pixal3d_mrp_mlx

MLX port of Pixal3D (TencentARC + Tsinghua, SIGGRAPH 2026, MIT) — pixel-aligned single-image 3D generation — for Apple Silicon.

Built on 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

Both of those are now done in the shared core.

Correction to the earlier scope

"the gap is two ops" was accurate about modules/sparse/ — the sparse primitives. It undercounted the model blocks, which the configs revealed:

Still needed	Where
RoPE positional embedding	all 4 flow models (pe_mode: "rope") — but rope_phases ships as a stored tensor, so phases are precomputed, not derived
qk_rms_norm on q and k	all 4 flow models
AdaLN modulation (share_mod: true)	all 4 flow models
image_attn_mode: "proj" conditioning	all 4 flow models
SparseConvNeXtBlock3d	shape_dec, tex_dec
SparseResBlockC2S3d (channel↔spatial)	shape_dec, tex_dec — uses SparseSpatial2Channel

Offsetting that, a genuine simplification the tensors revealed: the four flow models (~20 GB, the bulk of the download) contain ZERO 5-D tensors. ss_flow and the three slat_flow DiTs are pure transformers — they never touch sparse convolution, so they need none of the sparse core, just DiT blocks.

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

• Scoped against the shared core
• Weights downloaded (24 GB) — each ships a sibling .json with the exact config, so unlike LATO.2 there is no architecture to infer
• upsample (masked) + downsample(mode=) landed in the shared core
• Weight converter — decoders remap KRSC→[K³,in,out], dtype preserved, remap verified a pure permutation. Flow models pass through untouched.
• DiT blocks: RoPE (stored complex phases), qk_rms_norm, AdaLN modulation, proj conditioning
• All four flow models verified against upstream at correlation 1.00000000 — ss_flow (max diff 1.2e-5) and slat_flow (9.3e-6), 700/700 params each, on the real 1.3B checkpoints
• SparseConvNeXtBlock3d, SparseResBlockC2S3d, spatial2channel/channel2spatial — all in the shared core, round-trip and selective-growth tested
• ss_dec verified at correlation 1.00000000 (74/74), completing the whole structure stage: image -> ss_flow -> latent -> ss_dec -> 64^3 occupancy grid
• shape_dec / tex_dec model graphs (the two sparse decoders)
• End-to-end pipeline wiring

Numerical verification

The flow models contain no sparse convolution, which means upstream runs on CPU torch here — swap flash-attn for F.scaled_dot_product_attention and it loads the real checkpoint and runs. So unlike the sparse path (where spconv is uninstallable and the oracle had to be hand-written), these are diffed against upstream directly:

MLX      : mean +0.18490  std 0.86841
UPSTREAM : mean +0.18490  std 0.86841
max abs diff 1.216e-05   correlation 1.00000000

Getting there required finding three bugs that weight-key matching could not catch — the loader reported a perfect 700/700 with 0 missing and 0 unmapped through all of them:

1. A parameterless final LayerNorm between the last block and out_layer. It has no weights, so it leaves no trace in the checkpoint. Without it the output was ~200x too large (std 187 vs 0.87).
2. rope_phases is complex64, built with torch.polar. The rotation is a complex multiply and cos/sin are the phase's real/imaginary parts — taking cos() of a complex phase is meaningless.
3. qk RMS norm is applied BEFORE RoPE, not after. They do not commute. Reversed, a single block still correlated 0.9998 with upstream; over 30 blocks that compounds to 0.84. This one is invisible without an oracle.