Scope Pixal3D against the shared sparse core

Every SparseConv3d in the Pixal3D model is (channels, out, 3) - stride=1/padding=None,
so spconv dispatches to SubMConv3d, which trellis_sparse_mlx already implements and
benchmarks. attn_mode is 'full' throughout. The container split (VarLenTensor /
SparseTensor) and the get/register_spatial_cache spelling are already in the shared core.

Remaining gap is two ops: SparseUpsample and SparseSpatial2Channel, both cache-paired
with a matching downsample rather than recomputing structure.

.gitignore

@@ -0,0 +1,6 @@
+.venv
+upstream/
+ckpt/
+weights/
+__pycache__/
+*.pyc

CLAUDE.md

@@ -0,0 +1,29 @@
+# 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).

README.md

@@ -0,0 +1,56 @@
+# pixal3d_mrp_mlx
+
+MLX port of [Pixal3D](https://github.com/TencentARC/Pixal3D) (TencentARC + Tsinghua,
+SIGGRAPH 2026, MIT) — pixel-aligned single-image 3D generation — for Apple Silicon.
+
+Built on **[`trellis_sparse_mlx`](../trellis_sparse_mlx)**, the shared TRELLIS-lineage
+sparse core. Pixal3D and LATO.2 inherit the same sparse module from TRELLIS.2, so the
+expensive part — submanifold sparse convolution, which has no Metal implementation — is
+already done and tested there.
+
+## Why Pixal3D
+
+It back-projects pixel features directly into 3D rather than injecting them through
+attention, so silhouettes stay exact to the source image. Different failure mode from
+TRELLIS/Hunyuan, and complementary to them.
+
+Upstream needs ~24 GB VRAM, which is a wall on consumer Nvidia and a non-issue on a
+128 GB+ Ultra.
+
+## Scope, measured against the shared core
+
+| Need | Status |
+|---|---|
+| `SparseConv3d` — **every call is `(c, out, 3)`**, i.e. stride=1/padding=None → SubMConv3d | ✅ in shared core |
+| `attn_mode='full'` (the only mode used) | ✅ in shared core |
+| `SparseLinear`, norms, activations, ResBlock, transformer blocks | ✅ in shared core |
+| `SparseDownsample(2)` | ✅ in shared core |
+| `VarLenTensor` / `SparseTensor` split + `get/register_spatial_cache` | ✅ added to shared core |
+| dense `nn.Conv3d(.., 2, stride=2)` in `sparse_structure_vae` | ✅ maps to `mlx.nn.Conv3d` |
+| `SparseUpsample(2)` | ❌ **to do** — cache-paired inverse of a downsample |
+| `SparseSpatial2Channel(2)` | ❌ **to do** — sparse pixel-shuffle, spatial→channel |
+
+So the gap is **two ops**, both of which work through the spatial cache (they pair with a
+matching downsample rather than recomputing structure). Everything else is already
+covered by work done for LATO.2.
+
+## Model surface
+
+```
+pixal3d/models/
+  sparse_structure_vae.py     dense Conv3d — voxel structure
+  sparse_structure_flow.py    structure flow (SS)
+  structured_latent_flow.py   SLAT flow
+  sc_vaes/sparse_unet_vae.py  the only file using sparse conv
+```
+
+Weights: 24.04 GB across 19 files (1.3B DiTs at 512/1024 + shape/tex decoders).
+
+## Status
+
+- [x] Scoped against the shared core — gap is two ops
+- [x] Weights downloaded
+- [ ] `SparseUpsample`, `SparseSpatial2Channel`
+- [ ] Weight converter
+- [ ] Model graphs
+- [ ] End-to-end