message: Use average error of active tiles instead of maximum error
  This results in a smoother / more predictable function over time and
  thus a better estimate is possible when the image will be converged.
diff: @@ -66,6 +66,18 @@ namespace ospray {
  if (tiles <= 0)
  return inf;
  + float maxErr = 0.f;
  + float sumActErr = 0.f;
  + int actTiles = 0;
  + for (int i = 0; i < tiles; i++) {
  + maxErr = std::max(maxErr, tileErrorBuffer[i]);
  + if (tileErrorBuffer[i] > errorThreshold) {
  + sumActErr += tileErrorBuffer[i];
  + actTiles++;
  + }
  + }
  + const float error = actTiles ? sumActErr / actTiles : maxErr;
  +
  // process regions first, but don't process newly split regions again
  int regions = errorThreshold > 0.f ? errorRegion.size() : 0;
  for (int i = 0; i < regions; i++) {
  @@ -114,6 +126,6 @@ namespace ospray {
  }
  }
  - return *std::max_element(tileErrorBuffer, tileErrorBuffer + tiles);
  + return error;
  }
  } // ::ospray
  