// devlog

Log

CAD viewer October 28, 2026 · 11 posts Building a browser STEP/IGES/BREP viewer on OCCT WASM and WebGPU — shipped milestone by milestone.

• #1 We're building a CAD viewer for the browser

  First entry in the DKOPA devlog: a fast STEP viewer running entirely client-side on WASM, headed for WebGPU.

• #2 First triangle: STEP in, pixels out

  The viewer scaffold is live at /viewer — drop a .step file and it parses entirely in your browser. Here's how the pipeline works.

• #3 Parts panel, edge lines, and the CAD look

  Second milestone: the viewer now shows the part structure from your STEP file and renders teal edge outlines. Also — IGES and BREP support snuck in.

• #4 Cutting the model open: the section plane

  Third milestone: a clipping plane that sweeps through the model on any axis. Almost free in Three.js — here's how it works and what it doesn't do yet.

• #5 Face picking: mesh in the front, B-rep in the back

  Fourth milestone: click any face in the viewer and the actual B-rep face highlights — not the triangle you hit. Here's the data path that makes it work.

• #6 Measurements: two clicks, one distance

  Fifth milestone: the viewer measures point-to-point distances — two clicks on the model, a line between them, and a live label that follows you around the scene.

• #7 True B-rep edges — from topology, not heuristics

  Sixth milestone: edge lines now come from the B-rep's own face structure — every edge shown is a real CAD edge, and the extraction is one observation plus a hash map.

• #8 WebGPU, finally — and the honest tradeoffs

  Seventh milestone: the viewer now renders on WebGPU where available, falling back to WebGL2 automatically. The swap was three lines — here's why it took seven posts to get here.

• #9 Section caps: closing the cut

  Eighth milestone: clipped models now show solid faces where the section plane cuts through, instead of a hollow shell. It was the honest hard problem on the list — here's the mesh-level version.

• #10 Measuring edges, not pixels

  Ninth milestone: click an edge in the viewer and get its length — the whole B-rep edge chain, not the segment under your cursor. The interesting part is splitting the wireframe graph at junctions.

• #11 STL and OBJ: the mesh-mode tradeoffs

  Tenth milestone: the viewer takes .stl and .obj files. Easy to load — the interesting part is honestly degrading the features that need B-rep data the formats don't carry.