Heightmap Collision Detection in Three.js

You're experiencing a common issue with displacement maps in Three.js. The displacement map visually alters the geometry by moving vertices during rendering, but this doesn't automatically update the actual geometry data used for raycasting and collision detection.

The Problem

Based on your code:

JAVASCRIPTCopied
const geometry = new THREE.PlaneGeometry(20, 20, 100, 100);
const material = new THREE.MeshStandardMaterial({
    map: <my texture map>,
    displacementMap: <my greyscale heightmap>,
    displacementScale: 1,
});
const plane = new THREE.Mesh(geometry, material);
scene.add(plane);

The displacementMap is affecting the visual appearance of your mesh, but the underlying geometry vertices aren't being updated for physics calculations. As the documentation states:

The displacement map affects the position of the mesh's vertices, unlike other maps which only affect the light and shade of the material.

Solutions

Here are some approaches to improve your heightmap collision detection:

1. Apply Displacement to Actual Geometry

Instead of relying solely on the material's displacement effect, modify the actual geometry vertices:

JAVASCRIPTCopied
const geometry = new THREE.PlaneGeometry(20, 20, 100, 100);
const material = new THREE.MeshStandardMaterial({
    map: <my texture map>
});

// Load your heightmap as an image
const heightmapTexture = new THREE.TextureLoader().load('path/to/heightmap.png', (texture) => {
    // Create a canvas to read pixel data
    const canvas = document.createElement('canvas');
    const context = canvas.getContext('2d');
    canvas.width = texture.image.width;
    canvas.height = texture.image.height;
    context.drawImage(texture.image, 0, 0);
    const imageData = context.getImageData(0, 0, canvas.width, canvas.height).data;
    
    // Apply height data to geometry vertices
    const positions = geometry.attributes.position;
    for (let i = 0; i < positions.count; i++) {
        // Calculate texture coordinate
        const u = positions.getX(i) / 20 + 0.5;
        const v = positions.getY(i) / 20 + 0.5;
        
        // Sample height from texture
        const pixelX = Math.floor(u * canvas.width);
        const pixelY = Math.floor(v * canvas.height);
        const pixelIndex = (pixelY * canvas.width + pixelX) * 4;
        const height = imageData[pixelIndex] / 255; // Assuming grayscale
        
        // Apply height to vertex
        positions.setZ(i, height * displacementScale);
    }
    positions.needsUpdate = true;
    geometry.computeVertexNormals();
});

const plane = new THREE.Mesh(geometry, material);
scene.add(plane);

2. Reduce Collision Geometry Complexity

Create a simplified collision mesh with fewer triangles:

JAVASCRIPTCopied
// Visual high-detail mesh
const visualGeometry = new THREE.PlaneGeometry(20, 20, 100, 100);
const visualMaterial = new THREE.MeshStandardMaterial({
    map: <my texture map>,
    displacementMap: <my greyscale heightmap>,
    displacementScale: 1,
});
const visualPlane = new THREE.Mesh(visualGeometry, visualMaterial);
scene.add(visualPlane);

// Collision mesh with fewer triangles
const collisionGeometry = new THREE.PlaneGeometry(20, 20, 20, 20);
// Apply displacement to collision geometry vertices (similar to method 1)
// ...
const collisionMaterial = new THREE.MeshBasicMaterial({ visible: false });
const collisionPlane = new THREE.Mesh(collisionGeometry, collisionMaterial);
scene.add(collisionPlane);

// Use collisionPlane for raycasting

3. Height Sampling for Player Position

For FPS movement, you could directly sample the heightmap at the player's position:

JAVASCRIPTCopied
function updatePlayerHeight(x, z) {
    // Convert world position to UV coordinates on the heightmap
    const u = (x / 20) + 0.5;
    const v = (z / 20) + 0.5;
    
    // Sample height from heightmap
    const height = getHeightAt(u, v);
    
    // Set player Y position
    player.position.y = height + playerHeight;
}

// Call this in your game loop
updatePlayerHeight(player.position.x, player.position.z);

4. Use a Height Field

For more complex terrain, consider implementing a height field data structure that can be queried efficiently:

JAVASCRIPTCopied
class HeightField {
    constructor(width, depth, heightData) {
        this.width = width;
        this.depth = depth;
        this.heightData = heightData; // 2D array of height values
    }
    
    getHeight(x, z) {
        // Convert world coordinates to grid coordinates
        const gridX = Math.floor((x / this.width + 0.5) * (this.heightData[0].length - 1));
        const gridZ = Math.floor((z / this.depth + 0.5) * (this.heightData.length - 1));
        
        // Clamp to valid indices
        const clampedX = Math.max(0, Math.min(gridX, this.heightData[0].length - 1));
        const clampedZ = Math.max(0, Math.min(gridZ, this.heightData.length - 1));
        
        return this.heightData[clampedZ][clampedX];
    }
}

Performance Considerations

1. Reduce the number of raycasts by performing them less frequently
2. Use a lower resolution collision mesh
3. Implement spatial partitioning (quadtree/octree) for large terrains
4. Consider using a physics engine like Ammo.js or Cannon.js for more complex terrain

These approaches should help you achieve more accurate and efficient collision detection with your heightmap terrain.