ADR-0006: Rendering, LOD & streaming#

• Status: Accepted
• Date: 2026-10-01

Context#

Large projects (hundreds of km, billions of LiDAR points) must open fast and render smoothly (DESIGN_GOALS §1, §5). LOD must never alter the engineering model.

Decision#

• Renderer: wgpu (Vulkan / Metal / DX12, WebGPU for a future web client); one renderer for plan, profile (with vertical exaggeration) and 3D views.
• Precision: camera-relative rendering — f64 on the CPU, positions relative to the camera sent as f32 to the GPU (avoids jitter with UTM-scale coordinates).
• Chunks: fixed quadtree grid of map tiles in the project CRS (e.g. 256 m base tiles), independent of the alignment.
• Render distance per layer (terrain, point cloud, structures, wires, labels) + automatic mode targeting a frame rate.
• LOD:
  • Terrain: quadtree LOD with simplified meshes per level.
  • Point clouds: COPC octree streaming with a per-frame point budget and screen-space error.
  • Structures: GPU instancing; LOD0 full lattice → LOD1 simplified → LOD2 line/icon, chosen by screen-space size.
  • Wires: GPU receives catenary parameters and tessellates adaptively (distance, curvature) — instant update while dragging.
  • Labels: decluttering and distance fading.
• Culling: frustum + occlusion.
• Picking: GPU ID buffer.
• Threads: loading and meshing on background threads; the UI thread never blocks.

Consequences#

• Structure library items need several mesh LODs (auto-generated where possible).
• The chunk grid is shared by rendering, project file indexing (ADR-0004) and server interest management (ADR-0005).