Implementing Varying Opacity in InstancedMesh

Based on the information available, you can't directly set per-instance opacity in an InstancedMesh using the standard material properties. The opacity property in materials like MeshPhongMaterial applies globally to all instances.

Solution: Using an Alpha Map

The most effective approach would be to use an alpha map texture with custom shader material. Here's how you could implement this:

1. Create a custom shader material that supports per-instance opacity
2. Use instance attributes to pass opacity values to the shader

Example Implementation

JAVASCRIPTCopied
// Create a custom shader material
const material = new THREE.ShaderMaterial({
  uniforms: {
    // Your other uniforms
  },
  vertexShader: `
    attribute float instanceOpacity;
    varying float vOpacity;
    
    void main() {
      vOpacity = instanceOpacity;
      // Your standard vertex shader code
      gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
    }
  `,
  fragmentShader: `
    varying float vOpacity;
    
    void main() {
      // Your fragment shader code
      gl_FragColor = vec4(1.0, 1.0, 1.0, vOpacity);
    }
  `,
  transparent: true
});

// Create geometry with an additional buffer attribute for opacity
const geometry = new THREE.BoxBufferGeometry(1, 1, 1);
const opacityArray = new Float32Array(16); // For 16 instances

// Set opacity values
for (let i = 0; i < 16; i++) {
  const distance = i % 4;
  opacityArray[i] = Math.max(0, 1 - distance / 3);
}

// Add the opacity attribute to the geometry
geometry.setAttribute('instanceOpacity', new THREE.InstancedBufferAttribute(opacityArray, 1));

// Create the instanced mesh
const mesh = new THREE.InstancedMesh(geometry, material, 16);

// Set positions
for (let i = 0; i < 16; i++) {
  const matrix = new THREE.Matrix4().makeTranslation(
    i % 4, 
    0, 
    Math.floor(i / 4)
  );
  mesh.setMatrixAt(i, matrix);
}

mesh.instanceMatrix.needsUpdate = true;
scene.add(mesh);

Alternative Approach

If you prefer not to use custom shaders, another approach would be to use multiple meshes instead of an InstancedMesh, which would allow you to set individual opacity values:

JAVASCRIPTCopied
for (let i = 0; i < 16; i++) {
  const material = new THREE.MeshPhongMaterial({
    transparent: true,
    opacity: Math.max(0, 1 - (i % 4) / 3)
  });
  
  const geometry = new THREE.BoxBufferGeometry(1, 1, 1);
  const mesh = new THREE.Mesh(geometry, material);
  
  mesh.position.set(i % 4, 0, Math.floor(i / 4));
  scene.add(mesh);
}

However, this approach is less efficient for large numbers of objects compared to using InstancedMesh with custom shaders.

The custom shader approach is more complex but provides the best performance while allowing per-instance opacity control.