Improving Drag-and-Drop Placement in Three.js

Based on your description, you're looking to enhance your drag-and-drop functionality to intelligently place objects in empty spaces rather than always at the top of the structure. While the provided context doesn't include specific implementation details for this exact scenario, I can suggest an approach using Three.js DragControls and raycasting.

Suggested Approach

You could implement a grid-based or collision detection system that identifies empty spaces in your structure:

1. Create a Grid System:

   • Divide your placement area into a grid
   • Track occupied and empty grid cells

2. Use Raycasting During Drag:

   • During the drag event, use raycasting to determine potential placement positions
   • Check for intersections with existing objects

3. Find Nearest Empty Space:

   • When the user drags an object, calculate the nearest valid empty space
   • Highlight this space to provide visual feedback

Implementation Example

Here's a basic implementation approach:

JAVASCRIPTCopied
// Set up DragControls
const dragControls = new THREE.DragControls(draggableObjects, camera, renderer.domElement);

// Create a grid system to track occupied spaces
const grid = createGridSystem(sceneWidth, sceneHeight, cellSize);

// Track drag state
let isDragging = false;
let draggedObject = null;

// Event listeners
dragControls.addEventListener('dragstart', function(event) {
  isDragging = true;
  draggedObject = event.object;
  
  // Optional: Change appearance of dragged object
  draggedObject.material.opacity = 0.5;
});

dragControls.addEventListener('drag', function(event) {
  // Use raycasting to find potential placement positions
  const raycaster = new THREE.Raycaster();
  raycaster.setFromCamera(mouse, camera);
  
  // Find intersections with placement plane
  const intersects = raycaster.intersectObject(placementPlane);
  
  if (intersects.length > 0) {
    const point = intersects[0].point;
    
    // Find nearest empty grid cell
    const nearestEmptyCell = findNearestEmptyCell(grid, point);
    
    // Update visual indicator for placement
    updatePlacementIndicator(nearestEmptyCell);
  }
});

dragControls.addEventListener('dragend', function(event) {
  isDragging = false;
  
  // Reset appearance
  draggedObject.material.opacity = 1.0;
  
  // Get final placement position
  const finalPosition = getPlacementPosition();
  
  // Place object at final position
  draggedObject.position.copy(finalPosition);
  
  // Update grid to mark this cell as occupied
  updateGrid(grid, finalPosition);
  
  draggedObject = null;
});

// Helper function to find nearest empty cell
function findNearestEmptyCell(grid, point) {
  // Convert world position to grid coordinates
  const gridX = Math.floor(point.x / cellSize);
  const gridY = Math.floor(point.y / cellSize);
  
  // Search for nearest empty cell, prioritizing cells below the cursor
  // This is where you'd implement your gap-filling logic
  
  // Example: Simple search for empty cells below current position
  for (let y = gridY; y >= 0; y--) {
    if (!grid[gridX][y].occupied) {
      return grid[gridX][y];
    }
  }
  
  // If no empty cells found below, search in other directions
  // ...
}

Advanced Considerations

1. Physics-Based Placement:

   • Consider using a physics engine like Ammo.js or Cannon.js for more realistic placement

2. Visual Feedback:

   • Highlight potential placement spots
   • Show a ghost/preview of where the object will land

3. Collision Detection:

   • Implement more sophisticated collision detection to handle complex shapes

4. Optimization:

   • For large structures, consider spatial partitioning to improve performance

This approach should help you create a more intuitive drag-and-drop system that fills gaps rather than always placing objects at the top of the structure.