Create Cesium.Ray from Cesium.Cartesian3 and Cesium.HeadingPitchRoll

Yes, it is possible. These two kinds of objects hold different kinds of information, so be careful that you're not losing data in the conversion.

• A Ray has an origin (Cartesian position) and a direction which is just a unit vector, not the same as a full orientation.

• The HeadingPitchRoll is a full orientation. The Heading and Pitch work together to identify a particular "forward" direction, and Roll indicates a rotation about that direction vector. This Roll rotation does not affect the direction, and so will not have an effect on a Ray.

Here's the code:

// A Cesium Ray consists of an "origin" and a "direction".
// The origin is just a Cartesian3 from anywhere.
var origin = new Cesium.Cartesian3(1.0, 2.0, 3.0);

// For this question, "direction" comes from a HeadingPitchRoll.
// Let's establish an HPR here.
var heading = Cesium.Math.toRadians(0);  // YOUR VALUE HERE
var pitch = Cesium.Math.toRadians(0);    // YOUR VALUE HERE
var roll = Cesium.Math.toRadians(0);     // This one does nothing.

var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);

// Next we need a direction vector from the HPR.  We'll use
// a rotation matrix (3x3) to do the conversion.
var matrix3 = Cesium.Matrix3.fromHeadingPitchRoll(hpr);

// The UNIT_X vector is "forward" in Cesium.  We'll multiply the rotation
// matrix by that unit vector, to get our direction vector.
var direction = Cesium.Matrix3.multiplyByVector(matrix3, Cesium.Cartesian3.UNIT_X,
    new Cesium.Cartesian3());

// Now we can construct the Ray.
var ray = new Cesium.Ray(origin, direction);

console.log('Ray: ' + JSON.stringify(ray));

If you run this with the default zero heading/pitch shown here, you'll get the unit X vector back again:

Ray: {"origin":{"x":1,"y":2,"z":3},"direction":{"x":1,"y":0,"z":0}}

But if you set the pitch to 45 degress:

var heading = Cesium.Math.toRadians(0);
var pitch = Cesium.Math.toRadians(45);

You'll see the X direction has pitched up towards +Z.

Ray: {"origin":{"x":1,"y":2,"z":3},"direction":{"x":0.7071067811865476,"y":0,"z":0.7071067811865475}}

A positive heading will rotate the X vector towards -Y (clockwise as seen from +Z above).

var heading = Cesium.Math.toRadians(45);
var pitch = Cesium.Math.toRadians(0);

Ray: {"origin":{"x":1,"y":2,"z":3},"direction":{"x":0.7071067811865476,"y":-0.7071067811865475,"z":0}}

EDIT: The question was revised a bit. The Ray's direction needs to be in Earth-Centered Fixed frame, not local-up frame, for the intended use.

Here's a longer demo that shows how to handle this. You can also view a live demo on Sandcastle.

var viewer = new Cesium.Viewer("cesiumContainer");

const i = {
  id: 'test',
  longitude: -79,
  latitude: 40,
  altitude: 500
};

// Create the origin and hpr.
const origin = Cesium.Cartesian3.fromDegrees(i.longitude, i.latitude, i.altitude)

let hpr = Cesium.HeadingPitchRoll.fromDegrees(0, 0, 0)

// Cesium uses East-North-Up axes, where East is zero.  If you want North == Zero,
// you must subtract 90 degrees' worth of radians (pi/2).
hpr.heading -= Cesium.Math.PI_OVER_TWO

// We'll get the orientation, which is a quaternion.  This is specific to the
// location on the planet.  Also it comes in handy later for the 3D model vis.
const orientation = Cesium.Transforms.headingPitchRollQuaternion(origin, hpr);

// Get a Matrix3 version of that Quaternion.
const matrix3 = Cesium.Matrix3.fromQuaternion(orientation);

// The UNIT_X vector is "forward" in Cesium.  We'll multiply the rotation
// matrix by that unit vector, to get our direction vector.
const direction = Cesium.Matrix3.multiplyByVector(
  matrix3,
  Cesium.Cartesian3.UNIT_X,
  new Cesium.Cartesian3()
)


// Now we can construct the Ray.
const ray = new Cesium.Ray(origin, direction)

const finalPoint = Cesium.Ray.getPoint(ray, i.altitude)
const e = viewer.entities.add({
  name: i.id,
  polyline: {
    positions: [origin, finalPoint],
    width: 4,
    arcType: Cesium.ArcType.NONE,
    material: new Cesium.PolylineArrowMaterialProperty(Cesium.Color.RED),
  },
})

// Optionally, show an aircraft model with this HPR.
const m = viewer.entities.add({
  position: origin,
  orientation: orientation,
  model: {
    uri: "../SampleData/models/CesiumAir/Cesium_Air.glb",
    minimumPixelSize: 64,
  }
})

viewer.zoomTo(e)