color-limit.gdshader

shader_type canvas_item;
render_mode blend_mix;
// filter_nearest so you don't get weird ghost palette colors!!
uniform sampler2D palette: filter_nearest;
uniform sampler2D screen_texture : repeat_disable, filter_nearest;
uniform float alpha_thresh= 0.1;

void fragment()
{
    vec3 source_col = texture(screen_texture, SCREEN_UV).rgb;
    float alph = texture(screen_texture, SCREEN_UV).a;
    vec3 closest_col = vec3(0.0);
    float smallest_error = 10.0;

    int palette_size = textureSize(palette, 0).x;
    float palette_pixel_size = 1.0 / float(palette_size);
    for (int x = 0; x < palette_size; x++)
    {
        vec3 palette_col = texture(palette, vec2(float(x) * palette_pixel_size, 0.0)).rgb;
        float error = distance(source_col, palette_col);
        if (error < smallest_error)
        {
            closest_col = palette_col;
            smallest_error = error;
        }
    }
	if (alph >= alpha_thresh){
		alph = 1.0;
	}
	else {
		alph = 0.0;
	}
    COLOR.rgb = closest_col;
    COLOR.a = alph;
}