Using Stencil Buffer in Three.js for Card Cutout Effect

Based on your request, you want to create an effect where a 3D object (cube) appears to be "cutting through" a card, similar to the dragon example in the article you shared. This can be achieved using the stencil buffer in Three.js.

Here's how you can implement this effect:

Implementation Steps

1. Set up the renderer with stencil buffer enabled
2. Create your card and cube objects
3. Use the stencil buffer to create the cutout effect
4. Render the scene with proper stencil operations

Code Implementation

JAVASCRIPTCopied
// 1. Set up renderer with stencil buffer
const renderer = new THREE.WebGLRenderer({
  antialias: true,
  stencil: true  // Enable stencil buffer
});
renderer.setSize(window.innerWidth, window.innerHeight);
document.body.appendChild(renderer.domElement);

// Clear settings
renderer.autoClear = false;
renderer.autoClearStencil = false;

// Create scene, camera, etc.
const scene = new THREE.Scene();
const camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 1000);
camera.position.z = 5;

// Create card (plane)
const cardGeometry = new THREE.PlaneGeometry(4, 6);
const cardMaterial = new THREE.MeshBasicMaterial({ 
  color: 0x2266aa,
  side: THREE.DoubleSide
});
const card = new THREE.Mesh(cardGeometry, cardMaterial);
scene.add(card);

// Create cube
const cubeGeometry = new THREE.BoxGeometry(1, 1, 1);
const cubeMaterial = new THREE.MeshBasicMaterial({ color: 0xff0000 });
const cube = new THREE.Mesh(cubeGeometry, cubeMaterial);
scene.add(cube);

// Animation loop
function animate() {
  requestAnimationFrame(animate);
  
  // Rotate cube
  cube.rotation.x += 0.01;
  cube.rotation.y += 0.01;
  
  // Clear buffers
  renderer.clear();
  
  // FIRST PASS: Write to stencil buffer
  // Set up stencil operations
  renderer.clearStencil();
  
  // Disable color and depth writing, enable stencil
  cubeMaterial.colorWrite = false;
  cubeMaterial.depthWrite = false;
  renderer.state.buffers.stencil.setTest(true);
  renderer.state.buffers.stencil.setOp(
    THREE.ReplaceStencilOp,  // stencil fail
    THREE.ReplaceStencilOp,  // depth fail
    THREE.ReplaceStencilOp   // pass
  );
  renderer.state.buffers.stencil.setFunc(
    THREE.AlwaysStencilFunc, // function
    1,                       // ref value
    0xff                     // mask
  );
  
  // Render cube to stencil buffer only
  renderer.render(scene, camera);
  
  // SECOND PASS: Render the card with stencil test
  // Reset cube material
  cubeMaterial.colorWrite = true;
  cubeMaterial.depthWrite = true;
  
  // Set stencil test to only render where stencil value is 0 (not where the cube is)
  renderer.state.buffers.stencil.setFunc(
    THREE.NotEqualStencilFunc, // function
    1,                         // ref value
    0xff                       // mask
  );
  
  // Render card with stencil test
  renderer.render(scene, camera);
  
  // THIRD PASS: Render the cube normally
  renderer.state.buffers.stencil.setTest(false);
  renderer.render(scene, camera);
}

animate();

Key Concepts

1. Enable stencil buffer: Make sure to create the renderer with stencil: true
2. Control auto-clearing: Set renderer.autoClear = false and renderer.autoClearStencil = false to manually control clearing
3. Multiple render passes:
   • First pass: Write to stencil buffer where the cube is
   • Second pass: Render the card with a stencil test to create the "hole"
   • Third pass: Render the cube normally

Adjustments

You may need to adjust the stencil operations and functions based on your specific needs. The key is to:

1. Use the cube to mark areas in the stencil buffer
2. Use that stencil information to control where the card renders
3. Finally render the cube normally

This approach should give you the effect of the cube appearing to cut through the card, similar to the dragon example in the article you referenced.