Function paintAt

js/main.js:2159–2177

(e)

  const qx = ax+abx*v+acx*w-px, qy = ay+aby*v+acy*w-py, qz = az+abz*v+acz*w-pz;
  return qx*qx + qy*qy + qz*qz;
}

/**
 * BFS-along-adjacency circle brush (after PrusaSlicer's TriangleSelector).
 *
 * Starts at `seedTriIdx`, walks the mesh's neighbor graph, and invokes
 * cb(triIdx) for every triangle that:
 *   1. has at least one part inside the brush "cylinder" (the projection of
 *      a 3D distance-to-triangle test onto the plane perpendicular to
 *      `viewDir`), AND
 *   2. is reachable without crossing any back-facing triangle.
 *
 * Back-face culling at the BFS expansion step is what makes this both fast
 * and correct: the walk can't tunnel through a thin shell to its hidden
 * other side because the connecting wall faces away from the camera. Work
 * is bounded by the painted area, not the mesh size — no spatial index
 * required.
 */
function bfsBrushSelect(seedTriIdx, hitPt, r2, viewDir, cb) {
  const usePrecision = precisionMaskingEnabled && precisionGeometry;
  const adjacency  = usePrecision ? precisionAdjacency  : triangleAdjacency;
  const faceNormals = usePrecision ? precisionFaceNormals : triangleFaceNormals;