grunk/src/shaders/canvas_retro.gdshader

// CanvasItem Retro Post-processing
// Adapted from https://godotshaders.com/shader/retro-post-processing/

shader_type canvas_item;


// Handles the resolution changes, color depth, and dithering
uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_nearest;


group_uniforms resolution_and_colors;
uniform bool change_color_depth = false;
uniform int target_color_depth : hint_range(1, 8) = 5;
uniform bool dithering = false;
uniform bool scale_resolution = false;
uniform int target_resolution_scale = 3;


// Handles the LUTish recoloring
group_uniforms gradient_recoloring;
uniform bool enable_recolor = false;
uniform sampler2D to_gradient: hint_default_black;


int dithering_pattern(ivec2 fragcoord) {
	const int pattern[] = {
		-4, +0, -3, +1,
		+2, -2, +3, -1,
		-3, +1, -4, +0,
		+3, -1, +2, -2
	};

	int x = fragcoord.x % 4;
	int y = fragcoord.y % 4;

	return pattern[y * 4 + x];
}


vec3 rgb2hsv(vec3 rgb) { //Converts RGB values to HSV
	float r = rgb.r;
	float g = rgb.g;
	float b = rgb.b;


	float cmax = max(r,max(g,b));
	float cmin = min(r,min(g,b));
	float delta = cmax - cmin;


	float h = 0.f; //hue


	if (delta > 0.f){
		if (cmax == r){
			h = (g-b)/delta;
			h = mod(h,6.f);
		} else if (cmax == g){
			h = ((b - r) / delta) + 2.f;
		} else {
			h = ((r-g)/delta) + 4.f;
		}
		h = h * 60.f;
	}


	float s = 0.f; //saturation
	if (cmax > 0.f){
		s = delta / cmax;
	}


	return vec3(h,s,cmax); // Keep original alpha value


}


vec3 hsv2rgb(vec3 hsv) { //Converts HSV values to RGB
	float h = hsv.r;
	float s = hsv.g;
	float v = hsv.b;
	float c = v * s;
	//X = C × (1 - |(H / 60°) mod 2 - 1|)
	float x = h / 60.f;
	x = mod(x,2.f);
	x = abs(x - 1.f);
	x = c * (1.f - x);


	float m = v - c;


	vec3 rgb = vec3(0.f,0.f,0.f);


	if (h < 60.f) {
		rgb = vec3(c,x,0.f);
	} else if (h < 120.f){
		rgb = vec3(x,c,0.f);
	} else if (h < 180.f){
		rgb = vec3(0.f,c,x);
	} else if (h < 240.f){
		rgb = vec3(0.f,x,c);
	} else if (h < 300.f){
		rgb = vec3(x,0.f,c);
	} else if (h < 360.f){
		rgb = vec3(c,0.f,x);
	}
	rgb[0] = rgb[0] + m;
	rgb[1] = rgb[1] + m;
	rgb[2] = rgb[2] + m;


	return rgb;
}


void fragment() {

	vec2 iResolution = 1.0 / SCREEN_PIXEL_SIZE;
    vec2 q = FRAGCOORD.xy / iResolution.xy;
	ivec2 uv = ivec2(q);

	vec3 color = texture(screen_texture, vec2(q.x,q.y) ).xyz;

	if(scale_resolution){
		uv = ivec2(FRAGCOORD.xy / float(target_resolution_scale));
		color = texelFetch(screen_texture, uv * target_resolution_scale, 0).rgb;
	} else {
		uv = ivec2(FRAGCOORD.xy);
		color = texelFetch(screen_texture, uv, 0).rgb;
	}

	if(enable_recolor){
		vec3 hsv = rgb2hsv(color);
		float color_pos = (hsv.x / 360.0);
		vec3 new_color = texture(to_gradient, vec2((color_pos), 0.5)).rgb;
		vec3 new_hsv = rgb2hsv(new_color);
		hsv.x = new_hsv.x;
		vec3 final_rgb = hsv2rgb(hsv);


		color.rgb = final_rgb;
	}



	// Convert from [0.0, 1.0] range to [0, 255] range
	ivec3 c = ivec3(round(color * 255.0));

	// Apply the dithering pattern
	if (dithering) {
		c += ivec3(dithering_pattern(uv));
	}

	vec3 final_color;
	if(change_color_depth){
		// Truncate from 8 bits to color_depth bits
		c >>= (8 - target_color_depth);
		final_color = vec3(c) / float(1 << target_color_depth);
	} else {
		final_color = vec3(c) / float(1 << 8);
	}

	// Convert back to [0.0, 1.0] range
	COLOR.rgb = final_color;
}