In the world of 3D web experiences, users rarely want to just passively observe. They want to interact! Whether it's clicking on an object, dragging it around, or moving their mouse to control a camera, interactivity is key to engaging your audience. Three.js, when combined with browser event handling, provides powerful ways to bring your 3D scenes to life with user input.
The core idea behind implementing event listeners in Three.js is to leverage the browser's standard event system and then translate those events into actions within your 3D scene. This involves listening for events like 'click', 'mousemove', 'keydown', and 'keyup' on the HTML element that hosts your Three.js canvas. However, for more advanced 3D-specific interactions, especially those involving raycasting, we often need to work with Three.js's own event management utilities.
Let's start with the basics: listening for DOM events directly. You can attach event listeners to your renderer's DOM element, which is typically a <canvas>.
const canvas = renderer.domElement;
canvas.addEventListener('click', (event) => {
console.log('Canvas clicked!');
});This is a simple starting point. When the user clicks anywhere on the canvas, the message 'Canvas clicked!' will appear in the console. But how do we know what was clicked in the 3D scene?
This is where the concept of raycasting becomes crucial. Raycasting in Three.js allows you to simulate a ray of light emanating from a point (like the camera or a mouse cursor) into the 3D scene. You can then determine which objects the ray intersects with. This is the foundation for click detection on 3D objects.
graph TD
A[User Clicks Canvas] --> B{Get Mouse Coordinates}
B --> C[Create Raycaster]
C --> D[Set Raycaster Origin & Direction]
D --> E[Perform Intersection Test]
E --> F{Objects Intersected?}
F -- Yes --> G[Identify Clicked Object(s)]
F -- No --> H[No Object Clicked]
To implement raycasting for click detection, you'll need to: