Function handleZoom

packages/engine/Source/Scene/ScreenSpaceCameraController.js:559–983

(
  object,
  startPosition,
  movement,
  zoomFactor,
  distanceMeasure,
  unitPositionDotDirection,
)

};

function handleZoom(
  object,
  startPosition,
  movement,
  zoomFactor,
  distanceMeasure,
  unitPositionDotDirection,
) {
  let percentage = 1.0;
  if (defined(unitPositionDotDirection)) {
    percentage = CesiumMath.clamp(
      Math.abs(unitPositionDotDirection),
      0.25,
      1.0,
    );
  }

  const diff = movement.endPosition.y - movement.startPosition.y;

  // distanceMeasure should be the height above the ellipsoid.
  // When approaching the surface, the zoomRate slows and stops minimumZoomDistance above it.
  const approachingSurface = diff > 0;
  const minHeight = approachingSurface
    ? object.minimumZoomDistance * percentage
    : 0;
  const maxHeight = object.maximumZoomDistance;

  const minDistance = distanceMeasure - minHeight;
  let zoomRate = zoomFactor * minDistance;
  zoomRate = CesiumMath.clamp(
    zoomRate,
    object._minimumZoomRate,
    object._maximumZoomRate,
  );

  let rangeWindowRatio = diff / object._scene.canvas.clientHeight;
  rangeWindowRatio = Math.min(rangeWindowRatio, object.maximumMovementRatio);
  let distance = zoomRate * rangeWindowRatio;

  if (
    object.enableCollisionDetection ||
    object.minimumZoomDistance === 0.0 ||
    !defined(object._globe) // look-at mode
  ) {
    if (distance > 0.0 && Math.abs(distanceMeasure - minHeight) < 1.0) {
      return;
    }

    if (distance < 0.0 && Math.abs(distanceMeasure - maxHeight) < 1.0) {
      return;
    }

    if (distanceMeasure - distance < minHeight) {
      distance = distanceMeasure - minHeight - 1.0;
    } else if (distanceMeasure - distance > maxHeight) {
      distance = distanceMeasure - maxHeight;
    }
  }