How to Perform Slerp with Ammo.js Quaternions

Spherical linear interpolation (slerp) is a mathematical technique used in 3D computer graphics and physics to smoothly transition between two rotations. This article explains how to perform a slerp operation using quaternions in ammo.js, the JavaScript port of the Bullet physics engine. You will learn how to initialize the required quaternions, apply the interpolation formula using the built-in library methods, and properly manage WebAssembly memory.

To perform a slerp in ammo.js, you utilize the slerp method built into the btQuaternion class. This method calculates a quaternion that lies along the shortest spherical path between two given orientations based on an interpolation factor \(t\) (where \(0.0\) represents the start orientation and \(1.0\) represents the end orientation).

Step-by-Step Implementation

Here is how to set up and execute the slerp operation in JavaScript:

// 1. Initialize the start and end quaternions
// Example: qStart represents no rotation, qEnd represents a 180-degree rotation around the Y-axis
const qStart = new Ammo.btQuaternion(0, 0, 0, 1);
const qEnd = new Ammo.btQuaternion(0, 1, 0, 0);

// 2. Define the interpolation factor (t) between 0.0 and 1.0
const t = 0.5; // Represents the exact midpoint (90-degree rotation)

// 3. Perform the spherical linear interpolation
// The slerp method returns a new btQuaternion object on the WebAssembly heap
const qResult = qStart.slerp(qEnd, t);

// 4. Access the interpolated quaternion values
console.log(`Result: x=${qResult.x()}, y=${qResult.y()}, z=${qResult.z()}, w=${qResult.w()}`);

// 5. Clean up allocated memory to prevent memory leaks
Ammo.destroy(qStart);
Ammo.destroy(qEnd);
Ammo.destroy(qResult);

Important Memory Management

Because ammo.js is a WebAssembly/asm.js wrapper of C++ code, JavaScript’s automatic garbage collection does not clean up ammo.js objects. Every time you instantiate a quaternion using new Ammo.btQuaternion() or generate a new one via the slerp method, memory is allocated on the WebAssembly heap. You must explicitly call Ammo.destroy(object) on these quaternions once they are no longer needed to prevent memory leaks in your application.