Love2D Tile Shader

local tile_shader = love.graphics.newShader[[
    vec4 darkenPixel();

    vec4 darkenPixel( vec4 pixel, number darkness , number initial_lightness) {
        number r = pixel.r * darkness * initial_lightness;
        number g = pixel.g * darkness * initial_lightness;
        number b = pixel.b * darkness * initial_lightness;
        return vec4(r, g, b, pixel.a);
    }

    extern number initial_lightness;

    extern bool top;
    extern bool bottom;
    extern bool left;
    extern bool right;

    extern bool top_left;
    extern bool top_right;
    extern bool bottom_left;
    extern bool bottom_right;

    vec4 effect( vec4 colour, Image texture, vec2 texture_coords, vec2 screen_coords ) {
        number lightness_multiplier = 1.75;
        number spread = 2.3;

        number x = texture_coords.x;
        number y = texture_coords.y;

        number diffX = x - 0.5;
        number diffY = y - 0.5;

        vec4 pixel = Texel(texture, texture_coords);

        number magnitude = sqrt(pow(diffX, 2) + pow(diffY, 2));

        if (top && diffY <= 0 || bottom && diffY >= 0)
            magnitude = min(abs(0.5 - x), magnitude);

        if (left && diffX <= 0 || right && diffX >= 0)
            magnitude = min(abs(0.5 - y), magnitude);

        if (top && left && top_left && y < 1 - x || bottom && right && bottom_right && y > 1 - x) {
            magnitude = min(abs(x - y) / sqrt(2), magnitude);

            bool top_left_top = top && diffX <= 0 && y < x;
            bool top_left_left = left && diffY <= 0 && y > x;
            bool bot_right_bot = bottom && diffX >= 0 && y > x;
            bool bot_right_right = right && diffY >= 0 && y < x;

            if (top_left_top || top_left_left || bot_right_bot || bot_right_right)
                magnitude = 0;
        }

        if (top && right && top_right && y < x || bottom && left && bottom_left && y > x){
            magnitude = min(abs(x + y - 1) / sqrt(2), magnitude);

            bool top_right_top = top && diffX >= 0 && y < 1 - x;
            bool top_right_right = right && diffY <= 0 && y > 1 - x;
            bool bot_left_bot = bottom && diffX <= 0 && y > 1 - x;
            bool bot_left_left = left && diffY >= 0 && y < 1 - x;

            if (top_right_top || top_right_right || bot_left_bot || bot_left_left)
                magnitude = 0;
        }

        magnitude = pow(magnitude, 2) * spread;

        number darkness = 1 - ((1 - magnitude) / lightness_multiplier);
        vec4 newPixel = darkenPixel(pixel, darkness, initial_lightness);
        return newPixel;
    }
]]