Retro dither

A dithering shader for rendering retro polygonal meshes.

This is GLES2 which does not have loops, arrays or array access. In GLES3 this would be a lot shorter and simpler.

Also this does not handle alpha yet.

Shader code

shader_type spatial;
render_mode blend_mix, depth_draw_opaque, cull_back, diffuse_burley;

uniform vec4 color: hint_color = vec4(0.5, 0.4, 0.4, 1.0);
uniform float shade_tweak: float = 0.5;
uniform float light_tweak: float = 1.0;
uniform float sharpness: float = 1.0;
uniform float scale: float = 2.0;

const mat4 dither = mat4(
	vec4(0.0, 0.5333333333, 0.1333333333,  0.6666666667),
	vec4(0.8, 0.2666666667, 0.9333333333,  0.4),
	vec4(0.2, 0.7333333333, 0.06666666667, 0.6),
	vec4(1.0, 0.4666666667, 0.8666666667,  0.3333333333)
);

void fragment() {
	ALBEDO = COLOR.rgb * color.rgb;
}

float sample(vec2 coord, float alpha, float shade, float lit) {
	int x = int(mod(coord.x, 4));
	int y = int(mod(coord.y, 4));
	if (y == 0) {
		if (x == 0 && dither[0][0] >= alpha) {
			return shade;
		} else if (x == 1 && dither[0][1] >= alpha) {
			return shade;
		} else if (x == 2 && dither[0][2] >= alpha) {
			return shade;
		} else if (x == 3 && dither[0][3] >= alpha) {
			return shade;
		}
	} else if (y == 1) {
		if (x == 0 && dither[1][0] >= alpha) {
			return shade;
		} else if (x == 1 && dither[1][1] >= alpha) {
			return shade;
		} else if (x == 2 && dither[1][2] >= alpha) {
			return shade;
		} else if (x == 3 && dither[1][3] >= alpha) {
			return shade;
		}
	} else if (y == 2) {
		if (x == 0 && dither[2][0] >= alpha) {
			return shade;
		} else if (x == 1 && dither[2][1] >= alpha) {
			return shade;
		} else if (x == 2 && dither[2][2] >= alpha) {
			return shade;
		} else if (x == 3 && dither[2][3] >= alpha) {
			return shade;
		}
	} else if (y == 3) {
		if (x == 0 && dither[3][0] >= alpha) {
			return shade;
		} else if (x == 1 && dither[3][1] >= alpha) {
			return shade;
		} else if (x == 2 && dither[3][2] >= alpha) {
			return shade;
		} else if (x == 3 && dither[3][3] >= alpha) {
			return shade;
		}
	}
	return lit;
}

void light() {
	float f = min(ATTENUATION.r, min(ATTENUATION.g, ATTENUATION.b));
	float a = sample(
		FRAGCOORD.xy * (1.0 / scale),
		clamp(dot(NORMAL, LIGHT) + sharpness, 0.0, 1.0 + sharpness) * f,
		shade_tweak,
		light_tweak);
	DIFFUSE_LIGHT += ALBEDO * a;
}

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