godot-demo-projects/2d/screen_space_shaders/shaders/contrasted.shader

shader_type canvas_item;

void fragment() {
	vec3 c = textureLod(SCREEN_TEXTURE, SCREEN_UV, 0.0).rgb;
	c = mod(c + vec3(0.5), vec3(1.0));
	COLOR.rgb = c;
}