#version 300 es
precision highp float;

out vec4 outColor;

uniform sampler2D uScene;
uniform vec2 uInverseResolution;

const float EDGE_THRESHOLD_MINIMUM = 0.0312;
const float EDGE_THRESHOLD = 0.125;
const float DIRECTION_REDUCE_MINIMUM = 1.0 / 128.0;
const float DIRECTION_REDUCE_MULTIPLIER = 1.0 / 8.0;
const float MAXIMUM_SPAN = 8.0;

float luminance(vec3 color) {
  return dot(color, vec3(0.299, 0.587, 0.114));
}

void main() {
  vec2 uv = gl_FragCoord.xy * uInverseResolution;
  vec3 center = texture(uScene, uv).rgb;
  vec3 northwest = texture(uScene, uv + vec2(-1.0, 1.0) * uInverseResolution).rgb;
  vec3 northeast = texture(uScene, uv + vec2(1.0, 1.0) * uInverseResolution).rgb;
  vec3 southwest = texture(uScene, uv + vec2(-1.0, -1.0) * uInverseResolution).rgb;
  vec3 southeast = texture(uScene, uv + vec2(1.0, -1.0) * uInverseResolution).rgb;

  float lumaCenter = luminance(center);
  float lumaNorthwest = luminance(northwest);
  float lumaNortheast = luminance(northeast);
  float lumaSouthwest = luminance(southwest);
  float lumaSoutheast = luminance(southeast);
  float lumaMinimum = min(
    lumaCenter,
    min(min(lumaNorthwest, lumaNortheast), min(lumaSouthwest, lumaSoutheast))
  );
  float lumaMaximum = max(
    lumaCenter,
    max(max(lumaNorthwest, lumaNortheast), max(lumaSouthwest, lumaSoutheast))
  );
  float lumaRange = lumaMaximum - lumaMinimum;

  if (lumaRange < max(EDGE_THRESHOLD_MINIMUM, lumaMaximum * EDGE_THRESHOLD)) {
    outColor = vec4(center, 1.0);
    return;
  }

  vec2 direction;
  direction.x = -(
    (lumaNorthwest + lumaNortheast) - (lumaSouthwest + lumaSoutheast)
  );
  direction.y = (
    (lumaNorthwest + lumaSouthwest) - (lumaNortheast + lumaSoutheast)
  );

  float directionReduce = max(
    (lumaNorthwest + lumaNortheast + lumaSouthwest + lumaSoutheast)
      * (0.25 * DIRECTION_REDUCE_MULTIPLIER),
    DIRECTION_REDUCE_MINIMUM
  );
  float reciprocalMinimum = 1.0 / (
    min(abs(direction.x), abs(direction.y)) + directionReduce
  );
  direction = clamp(
    direction * reciprocalMinimum,
    vec2(-MAXIMUM_SPAN),
    vec2(MAXIMUM_SPAN)
  ) * uInverseResolution;

  vec3 filteredNarrow = 0.5 * (
    texture(uScene, uv + direction * (1.0 / 3.0 - 0.5)).rgb
    + texture(uScene, uv + direction * (2.0 / 3.0 - 0.5)).rgb
  );
  vec3 filteredWide = filteredNarrow * 0.5 + 0.25 * (
    texture(uScene, uv - direction * 0.5).rgb
    + texture(uScene, uv + direction * 0.5).rgb
  );
  float wideLuminance = luminance(filteredWide);
  vec3 filtered = wideLuminance < lumaMinimum || wideLuminance > lumaMaximum
    ? filteredNarrow
    : filteredWide;

  outColor = vec4(filtered, 1.0);
}