Skeletal models skinned on the SPEs (2026-10-17)

Moved on 2026-10-26 from docs/SPU.md (“BananaBread’s skinning”), where libspeq’s skinning op stays described. Measured in BananaBread; the patches are the Cube 2 engine’s.

Symptom

The engine skins its skeletal models (players, the hud gun, monsters) either on the GPU (gpuskel 1) or on the CPU. GPU skinning runs on nv30 without a wedge but at 11 fps against 28 with CPU skinning: nv30 turns the vertex program’s bone lookups into if/else trees. So cube2 0002 makes CPU skinning the default and drops the GPU skinning shaders, and the skinning becomes PPE time.

Evidence and fix

libspeq has a skinning op (speq_skin_*_v1, SPEQ_SKIN_ABI 1, dual-quaternion palettes, one SPU pass per mesh, run on the pool; design and tests in packages/walnut/tools/ps3-spu/doc/skin.md), and cube2 patch 0006-CPU-skinning-through-libspeq-on-the-PS3-s-SPEs.patch hooks it in. The modes are SPEQ_SKIN=auto|spu|ppe|off; the engine’s own loop remains the default in the package.

Measured (BananaBread spawn and gun, one run each, same boot, batch 2026-10-17-luanti-neverball-batching):

So libspeq’s PPE path is the fastest, and the SPU path adds hitches. The expected gain was small anyway (about 1% on map cave, where only the hud gun is skinned, around 3200 vertices). Skinning stays selectable and off the default; it is one of the switch-gated changes in docs/TESTS.md’s keep-or-drop re-test.