Update Three.js Camera Projection Matrix Dynamically

When working with Three.js, modifying camera properties like aspect ratio, field of view (FOV), near plane, or far plane at runtime will not automatically update the rendered scene. To apply these changes, you must manually trigger an update to the camera’s projection matrix. This article explains why this manual update is necessary, identifies the key properties that require it, and provides clear code examples demonstrating how to implement the update in your projects.

Why You Must Update the Projection Matrix

Three.js caches the camera’s projection matrix to optimize rendering performance. Computing a projection matrix involves complex trigonometric and algebraic calculations. Rather than recalculating this matrix on every single frame, Three.js only computes it once when the camera is created.

If you dynamically change any property that affects the camera’s frustum (the visible volume of the 3D space), Three.js will continue to use the old, cached projection matrix. This results in stretched, distorted, or incorrectly clipped visuals until you explicitly tell the library to recalculate the matrix.

The Solution: updateProjectionMatrix()

To apply changes made to your camera properties, you must call the .updateProjectionMatrix() method on your camera object immediately after making the modifications.

Common Properties Requiring an Update:

Code Example: Handling Window Resize

The most common scenario for updating the camera projection matrix is handling a browser window resize event. When the window size changes, the camera’s aspect ratio must be recalculated to prevent the 3D scene from stretching.

// Initialize camera
const camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 1000);

// Event listener for window resize
window.addEventListener('resize', () => {
    const width = window.innerWidth;
    const height = window.innerHeight;

    // 1. Update the renderer size
    renderer.setSize(width, height);

    // 2. Update the camera's aspect ratio property
    camera.aspect = width / height;

    // 3. Recalculate the projection matrix to apply the change
    camera.updateProjectionMatrix();
});

Code Example: Dynamically Changing FOV (Zooming)

You can also use this method to create a smooth zoom effect by dynamically changing the camera’s Field of View (FOV) based on user input, such as scrolling.

window.addEventListener('wheel', (event) => {
    // Decrease FOV to zoom in, increase to zoom out
    if (event.deltaY < 0) {
        camera.fov = Math.max(30, camera.fov - 2); // Cap minimum FOV
    } else {
        camera.fov = Math.min(100, camera.fov + 2); // Cap maximum FOV
    }

    // Apply the FOV change
    camera.updateProjectionMatrix();
});

By calling camera.updateProjectionMatrix() whenever you alter these key properties, you ensure that Three.js accurately reflects your camera adjustments in real time.