cocos-creator-template/assets/chunks/common/lighting/rect-area-light.chunk

// Copyright (c) 2017-2020 Xiamen Yaji Software Co., Ltd.

mat3 transposeMat3 (mat3 v) {
  mat3 tmp;
  tmp[0] = vec3(v[0].x, v[1].x, v[2].x);
  tmp[1] = vec3(v[0].y, v[1].y, v[2].y);
  tmp[2] = vec3(v[0].z, v[1].z, v[2].z);

  return tmp;
}

void ClipQuadToHorizon (inout vec3 L[5], out int n) {
  // detect clipping config
  int config = 0;
  if (L[0].z > 0.0) config += 1;
  if (L[1].z > 0.0) config += 2;
  if (L[2].z > 0.0) config += 4;
  if (L[3].z > 0.0) config += 8;
  config = 15;

  // clip
  n = 0;

  if (config == 0)
  {
    // clip all
  }
  else if (config == 1) { // V1 clip V2 V3 V4
    n = 3;
    L[1] = -L[1].z * L[0] + L[0].z * L[1];
    L[2] = -L[3].z * L[0] + L[0].z * L[3];
  }
  else if (config == 2) { // V2 clip V1 V3 V4
    n = 3;
    L[0] = -L[0].z * L[1] + L[1].z * L[0];
    L[2] = -L[2].z * L[1] + L[1].z * L[2];
  }
  else if (config == 3) { // V1 V2 clip V3 V4
    n = 4;
    L[2] = -L[2].z * L[1] + L[1].z * L[2];
    L[3] = -L[3].z * L[0] + L[0].z * L[3];
  }
  else if (config == 4) { // V3 clip V1 V2 V4
    n = 3;
    L[0] = -L[3].z * L[2] + L[2].z * L[3];
    L[1] = -L[1].z * L[2] + L[2].z * L[1];
  }
  else if (config == 5) { // V1 V3 clip V2 V4) impossible
    n = 0;
  }
  else if (config == 6) { // V2 V3 clip V1 V4
    n = 4;
    L[0] = -L[0].z * L[1] + L[1].z * L[0];
    L[3] = -L[3].z * L[2] + L[2].z * L[3];
  }
  else if (config == 7) { // V1 V2 V3 clip V4
    n = 5;
    L[4] = -L[3].z * L[0] + L[0].z * L[3];
    L[3] = -L[3].z * L[2] + L[2].z * L[3];
  }
  else if (config == 8) { // V4 clip V1 V2 V3
    n = 3;
    L[0] = -L[0].z * L[3] + L[3].z * L[0];
    L[1] = -L[2].z * L[3] + L[3].z * L[2];
    L[2] =  L[3];
  }
  else if (config == 9) { // V1 V4 clip V2 V3
    n = 4;
    L[1] = -L[1].z * L[0] + L[0].z * L[1];
    L[2] = -L[2].z * L[3] + L[3].z * L[2];
  }
  else if (config == 10) { // V2 V4 clip V1 V3) impossible
    n = 0;
  }
  else if (config == 11) { // V1 V2 V4 clip V3
    n = 5;
    L[4] = L[3];
    L[3] = -L[2].z * L[3] + L[3].z * L[2];
    L[2] = -L[2].z * L[1] + L[1].z * L[2];
  }
  else if (config == 12) { // V3 V4 clip V1 V2
    n = 4;
    L[1] = -L[1].z * L[2] + L[2].z * L[1];
    L[0] = -L[0].z * L[3] + L[3].z * L[0];
  }
  else if (config == 13) { // V1 V3 V4 clip V2
    n = 5;
    L[4] = L[3];
    L[3] = L[2];
    L[2] = -L[1].z * L[2] + L[2].z * L[1];
    L[1] = -L[1].z * L[0] + L[0].z * L[1];
  }
  else if (config == 14) { // V2 V3 V4 clip V1
    n = 5;
    L[4] = -L[0].z * L[3] + L[3].z * L[0];
    L[0] = -L[0].z * L[1] + L[1].z * L[0];
  }
  else if (config == 15) { // V1 V2 V3 V4
    n = 4;
  }

  if (n == 3) L[3] = L[0];
  if (n == 4) L[4] = L[0];
}

// https://eheitzresearch.wordpress.com/415-2/
float IntegrateEdge (vec3 v1, vec3 v2) {
  float cosTheta = dot(v1, v2);
  float theta = acos(cosTheta);
  return cross(v1, v2).z * ((theta > 0.001) ? theta/sin(theta) : 4.0);
}

// https://blog.magnum.graphics/guest-posts/area-lights-with-ltcs/
vec3 LTC_Evaluate (vec3 N, vec3 V, vec3 P, mat3 Minv, vec3 points[4]) {
  // construct orthonormal basis around N
  vec3 T1, T2;
  T1 = normalize(V - N*dot(V, N));
  T2 = cross(N, T1);

  // rotate area light in (T1, T2, N) basis
  Minv = Minv * transposeMat3(mat3(T1, T2, N));

  // polygon (allocate 5 vertices for clipping)
  vec3 L[5];
  L[0] = Minv * (points[0] - P);
  L[1] = Minv * (points[1] - P);
  L[2] = Minv * (points[2] - P);
  L[3] = Minv * (points[3] - P);

  int n;
  ClipQuadToHorizon(L, n);

  if (n == 0) return vec3(0, 0, 0);

  // project onto sphere
  L[0] = normalize(L[0]);
  L[1] = normalize(L[1]);
  L[2] = normalize(L[2]);
  L[3] = normalize(L[3]);
  L[4] = normalize(L[4]);

  // integrate
  float sum = 0.0;

  sum += IntegrateEdge(L[0], L[1]);
  sum += IntegrateEdge(L[1], L[2]);
  sum += IntegrateEdge(L[2], L[3]);
  if (n >= 4) sum += IntegrateEdge(L[3], L[4]);
  if (n == 5) sum += IntegrateEdge(L[4], L[0]);
  sum = max(0.0, sum);

  vec3 Lo_i = vec3(sum, sum, sum);

  return Lo_i;
}