Custom heightmap terrain provider

[IanLilleyT]

This PR adds CustomHeightmapTerrainProvider, a TerrainProvider that lets you set height values inside a callback function. This way you can create procedural terrain very easily without having to create a whole new TerrainProvider subclass. However, there are some limitations such as no water mask, no vertex normals, and no availability, so a full-fledged TerrainProvider is still the right thing to do a lot of the time.

One example: Procedural terrain in CustomHeightmapTerrainProvider dropdown of the Terrain Sandcastle (localhost).

Another example: Sine wave

Local Sandcastle

This is the code that does it:

var width = 32;
var height = 32;

var viewer = new Cesium.Viewer("cesiumContainer", {
  terrainProvider: new Cesium.CustomHeightmapTerrainProvider({
    callback: function (x, y, level) {
      var buffer = new Float32Array(width * height);
      for (var yy = 0; yy < height; yy++) {
        for (var xx = 0; xx < width; xx++) {
          var v = (y + yy / (height - 1)) / Math.pow(2, level);
          var heightValue = 8000 * (Math.sin(4000 * v) * 0.5 + 0.5);
          var index = yy * width + xx;
          buffer[index] = heightValue;
        }
      }
      return buffer;
    },
    width: width,
    height: height,
  }),
});

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[IanLilleyT]

@lilleyse This is ready for another review

[lilleyse]

The updates look good. eslint and tests pass locally

We're still figuring out why all branches are suddenly failing CI after the travis-ci.org -> travis.ci.com switch. That doesn't need to hold up this PR.