Alright, let's break down your questions one by one and fix the camera navigation issue you're working on with Forge Viewer and Three.js:

1. How to Navigate Camera Along the A-B Path Direction #

Forge Viewer's camera is actually a Three.js PerspectiveCamera under the hood, so you can directly manipulate it or use Viewer's built-in navigation utilities. Here's a practical way to animate the camera along your A-B line:

First, grab the start (A) and end (B) points (you already have these from bounding box centers):

const pointA = getBoundingBox([2112]).center(); // Your existing center point
const pointB = getBoundingBox([2109]).center();
const camera = NOP_VIEWER.navigation.getCamera();
const direction = pointB.clone().sub(pointA).normalize(); // Direction vector from A to B

Then, create an animation loop to move the camera incrementally. We'll use requestAnimationFrame and linear interpolation for smooth movement:

let progress = 0;
const duration = 3000; // 3 seconds to travel from A to B
const startTime = Date.now();

function animateCamera() {
  const elapsed = Date.now() - startTime;
  progress = Math.min(elapsed / duration, 1);
  
  // Interpolate camera position between A and B
  const newPosition = pointA.clone().lerp(pointB, progress);
  
  // Update camera position, keep looking at pointB (adjust target as needed)
  camera.position.copy(newPosition);
  camera.lookAt(pointB);
  
  // Force Viewer to render the updated view
  NOP_VIEWER.impl.invalidate(true);
  
  if (progress < 1) {
    requestAnimationFrame(animateCamera);
  }
}

// Start the animation
animateCamera();

To prevent user input from interrupting the animation, temporarily lock Viewer's default navigation controls:

NOP_VIEWER.navigation.setIsLocked(true);
// Re-enable after animation ends: NOP_VIEWER.navigation.setIsLocked(false);

2. Get All Vectors/Points Along the Path for Camera Position, Target, and UpVector #

Since your path is a straight line between A and B, you don't need a precomputed list of points—generate them on the fly with linear interpolation (like we did above). For camera姿态:

• Position: Use pointA.lerp(pointB, t) where t ranges from 0 (start) to 1 (end)
• Target: Keep looking at pointB the whole time, or interpolate to look ahead along the path: newPosition.clone().add(direction.multiplyScalar(10))
• UpVector: By default, Viewer uses new THREE.Vector3(0, 1, 0) (Y-up). If your path is tilted, compute a perpendicular "upright" vector:

// Compute up vector perpendicular to the path direction
const up = new THREE.Vector3(0, 1, 0);
if (Math.abs(direction.dot(up)) > 0.9) {
  // Fallback to X-axis if path is almost vertical
  up.set(1, 0, 0);
}
up.cross(direction).normalize().cross(direction).normalize();
camera.up.copy(up);

3. Retrieve Vectors/Points Between A and B via Forge Viewer API (Using DBIDs) #

Forge Viewer doesn't have a built-in API to generate intermediate points between two DBID bounding box centers—since it's a straight line, compute these manually with interpolation (as shown above).

If you need the actual geometric vertices of the objects (not just bounding box centers), extract fragment geometry using the Viewer's FragmentList:

function getFragmentVertices(dbId) {
  const model = NOP_VIEWER.model;
  const fragList = model.getFragmentList();
  const fragIds = model.getFragmentList().getFragIds(dbId);
  const vertices = [];
  
  fragIds.forEach(fragId => {
    const geometry = fragList.getGeometry(fragId);
    const positionAttr = geometry.attributes.position;
    for (let i = 0; i < positionAttr.count; i++) {
      const vertex = new THREE.Vector3().fromBufferAttribute(positionAttr, i);
      fragList.transformVertex(fragId, vertex); // Convert to world space
      vertices.push(vertex);
    }
  });
  
  return vertices;
}

For your camera path use case, bounding box centers are sufficient unless you need to follow the actual surface of the objects.

4. Fixing Three.js + Forge Viewer Integration for Camera Path Movement #

Your existing Three.js dashed line code works great for visualizing the path, but remember: you don't need to manipulate Three.js scenes directly to move the camera—Viewer exposes its Three.js camera via NOP_VIEWER.navigation.getCamera(), so work with that directly.

Key integration tips:

• Always call NOP_VIEWER.impl.invalidate(true) after updating the camera to trigger a render
• Lock navigation controls during animations to avoid conflicts with user input
• When adding Three.js objects (like your dashed line), use NOP_VIEWER.impl.sceneAfter (which you're already doing) to ensure they render on top of the model

Full Combined Example #

Here's how to tie it all together: visualize the dashed path and animate the camera along it:

// 1. Create dashed line (your existing code)
function createDashedLine() {
  const geometry = new THREE.Geometry();
  const pointA = getBoundingBox([2112]).center();
  const pointB = getBoundingBox([2109]).center();
  geometry.vertices.push(pointA, pointB);
  
  const material = new THREE.LineDashedMaterial({
    color: 0xFF0000,
    dashSize: 3,
    gapSize: 1,
    transparent: true,
    depthWrite: false,
    depthTest: true
  });
  
  geometry.computeLineDistances();
  geometry.lineDistancesNeedUpdate = true;
  
  NOP_VIEWER.impl.matman().addMaterial('dashed-path-material', material, true);
  const line = new THREE.Line(geometry, material);
  
  NOP_VIEWER.impl.sceneAfter.skipDepthTarget = true;
  NOP_VIEWER.impl.sceneAfter.skipIdTarget = true;
  NOP_VIEWER.impl.sceneAfter.add(line);
  
  return { pointA, pointB };
}

// 2. Animate camera along path
function animateCameraAlongPath(pointA, pointB) {
  const camera = NOP_VIEWER.navigation.getCamera();
  const duration = 3000;
  const startTime = Date.now();
  const direction = pointB.clone().sub(pointA).normalize();
  
  // Compute upright up vector
  const up = new THREE.Vector3(0, 1, 0);
  if (Math.abs(direction.dot(up)) > 0.9) up.set(1, 0, 0);
  up.cross(direction).normalize().cross(direction).normalize();
  camera.up.copy(up);
  
  NOP_VIEWER.navigation.setIsLocked(true);
  
  function animate() {
    const elapsed = Date.now() - startTime;
    const progress = Math.min(elapsed / duration, 1);
    const newPos = pointA.clone().lerp(pointB, progress);
    
    camera.position.copy(newPos);
    camera.lookAt(pointB); // Or look ahead: newPos.clone().add(direction.multiplyScalar(50))
    
    NOP_VIEWER.impl.invalidate(true);
    
    if (progress < 1) {
      requestAnimationFrame(animate);
    } else {
      NOP_VIEWER.navigation.setIsLocked(false);
    }
  }
  
  animate();
}

// Run everything
const pathData = createDashedLine();
animateCameraAlongPath(pathData.pointA, pathData.pointB);

内容的提问来源于stack exchange，提问作者Ronak Shetiya