Function vm_GetCentroid

vecmat/vector.cpp:753–803

Gets the real center of a polygon Returns the size of the passed in stuff

// Gets the real center of a polygon
// Returns the size of the passed in stuff
float vm_GetCentroid(vector *centroid, vector *src, int nv) {
  ASSERT(nv > 2);
  vector normal;
  float area, total_area;
  int i;
  vector tmp_center;

  vm_MakeZero(centroid);

  // First figure out the total area of this polygon
  vm_GetPerp(&normal, &src[0], &src[1], &src[2]);
  total_area = (vm_GetMagnitude(&normal) / 2);

  for (i = 2; i < nv - 1; i++) {
    vm_GetPerp(&normal, &src[0], &src[i], &src[i + 1]);
    area = (vm_GetMagnitude(&normal) / 2);
    total_area += area;
  }

  // Now figure out how much weight each triangle represents to the overall
  // polygon
  vm_GetPerp(&normal, &src[0], &src[1], &src[2]);
  area = (vm_GetMagnitude(&normal) / 2);

  // Get the center of the first polygon
  vm_MakeZero(&tmp_center);
  for (i = 0; i < 3; i++) {
    tmp_center += src[i];
  }
  tmp_center /= 3;

  *centroid += (tmp_center * (area / total_area));

  // Now do the same for the rest
  for (i = 2; i < nv - 1; i++) {
    vm_GetPerp(&normal, &src[0], &src[i], &src[i + 1]);
    area = (vm_GetMagnitude(&normal) / 2);

    vm_MakeZero(&tmp_center);

    tmp_center += src[0];
    tmp_center += src[i];
    tmp_center += src[i + 1];

    tmp_center /= 3;

    *centroid += (tmp_center * (area / total_area));
  }

  return total_area;
}

// Gets the real center of a polygon, but uses fast magnitude calculation
// Returns the size of the passed in stuff