Camera can't be put low

[alexander-entin]

Load the code into Sandcastle and try looking at the model from below.
Camera can't be put lower than 12.6 meters.

var point = 
     { lon: -2.434, lat: -0.248 } // Case 1
//    { lon: -2.137, lat: 0.66 } // Case 2
;

var viewer = new Cesium.Viewer('cesiumContainer', {
    baseLayerPicker: false
  , terrainProvider: new Cesium.CesiumTerrainProvider({
        url : '//cesiumjs.org/tilesets/terrain/smallterrain'
    })
});
viewer.scene.globe.depthTestAgainstTerrain = true

viewer.camera.flyTo({
    destination : Cesium.Cartesian3.fromRadians(point.lon, point.lat, 100)
  , duration: 0
});

// put the model at terrain height
Cesium.sampleTerrain(viewer.terrainProvider, 11, [viewer.camera.positionCartographic.clone()])
.then(function(cartographic) {
    var alt = cartographic[0].height || 0
    console.log(alt)

    var czml = new Cesium.CzmlDataSource()
    czml.load([{ id: "document", version: "1.0" }, {
        position: {
            cartographicRadians: [point.lon, point.lat, alt]
        }
      , model: { 
          gltf: '../../SampleData/models/CesiumAir/Cesium_Air.gltf' 
      }
    }])
    viewer.dataSources.add(czml)
})

// to monitor camera height
setInterval(function() {
   console.log(viewer.camera.positionCartographic.height)
}, 3000)    

In "Case 2" (un-comment line 3) terrain height is -32.4 but camera can't be put lower than -12.6 m (again! now negative).

"Case 2" also illustrates #2271 - when camera height becomes negative terrain and sky disappear.

Also got an exception several times while manipulating the camera with middle mouse button pressed:

An error occurred while rendering. Rendering has stopped.
RangeError: Invalid array length
RangeError: Invalid array length
    at updateFrustums (http://cesiumjs.org/Cesium/Source/Scene/Scene.js:821:36)
    at createPotentiallyVisibleSet (http://cesiumjs.org/Cesium/Source/Scene/Scene.js:971:13)
    at render (http://cesiumjs.org/Cesium/Source/Scene/Scene.js:1373:9)
    at Scene.render (http://cesiumjs.org/Cesium/Source/Scene/Scene.js:1413:13)
    at CesiumWidget.render (http://cesiumjs.org/Cesium/Source/Widgets/CesiumWidget/CesiumWidget.js:641:25)
    at render (http://cesiumjs.org/Cesium/Source/Widgets/CesiumWidget/CesiumWidget.js:68:32)

[pjcozzi]

Thanks for the report. @bagnell can probably suggest a workaround.

[mramato]

There are a couple of things at work here.

1. When in the camera's default mode, the closest you can get to the terrain is specified by the viewer.scene.screenSpaceCameraController.minimumZoomDistance property. I believe it defaults to 20 (mainly because of the sky disappearing issue I think).
2. If you are in a different camera reference frame (i.e. you aren't looking at the center of the earth), you can get as close to whatever you are looking at via the above property as well. The 3D Models example is a good illustration of that (use the combo box and select skinned model). You are only about 1 meter off the ground in that example.
3. The limitation of 2, is that in this mode we currently don't keep the camera above terrain. (which is on the todo list).

[pjcozzi]

@alexander-entin thanks for the report. This is fixed in master, and will be in Cesium 1.14 on October 1.