Hidden-Line Elimination

CPU-side orthographic depth-buffer rasterisation and edge visibility tests for the drawings pipeline.

Rasterises source SceneModel triangles into an orthographic HLEDepthBuffer along an arbitrary projection basis. Downstream stages use the resulting point and edge tests to drop occluded geometry.

The depth buffer carries "distance-toward-camera" values in row-major pixel order. When built with withOwners: true, it also carries a parallel per-pixel owner buffer that maps every frontmost pixel back to the SceneMesh that produced it; the sibling fills extractor reads this owner buffer to trace per-object silhouettes.

Entry points

• buildHLEDepthBuffer — rasterise the source model into a depth buffer (and optionally an owner buffer).
• isPointVisible — test a single world-space point against the buffer.
• visibleEdgeSegments — sample an edge and return its visible sub-segments as world-space endpoint pairs.

Usage

buildDrawing calls this internally. Use it directly when custom annotation or overlay geometry needs the same visibility test.

import {
  buildHLEDepthBuffer,
  visibleEdgeSegments
} from "@xeokit/sdk/presentations/drawings/hle";

// Top-down basis: looking along -Y, +X reads as right, -Z as up.
const basis = {
  right:   [1,  0,  0],
  up:      [0,  0, -1],
  forward: [0, -1,  0]
};

const sourceModel = scene.models["myModel"];
const aabb        = sourceModel.aabb;

const buffer = await buildHLEDepthBuffer(sourceModel, basis, aabb, {
  resolution:  2048,
  withOwners:  false
});

// Test an annotation line for visibility against the model.
const segments = visibleEdgeSegments(
  buffer,
  [ 10, 5,  0 ],  // edge start
  [ 10, 5, 20 ],  // edge end
  8,              // sample count
  0.01            // depth-bias tolerance (world units)
);

for (const { a, b } of segments) {
  // Emit each visible sub-segment as a LinesPrimitive geometry.
}
Copy

Resolution and memory

Buffer memory is 4 × width × height bytes (Float32 depths) plus an equal-size Int32 owner buffer when withOwners: true. At the default 2048-pixel longer axis, peak memory is ~16 MB for the depth buffer and another ~16 MB for owners. For high-resolution drawings on dense models, use the tile-at-a-time fills extractor to keep peak memory bounded at O(tileSize²) regardless of resolution.