# pixal3d_mrp_mlx — working notes

MLX port of TencentARC Pixal3D, on top of the shared `trellis_sparse_mlx` core.

## Measured facts (don't re-derive)
- Pixal3D inherits TRELLIS.2's sparse module, same as LATO.2. `conv_spconv.py` is the
  same code: `stride==1 and padding is None -> spconv.SubMConv3d`.
- EVERY SparseConv3d in the model is `(channels, out, 3)` -> submanifold only. The
  shared core's SubMConv3d covers all of them.
- attn_mode is 'full' everywhere -> SDPA. No flash-attn/xformers needed.
- Container differs from LATO.2: `SparseTensor(VarLenTensor)`. VarLenTensor is feats +
  explicit slice-per-batch layout, no coords. Both are in the shared core now.
- Cache API spelling is `get_spatial_cache`/`register_spatial_cache` (LATO.2 uses
  cache_get/cache_put). Shared core exposes both.
- `_scale` uses `Fraction`, not int, upstream. Shared core keeps scale as an opaque
  tuple so either works.
- Only sparse-conv-using model file is `models/sc_vaes/sparse_unet_vae.py`.

## Remaining ops to port
- `SparseUpsample(2)` — pixal3d/modules/sparse/spatial/basic.py. Cache-paired: needs the
  subdivision stored by its matching SparseDownsample, or an explicit subdivision tensor.
- `SparseSpatial2Channel(2)` — pixal3d/modules/sparse/spatial/spatial2channel.py.
  Sparse pixel-shuffle; also cache-backed.

## Conventions
- Depends on ../trellis_sparse_mlx (editable install). Do NOT vendor a second copy of
  the sparse core.
- `upstream/Pixal3D` is vendored read-only reference; never edit it.
- Weights live at MODELBEAST/vendor/pixal3d-weights (24GB, outside this repo).