Function SnapTicksToRoundValues

Rendering/Annotation/vtkAxisActor2D.cxx:222–247

* Compute an interval that split range depending on targetedNumTicks, * while being "rounded" for nice display. Actually, we snap the labels * to be one of the following, multiply by a power of ten: * [10, 12, 15, 18, 20, 25, 30, 40, 50, 60, 80, 90] * (see ACCEPTABLE_LABELS) * * Return the computed number of ticks, that may differ from the target. */

 * Return the computed number of ticks, that may differ from the target.
 */
int SnapTicksToRoundValues(double range[2], int targetedNumTicks, double& interval)
{
  double delta = std::fabs(range[1] - range[0]);
  double roughInterval = delta / targetedNumTicks;

  // get order of magnitude of the range
  int rootPower = static_cast<int>(std::floor(std::log10(roughInterval) - 1));
  double root = std::pow(10.0, rootPower);

  // roundedInterval will be between 10 and 100 inclusive of 10 but not 100
  // and has 2 significant digits
  int roundedInterval = roughInterval / root;

  auto resulting =
    std::lower_bound(ACCEPTABLE_LABELS.begin(), ACCEPTABLE_LABELS.end(), roundedInterval);
  if (resulting != ACCEPTABLE_LABELS.end())
  {
    roundedInterval = *resulting;
  }

  // scale back rounded interval to actual range
  interval = roundedInterval * root;
  int numTicks = (delta / interval) + 1;

  return numTicks;
}

/**
 * Update Range, so outRange can be split into `numberOfTick`