Jitter when clamping to ground

[mramato]

Noticed while reviewing #3903. Here's a sandcastle example which creates a circular flight with altitude of 0 and then clamps to terrain. There is really jerky camera or entity motion as it animates.

var viewer = new Cesium.Viewer('cesiumContainer', {
    shouldAnimate: true,
    terrainProviderViewModels : [], //Disable terrain changing
    infoBox : false //Disable InfoBox widget
});

//Enable lighting based on sun/moon positions
viewer.terrainProvider = Cesium.createWorldTerrain();

//Use STK World Terrain
viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
    url : 'https://assets.agi.com/stk-terrain/world',
    requestWaterMask : true,
    requestVertexNormals : true
});

//Enable depth testing so things behind the terrain disappear.
viewer.scene.globe.depthTestAgainstTerrain = true;

//Set the random number seed for consistent results.
Cesium.Math.setRandomNumberSeed(3);

//Set bounds of our simulation time
var start = Cesium.JulianDate.fromDate(new Date(2015, 2, 25, 16));
var stop = Cesium.JulianDate.addSeconds(start, 360, new Cesium.JulianDate());

//Make sure viewer is at the desired time.
viewer.clock.startTime = start.clone();
viewer.clock.stopTime = stop.clone();
viewer.clock.currentTime = start.clone();
viewer.clock.clockRange = Cesium.ClockRange.LOOP_STOP; //Loop at the end
viewer.clock.multiplier = 1;

//Set timeline to simulation bounds
viewer.timeline.zoomTo(start, stop);

//Generate a random circular pattern with varying heights.
function computeCirclularFlight(lon, lat, radius) {
    var property = new Cesium.SampledPositionProperty();
    for (var i = 0; i <= 360; i += 45) {
        var radians = Cesium.Math.toRadians(i);
        var time = Cesium.JulianDate.addSeconds(start, i, new Cesium.JulianDate());
        var position = Cesium.Cartesian3.fromDegrees(lon + (radius * 1.5 * Math.cos(radians)), lat + (radius * Math.sin(radians)), 0);
        property.addSample(time, position);

        //Also create a point for each sample we generate.
        viewer.entities.add({
            position : position,
            point : {
                pixelSize : 8,
                color : Cesium.Color.TRANSPARENT,
                outlineColor : Cesium.Color.YELLOW,
                outlineWidth : 3
            }
        });
    }
    return property;
}

//Compute the entity position property.
var position = computeCirclularFlight(-112.110693, 36.0994841, 0.03);

//Actually create the entity
var entity = viewer.entities.add({

    //Set the entity availability to the same interval as the simulation time.
    availability : new Cesium.TimeIntervalCollection([new Cesium.TimeInterval({
        start : start,
        stop : stop
    })]),

    //Use our computed positions
    position : position,

    //Automatically compute orientation based on position movement.
    orientation : new Cesium.VelocityOrientationProperty(position),

    //Load the Cesium plane model to represent the entity
    model : {
        uri : '../../SampleData/models/CesiumAir/Cesium_Air.gltf',
        minimumPixelSize : 64,
        heightReference: Cesium.HeightReference.CLAMP_TO_GROUND
    }
});

viewer.trackedEntity = entity;

[pjcozzi]

Looks like this happens on the first terrain height request of a new tile. To see it more clearly:

• Zoom out a bit so lower LOD tiles are loaded
• Make the animation speed something like 0.5x
• Use the Cesium inspector to turn on Terrain -> Show tile coordinates

viewer.extend(Cesium.viewerCesiumInspectorMixin);

[srcejon]

It seems there can be a lot of jitter even if we don't have any terrain loaded.

Sandcastle example

[srcejon]

There's also jitter if RELATIVE_TO_GROUND is used:

Sandcastle example

[srcejon]

Looks like this happens on the first terrain height request of a new tile. To see it more clearly:

• Zoom out a bit so lower LOD tiles are loaded
• Make the animation speed something like 0.5x
• Use the Cesium inspector to turn on Terrain -> Show tile coordinates

viewer.extend(Cesium.viewerCesiumInspectorMixin);

The amount of jitter seems to be reduced if "Suspend LOD update" is checked. However, there's still some larger jitter at the end of this clip:

It looks like it's the camera that jitters rather than the model, as the motion appears smooth when the camera isn't tracking the entity.