3D Model Selection with Rectangle in Three.js

To implement a rectangular selection tool in Three.js, you need to determine which 3D objects are contained within your 2D selection rectangle on the screen. There are two main approaches you can take:

Approach 1: Screen-Space Selection

This approach checks if the screen-space coordinates of objects fall within your selection rectangle:

JAVASCRIPTCopied
function isInside(object, selectionRectangle) {
  // Get the camera and renderer dimensions
  const camera = yourCamera; // Your Three.js camera
  const renderer = yourRenderer; // Your Three.js renderer
  
  // Get the object's position in world space
  const position = new THREE.Vector3();
  object.getWorldPosition(position);
  
  // Project the 3D position to 2D screen space
  const screenPosition = position.clone().project(camera);
  
  // Convert to pixel coordinates
  const x = (screenPosition.x + 1) * renderer.domElement.width / 2;
  const y = (-screenPosition.y + 1) * renderer.domElement.height / 2;
  
  // Get rectangle bounds
  const rectBounds = selectionRectangle.getBoundingClientRect();
  // Or calculate from your selection rectangle's position and dimensions
  
  // Check if the point is inside the rectangle
  return (
    x >= rectBounds.left &&
    x <= rectBounds.right &&
    y >= rectBounds.top &&
    y <= rectBounds.bottom
  );
}

Approach 2: Raycasting with Frustum

For more accurate selection, especially with complex objects:

JAVASCRIPTCopied
function getObjectsInRectangle(objects, selectionRectangle) {
  // Get rectangle corners in normalized device coordinates (-1 to 1)
  const rect = selectionRectangle.getBoundingClientRect();
  const renderer = yourRenderer;
  
  const left = (rect.left / renderer.domElement.clientWidth) * 2 - 1;
  const right = (rect.right / renderer.domElement.clientWidth) * 2 - 1;
  const top = -(rect.top / renderer.domElement.clientHeight) * 2 + 1;
  const bottom = -(rect.bottom / renderer.domElement.clientHeight) * 2 + 1;
  
  // Create frustum from the selection rectangle
  const frustum = new THREE.Frustum();
  const camera = yourCamera;
  
  // Create camera for the selection area
  const selectionCamera = camera.clone();
  
  // Adjust the projection matrix to match the selection area
  selectionCamera.projectionMatrix.makeOrthographic(
    left, right, top, bottom, camera.near, camera.far
  );
  
  // Update the frustum with the selection camera's matrix
  frustum.setFromProjectionMatrix(
    new THREE.Matrix4().multiplyMatrices(
      selectionCamera.projectionMatrix,
      selectionCamera.matrixWorldInverse
    )
  );
  
  // Filter objects that are inside the frustum
  return objects.filter(object => {
    // For better accuracy, check if the object's bounding box/sphere is in frustum
    if (!object.geometry) return false;
    
    if (!object.geometry.boundingSphere) {
      object.geometry.computeBoundingSphere();
    }
    
    const boundingSphere = object.geometry.boundingSphere.clone();
    boundingSphere.applyMatrix4(object.matrixWorld);
    
    return frustum.intersectsSphere(boundingSphere);
  });
}

Complete Implementation Example

Here's a more complete implementation that combines both approaches:

JAVASCRIPTCopied
function getObjectsInRectangle(objects, startX, startY, endX, endY, camera, renderer) {
  // Normalize coordinates to -1 to 1
  const canvasRect = renderer.domElement.getBoundingClientRect();
  
  const normalizedStartX = ((startX - canvasRect.left) / canvasRect.width) * 2 - 1;
  const normalizedStartY = -((startY - canvasRect.top) / canvasRect.height) * 2 + 1;
  const normalizedEndX = ((endX - canvasRect.left) / canvasRect.width) * 2 - 1;
  const normalizedEndY = -((endY - canvasRect.top) / canvasRect.height) * 2 + 1;
  
  // Create selection frustum
  const selectionCamera = camera.clone();
  const minX = Math.min(normalizedStartX, normalizedEndX);
  const maxX = Math.max(normalizedStartX, normalizedEndX);
  const minY = Math.min(normalizedStartY, normalizedEndY);
  const maxY = Math.max(normalizedStartY, normalizedEndY);
  
  // Adjust projection matrix for the selection area
  if (camera.isOrthographicCamera) {
    selectionCamera.left = minX;
    selectionCamera.right = maxX;
    selectionCamera.top = maxY;
    selectionCamera.bottom = minY;
    selectionCamera.updateProjectionMatrix();
  } else {
    // For perspective camera
    selectionCamera.projectionMatrix.makeOrthographic(
      minX, maxX, maxY, minY, camera.near, camera.far
    );
  }
  
  const frustum = new THREE.Frustum();
  const matrix = new THREE.Matrix4().multiplyMatrices(
    selectionCamera.projectionMatrix,
    camera.matrixWorldInverse
  );
  frustum.setFromProjectionMatrix(matrix);
  
  // Filter objects
  return objects.filter(object => {
    // Skip objects without geometry
    if (!object.geometry) return false;
    
    // Ensure bounding sphere exists
    if (!object.geometry.boundingSphere) {
      object.geometry.computeBoundingSphere();
    }
    
    // Check if object's bounding sphere intersects the selection frustum
    const boundingSphere = object.geometry.boundingSphere.clone();
    boundingSphere.applyMatrix4(object.matrixWorld);
    
    return frustum.intersectsSphere(boundingSphere);
  });
}

This implementation creates a frustum from your selection rectangle and checks which objects' bounding spheres intersect with it, providing accurate selection in 3D space.