uniform highp float saturation; // Between 0 and 2;
uniform sampler2D SCREEN_TEXTURE;

void fragment() {
    highp vec4 color = texture(SCREEN_TEXTURE, UV);

    highp float brightness = (color.r + color.g + color.b) / 3.0;

    color.rgb = mix(vec3(brightness), color.rgb, saturation);

    COLOR = color;
}