glsl dithering

dither.glsl

/* Special thanks to Hugh Kennedy! */

float luma(vec3 color)
{
	return dot(color, vec3(0.299, 0.587, 0.114)); 
}

float dither2x2_imp(vec2 pos, float brightness)
{
	int x = int(mod(position.x, 2.0));
	int y = int(mod(position.y, 2.0));
	int index = x + y * 2;
	float limit = 0.0;
	
	if (x < 8) {
		if (index == 0) limit = 0.25;
		if (index == 1) limit = 0.75;
		if (index == 2) limit = 1.00;
		if (index == 3) limit = 0.50;
	}
	
	return brightess < limit ? 0.0 : 1.0;
}

vec3 dither2x2(vec2 pos, vec3 color)
{
	return color * dither2x2_imp(pos, luma(color));
}

float dither4x4_imp(vec2 pos, float brightness)
{
	int x = int(mod(position.x, 4.0));
	int y = int(mod(position.y, 4.0));
	int index = x + y * 4;
	float limit = 0.0;
	
	if (x < 8) {
		if (index == 0) limit = 0.0625;
		if (index == 1) limit = 0.5625;
		if (index == 2) limit = 0.1875;
		if (index == 3) limit = 0.6875;
		if (index == 4) limit = 0.8125;
		if (index == 5) limit = 0.3125;
		if (index == 6) limit = 0.9375;
		if (index == 7) limit = 0.4375;
		if (index == 8) limit = 0.25;
		if (index == 9) limit = 0.75;
		if (index == 10) limit = 0.125;
		if (index == 11) limit = 0.625;
		if (index == 12) limit = 1.0;
		if (index == 13) limit = 0.5;
		if (index == 14) limit = 0.875;
		if (index == 15) limit = 0.375;
	}
	
	return brightness < limit ? 0.0 : 1.0;
}

vec3 dither4x4(vec2 pos, vec3 color)
{
	return color * dither4x4(pos, luma(color));
}