Trying to implement an entity pov/third person view

CesiumJS
1

Hi all,

we are trying to implement a POV view for a tracking test.

The goal is to have an entity visualized by a model and the camera following a short distance behind, like a third person view where the model is still visble.

We have everything working, the path, the model moving smoothly along the path, but when trying to see what the model sees we encounter some ugly jumps and ~180 deg turns of the camera.

// The entity

var entity = this.cesium.viewer.entities.add({

availability: new Cesium.TimeIntervalCollection([new Cesium.TimeInterval({

start: startTime,

stop: stopTime
})]),

position: new Cesium.SampledPositionProperty(),

show: true,

orientation: new Cesium.VelocityOrientationProperty(entity.position), // this is reset each time the position of the entity is updated

model: {

uri: ‘/model.gltf’,

minimumPixelSize: 64

},

path: {

resolution: 5,

material: new Cesium.PolylineOutlineMaterialProperty({

color: cesiumColor

}),

width: 5,

leadTime: 0,

trailTime: settings.trailTime,

heightReference: Cesium.HeightReference.CLAMP_TO_GROUND

}

});

// --------------------------------------------------------
// This is running each camera tick

var direction = entity.orientation.getValue(time);

var angle = Cesium.Quaternion.computeAngle(direction);

// bad way to turn camera by 180 deg

angle = Cesium.Math.toRadians(Cesium.Math.toDegrees(angle) + 180);

camera.lookAt(position, new Cesium.HeadingPitchRange(angle, cesium.viewer.camera.pitch, 10));

``

I looked at the orientation value for the time when the jumps occur and the Quaternions “w” value (which is used to calculate the angle) is jumping there from e.g. 0.4xxxx to -0.xxxx,

so most probably the problem is in the entities orientation - still I’m wondering why the model then moves smoothly.

I uploaded a small video showcasing what I’m talking about: https://youtu.be/F6RiGhSY5ws
Jumps happen at ~12sec and ~28sec.

Somewhere I heard it’s too possible to take the models matrix and move the camera with that information, but I’m not familiar enough how that would work.

Side question: How would i get the pitch of the model/path? That would be our next step after fixing this issue.

Would appreciate any help, thank you in advance.

2

I recently had to implement something like this, and I believe I ran into similar issues with the entity orientation. I believe the issue is happening from how the
VelocityOrientationProperty samples the position, (either sampling too frequently or not enough). If you can recreate this issue in a Sandcastle example, we can open a GitHub issue along with that video as a bug report.

In the mean time, what solved this problem for me was following the example in this VR Sandcastle:

This makes the camera smoothly follow an Entity’s path, with a fixed offset (in terms of position and orientation). You can see where it sets that up right around line 68.

Let me know if that helps. This sounds like a cool project you’re working on!