Hidden-Line Elimination

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

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Rasterises every triangle of a source SceneModel into an orthographic HLEDepthBuffer along an arbitrary projection basis, then exposes point and edge visibility tests downstream stages use 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

The orchestrating buildDrawing entry point hides these calls, but they are also useful standalone — for instance to test custom annotation geometry against a model's visibility along an arbitrary direction.

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 — fine for a single drawing on a desktop. For very-high-resolution drawings on dense BIM models, prefer the tile-at-a-time fills extractor which keeps peak memory bounded at O(tileSize²) regardless of resolution.