
}