Fix zooming out speed after reaching minimum zoom distance.

[puckey]

The minimum zoom distance causes the zoom speed to slow as you arrive at the minimum zoom. This feels right, but when you zoom back out again from that minimum zoom, the zooming takes too long to get up to speed.

In the following demo, use your scroll wheel to zoom in as far as you can and then try zooming out again:

Sandcastle demo: https://sandcastle.cesium.com/#c=bY5PSwMxEMW/SsipBUkoBSuYLsL2KHgoeJBcptlRByfJkj9b6qc3u72Idg4Db+b33swESUyEZ0xiLwKeRY+Zqlevy2xlpVt0H0MBCpisXD/acHWo7DBg6wkxHEdw2IPHBDOcInMjPAXy1b/F6A+UCwSH7c79dvew2e5akLyTJpcLY2eDWOqJ/BhTETXxSild0I8MBbM+VfeFRbmc5w9m1OjfVjPQJGjY33hZOIac2+a9Mh/pG63sjG78PytHGCh8vEyYGC4z9rnpnq9DpZTRTd52lhj5BOlP8g8

When specifying a minimum zoom distance like so:

var viewer = new Cesium.Viewer("cesiumContainer");
viewer.scene.screenSpaceCameraController.minimumZoomDistance = 6378137;

This pull request avoids slowing down zooming as you zoom out.

[cesium-concierge]

Thanks for the pull request @puckey!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[youlinaa]

any news on this?

[puckey]

Note that you can see this in production on http://radio.garden - zoom in all the way and out again

[youlinaa]

Yeah, that's great, but is there any reason to stop merging to master, can you fix that?

[ggetz]

Thanks @puckey! I was able to test this out with both the mouse and touch controls. Both are now snappier once the minimum has been reached.

Since this changes existing behavior, can you add a note to CHANGES.md?

[ggetz]

For readability, can you either 1) break this out into a descriptive variable, or 2) expand on the comment above?

[puckey]

Changed it to:

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

[puckey]

@ggetz Thanks for reviewing – I added a note about the changes to CHANGES.md

[ggetz]

Awesome, thanks again @puckey!