Hey there! Let's tackle that T-junction crack issue in your quad tree terrain LOD—super common problem, but totally fixable once you know the right tricks.

First, let's break down why this is happening: your current setup gives each quad tree node independent vertices, so when adjacent nodes are at different LOD levels, the subdivided edges of a higher-LOD node don't line up with the coarser edges of its lower-LOD neighbor. That mismatch creates the T-junctions and visible cracks.

Here are actionable solutions you can implement, ordered by practicality:

1. Enforce Maximum LOD Difference Between Neighbors #

The simplest way to reduce T-junctions is to ensure adjacent nodes never differ by more than one LOD level. This limits the complexity of fixing gaps, as you only have to handle small, predictable mismatches.

• Modify your fillTree method to check the LOD depth of neighboring nodes before subdividing a current node.
• If a neighbor is one level higher (more subdivided), force the current node to subdivide its shared boundary to match. If a neighbor is one level lower, keep the current node's boundary at the coarser level to align.

2. Share Boundary Vertex Indices #

Your current approach of having independent vertices per node is a root cause—identical boundary positions are treated as separate vertices with different indices. Fix this by:

• Creating a global vertex pool (a dictionary mapping coordinate tuples to vertex indices) when building your tree.
• When generating a node's boundary vertices, first check if the coordinate already exists in the pool. If yes, reuse the existing index; if not, add the new vertex to the pool and use its new index.
• Pro tip: Add a small epsilon (like 1e-6) when comparing coordinates to avoid false mismatches from floating-point precision errors.

3. Add Stitching Edges/Faces #

For cases where you can't avoid small LOD differences, generate "stitching" geometry to fill the gap between mismatched edges:

• After building a node's children, check each edge against its neighboring node's edge.
• If the neighbor is higher-LOD, create additional triangles or lines that connect the coarser node's boundary vertices to the subdivided vertices of the higher-LOD neighbor.
• Example: If your parent node's right edge is a single line, but its right neighbor is a subdivided child with two right-edge vertices, add two triangles connecting the parent's right edge endpoints to the neighbor's middle vertex.

4. Adjust Your Node Partition Logic #

Tweak how you split nodes to prioritize boundary alignment:

• When partitioning a node into four children, ensure the boundary vertices of the children exactly match the parent's boundary vertices (and any neighbors' boundaries).
• Store neighbor references in each node (e.g., node.neighbors = { top: ..., right: ... }) so you can quickly access adjacent nodes during the fill process.

Here's a quick snippet showing how you might add neighbor checks to your fillTree method:

QuadTree.prototype.fillTree = function(currentNode, depth, currentBox, currentIndex, neighbors = {}) {
  // Stop subdividing if we hit max depth or can't partition, but first adjust boundaries
  if (depth === 0 || !currentBox.checkPartition()) {
    this.adjustBoundaryToMatchNeighbors(currentNode, neighbors);
    return currentNode;
  }

  const node = { 
    vertices: [], 
    lines: [], 
    children: [], 
    box: currentBox,
    neighbors: neighbors // Track adjacent nodes for later checks
  };

  // Split into child boxes and generate neighbor references for each child
  const childBoxes = currentBox.partition();
  const childNeighbors = this.generateChildNeighbors(node, neighbors);

  // Recursively fill child nodes
  for (let i = 0; i < 4; i++) {
    const child = this.fillTree({}, depth - 1, childBoxes[i], currentIndex + i, childNeighbors[i]);
    node.children.push(child);
  }

  // Stitch child nodes to avoid gaps at boundaries
  this.stitchChildNodeBoundaries(node);

  return node;
};

Start with enforcing max LOD difference and sharing boundary vertices—those two changes alone will eliminate most cracks. Add stitching edges if you still have minor gaps left.

内容的提问来源于stack exchange，提问作者DomainFlag