Cesium function to transform relative heading-pitch-roll to absolute rotation Quaternion

relativeOrientation.ts

import {
    Cartesian3,
    HeadingPitchRoll,
    Matrix3,
    Matrix4,
    Quaternion,
    Transforms,
} from "cesium";

/**
 * Convert a local heading/pitch/range rotation to an earth-axis-relative Quaternion
 * @param position a Cartesian3 point in space
 * @param heading degrees from North, rotation around Earth-normal vector
 * @param pitch degrees from horizontal, rotation around earth-tangent X axis
 * @param roll degrees from vertical, rotation around earth-tangent Y axis
 */
export function relativeOrientation(position: Cartesian3,
        heading: number, pitch: number, roll: number): Quaternion {
    // Build a Quaternion that represents the rotation between absolute and
    // local-surface frames of reference
    const transform4 = Transforms.eastNorthUpToFixedFrame(position);
    const transform3 = new Matrix3();
    Matrix4.getRotation(transform4, transform3);
    const transformQ = Quaternion.fromRotationMatrix(transform3);

    // Convert the h/p/r values to a (local-reference) Quaternion
    const localHpr = HeadingPitchRoll.fromDegrees(heading, pitch, roll);
    const localQ = Quaternion.fromHeadingPitchRoll(localHpr);

    // Result is local times transform
    const ret = new Quaternion();
    Quaternion.multiply(localQ, transformQ, ret);
    return ret;
}