Handling user input is a central part of building interactive 3D applications. In React, you typically handle user events such as mouse clicks, movement, and keyboard input using event handlers attached to DOM elements. When working with Three.js inside a React app, you must bridge the gap between standard DOM events and your 3D scene. React allows you to attach event listeners to the canvas element where your Three.js scene is rendered. For instance, you can use the onClick, onMouseMove, or onPointerDown props on the canvas to capture user actions. Once you capture these events, you need to translate the 2D screen coordinates of the event (such as the mouse position) into the 3D space of your scene. This is where raycasting becomes essential.

Raycasting is a technique that lets you detect which 3D objects in your scene are under a specific point on the screen, such as the mouse cursor. When a user interacts with the canvas, you can use the mouse position to create a ray that projects from the camera into your 3D scene. The raycaster checks which objects intersect with this ray, allowing you to determine, for example, which mesh the user clicked on. This is especially powerful in Three.js, where you can create a THREE.Raycaster instance, set its origin and direction based on the mouse position and camera, and then call its intersectObjects method to get a list of intersected meshes. By connecting these intersections to your React state or event handlers, you can trigger animations, highlight objects, or perform other interactive effects in response to user input.