Maintaining Constant Visual Size with OrthographicCamera

When working with an OrthographicCamera in Three.js, maintaining a constant visual size for objects during zoom operations requires a specific approach. Since you're using OrbitControls with an orthographic camera, you'll need to adjust the scale of your sphere inversely proportional to the camera's zoom level.

Solution Approach

For an orthographic camera, the relationship between zoom and apparent size is direct. When you zoom in (increasing the zoom value), objects appear larger. To maintain a constant visual size, you need to scale your object down by the same factor.

Here's how you can implement this:

JAVASCRIPTCopied
// Create your sphere
const sphereGeometry = new THREE.SphereGeometry(1, 32, 32);
const sphereMaterial = new THREE.MeshBasicMaterial({ color: 0xff0000 });
const sphere = new THREE.Mesh(sphereGeometry, sphereMaterial);
scene.add(sphere);

// Set initial size
const initialZoom = camera.zoom;
const initialSize = 1; // Your desired visual size

// In your animation/render loop
function animate() {
    requestAnimationFrame(animate);
    
    // Calculate scale factor based on camera zoom
    const scaleFactor = initialZoom / camera.zoom;
    
    // Apply scale to maintain constant visual size
    sphere.scale.set(scaleFactor, scaleFactor, scaleFactor);
    
    renderer.render(scene, camera);
}

Alternative Approach: Using a Separate Scene

Although you mentioned preferring not to use a separate scene, it's worth noting that this is actually a common approach for UI elements that need to maintain constant size. You would:

1. Create a separate scene and camera for your constant-size elements
2. Render this scene on top of your main scene
3. The separate camera wouldn't be affected by zoom operations

Using ArcballControls

If you're open to switching from OrbitControls to ArcballControls, you could take advantage of its properties to help with this issue. ArcballControls has properties like enableZoom and scaleFactor that can be configured to help maintain visual consistency.

For example:

JAVASCRIPTCopied
const controls = new THREE.ArcballControls(camera, renderer.domElement);
controls.enableZoom = true;
controls.scaleFactor = 1.1; // Adjust as needed

// Then in your update function, you can adjust your sphere based on controls state

Handling Window Resize

Don't forget to handle window resize events, as they can also affect the apparent size of objects with an orthographic camera:

JAVASCRIPTCopied
window.addEventListener('resize', () => {
    // Update camera aspect ratio and other properties
    camera.left = -window.innerWidth / 2;
    camera.right = window.innerWidth / 2;
    camera.top = window.innerHeight / 2;
    camera.bottom = -window.innerHeight / 2;
    camera.updateProjectionMatrix();
    
    // Recalculate object scale if needed
    const scaleFactor = initialZoom / camera.zoom;
    sphere.scale.set(scaleFactor, scaleFactor, scaleFactor);
    
    renderer.setSize(window.innerWidth, window.innerHeight);
});

This approach should give you a more accurate way to maintain constant visual size for your sphere regardless of zoom level.