ECEF (EPSG:4978) glTF model doesn't show up in Cesium

Question

The issue:

I have a glTF (ver. 2.0) mesh (converted from ply to dae to glTF) with triangles coordinates in EPSG:4978 that doesn't show up in Cesium (ver. 1.42).

The code:

This is the code in the main js app:

var model = scene.primitives.add(Cesium.Model.fromGltf({
    url : 'meshes/MESH_4978.gltf', // MESH_3857.gltf
    show: true,
    modelMatrix : Cesium.Matrix4.IDENTITY,
    scale : 1.0,
    debugWireframe: true
}));

The glTF file itself is well rendered here: https://gltf-viewer.donmccurdy.com/ (but the horizontal plane is vertical at start).

And I can see that the glTF file is well loaded with a Status=200 in the Firefox browser network monitor (or in Chrome).

The question:

So, where am I wrong?
Which coordinates system does Cesium use?

Other try:

I also tried with mesh coordinates in EPSG:3857; the exact same thing happens, i.e. file is loaded but nothing shows up.

Answer 1

Cesium's lowest-level internal coordinate system is Cartesian3 coordinates: the Earth-fixed frame, typically with a WGS84 ellipsoid, optionally with terrain on top, expressed in 3D Cartesian coordinates.

So your modelMatrix : Cesium.Matrix4.IDENTITY statement is placing the origin of your glTF file at the center of the Earth's core.

You can try placing the model at a specific location like this:

// Radians, not degrees, for HPR here
var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);

// Degrees (since we asked) and meters, WGS84 lon/lat/alt here.
var origin = Cesium.Cartesian3.fromDegrees(-123.0744619, 44.0503706, height);

var modelMatrix = Cesium.Transforms.headingPitchRollToFixedFrame(origin, hpr);