Paper 3d Utonia

Method Overview

Utonia is a cross-domain pre-trained PTv3 encoder that harmonizes point clouds with distinct sensing geometries, densities, and modality priors. The pipeline is:

1. Data preparation: Raw point clouds (coord, color, normal) are normalized and grid-sampled at 0.01m resolution. A domain-dependent scale parameter controls Perceptual Granularity Rescale -- the key mechanism for unifying different point cloud densities.
2. Serialization: Grid coordinates are serialized via z-order or Hilbert curves to create 1D sequences for efficient attention.
3. Encoder: A 5-stage hierarchical PTv3 with serialized patch attention, 3D RoPE positional encoding, and sparse convolution CPE. Stages are connected by GridPooling (stride-2 downsampling).
4. Output: The encoder produces a hierarchical Point object; features can be unpooled back to the original resolution via pooling_inverse and pooling_parent chains.

Key insight: A single encoder can learn universal 3D representations across wildly different domains by rescaling point cloud granularity through a per-domain scale factor applied before grid sampling.

Scale / Domain Table (CRITICAL)

The scale parameter in the transform pipeline controls the effective voxel resolution. Choosing the right scale per domain is essential:

Paper-Code Mapping

Setup

Dependencies

• Python 3.10+
• PyTorch 2.5.0+ with CUDA 12.4+
• FlashAttention (strongly recommended, required for default config)
• Key packages: spconv-cu124, torch-scatter, flash-attn, huggingface_hub, timm, addict, scipy
• Demo extras: open3d (requires numpy<=1.26.4), fast_pytorch_kmeans, psutil, camtools, trimesh, gradio, natsort, opencv-python

Installation

Standalone mode (conda env with everything):

git clone https://github.com/Pointcept/Utonia.git
cd Utonia

# unset CUDA_PATH if you have local CUDA installed
conda env create -f environment.yml --verbose
conda activate utonia

Package mode (inject into existing env):

# CUDA_VERSION e.g. 124, TORCH_VERSION e.g. 2.5.0
pip install spconv-cu${CUDA_VERSION}
pip install torch-scatter -f https://data.pyg.org/whl/torch-${TORCH_VERSION}+cu${CUDA_VERSION}.html
pip install git+https://github.com/Dao-AILab/flash-attention.git
pip install huggingface_hub timm addict scipy

# Install utonia as a package (or just copy utonia/ folder to your project)
cd Utonia
python setup.py install

Pre-trained Weights

Weights are auto-downloaded from HuggingFace on first utonia.model.load() call.

Available models on Pointcept/Utonia:

• utonia.pth -- main encoder (enc_mode=True, encoder-only PTv3)
• utonia_linear_prob_head_sc.pth -- linear probe segmentation head for ScanNet 20-class

Cache location: ~/.cache/utonia/ckpt (customizable via download_root param).

Usage Scenarios

Scenario 1: Feature Extraction (Indoor Scene)

export PYTHONPATH=./
python demo/0_pca_indoor.py
# With --wo_color or --wo_normal to test modality ablation

Scenario 2: Semantic Segmentation (Linear Probe)

python demo/2_sem_seg.py

Scenario 3: Batched Inference

python demo/3_batch_forward.py

Scenario 4: Outdoor LiDAR

python demo/7_pca_outdoor.py

Scenario 5: Object-level

python demo/6_pca_object.py

Scenario 6: Robotic Manipulation

python demo/5_pca_manipulation.py

Scenario 7: Video-lifted Point Clouds (requires VGGT)

git clone https://github.com/facebookresearch/vggt.git && cd vggt && pip install -e . && cd ..
python demo/8_pca_video.py --input_video ${VIDEO_PATH} --conf_thres 0 \
  --prediction_mode "Depthmap and Camera Branch" --if_TSDF --pca_start 1 --pca_brightness 1.2

Key Config Flags

Code Integration Guide

Minimal Imports

import utonia
# or individually:
from utonia.model import PointTransformerV3, load
from utonia.transform import Compose, default
from utonia.data import collate_fn
from utonia.structure import Point
from utonia.utils import offset2batch, batch2offset, offset2bincount, set_seed

Model Instantiation & Inference

import torch
import utonia

# Load pre-trained encoder from HuggingFace
model = utonia.model.load("utonia", repo_id="Pointcept/Utonia").cuda()
model.eval()

# Alternative: HuggingFace Hub API
# from utonia.model import PointTransformerV3
# model = PointTransformerV3.from_pretrained("Pointcept/Utonia").cuda()

# Without flash_attn:
model = utonia.model.load("utonia", repo_id="Pointcept/Utonia",
    custom_config=dict(enable_flash=False, enc_patch_size=[1024]*5)).cuda()

# From local checkpoint:
model = utonia.model.load("path/to/utonia.pth").cuda()

# Inspect checkpoint config:
ckpt = utonia.model.load("utonia", ckpt_only=True)
print(ckpt["config"])  # dict with all PointTransformerV3 constructor args

Data Format

Input (before transform): a dict with numpy arrays.

Input (after transform, ready for model): a dict with torch tensors.

Output: a Point object (subclass of addict.Dict).

Default encoder channel dims per stage: enc_channels=(48, 96, 192, 384, 512).

Feature Unpooling (CRITICAL)

The encoder output is at the deepest (most downsampled) stage. To recover features at higher resolutions:

# Unpool with concatenation (first K levels for richer features)
for _ in range(K):  # K=2 for visualization, K=4 for full unpool
    parent = point.pop("pooling_parent")
    inverse = point.pop("pooling_inverse")
    parent.feat = torch.cat([parent.feat, point.feat[inverse]], dim=-1)
    point = parent

# Unpool remaining levels with simple nearest-neighbor copy
while "pooling_parent" in point.keys():
    parent = point.pop("pooling_parent")
    inverse = point.pop("pooling_inverse")
    parent.feat = point.feat[inverse]
    point = parent

# Map back to original (pre-GridSample) resolution
feat_original = point.feat[point.inverse]  # (N, D) at original point count

Feature dims after concat-unpooling:

• K=2: 48 + 96 + 192 = 336 (stage0_feat + stage1_upcast + stage2_upcast)
• K=4 (all stages): 48 + 96 + 192 + 384 + 512 = 1232

Global Feature Extraction (for RL, classification)

For tasks needing a single feature vector per point cloud:

# After model forward (before unpooling), max-pool over deepest features
point = model(data_dict)
# point.feat is (M', 512) at deepest stage

# Simple max-pool for single sample:
global_feat = point.feat.max(dim=0).values  # (512,)

# Batched max-pool using offset:
from utonia.utils import offset2bincount
import torch_scatter
bincount = offset2bincount(point.offset)
idx_ptr = torch.cat([bincount.new_zeros(1), torch.cumsum(bincount, dim=0)])
_, indices = torch.sort(torch.arange(point.feat.shape[0], device=point.feat.device))
global_feat = torch_scatter.segment_csr(point.feat[indices], idx_ptr, reduce="max")
# global_feat: (B, 512)

Integration Notes

• The utonia package is self-contained (no dependency on full Pointcept). Just copy the utonia/ folder or pip install via setup.py.
• feat_keys=("coord", "color", "normal") in the Collect transform concatenates these into the feat tensor (9-dim). The model's in_channels in the checkpoint config determines what the embedding layer expects.
• When color or normal is absent, fill with zeros (same shape as coord). Do NOT omit the key.
• The model uses spconv.SubMConv3d for the convolutional position encoding (CPE). This is handled internally by Point.sparsify().
• FlashAttention is strongly recommended. Without it, set enable_flash=False via custom_config and optionally reduce enc_patch_size.
• The Point class inherits from addict.Dict -- attribute access works like dict access (point.feat == point["feat"]).
• GridPooling sets traceable=True by default, which stores pooling_parent and pooling_inverse for unpooling.

Core Architecture

Input dict {coord, color, normal}
    |
    v
[Embedding] Linear(in_channels -> 48) + LN + GELU
    |
    v
[Serialization] z-order / z-trans curve encoding
[Sparsify] -> SparseConvTensor for CPE
    |
    v
+-- Enc Stage 0: 3x Block(48, heads=3, patch=1024) ----> pooling_parent chain
    |  Block = CPE(SubMConv3d) + SerializedAttention(3D_RoPE) + MLP(ratio=4)
    v
[GridPooling stride=2, max-reduce] 48 -> 96
    |
+-- Enc Stage 1: 3x Block(96, heads=6) ----> pooling_parent chain
    v
[GridPooling stride=2] 96 -> 192
    |
+-- Enc Stage 2: 3x Block(192, heads=12) ----> pooling_parent chain
    v
[GridPooling stride=2] 192 -> 384
    |
+-- Enc Stage 3: 12x Block(384, heads=24) ----> pooling_parent chain
    v
[GridPooling stride=2] 384 -> 512
    |
+-- Enc Stage 4: 3x Block(512, heads=32)
    |
    v
Output: Point with feat(512), pooling_parent/pooling_inverse chain
        (enc_mode=True: no decoder, manual unpool via chain)

Repo Structure

Tips & Gotchas

• Scale is the most important hyperparameter. Wrong scale = garbage features. See the Scale / Domain Table above. Indoor=0.5, manipulation=4.0, outdoor=0.2, object=1.0.
• Outdoor LiDAR: Must set apply_z_positive=False so CenterShift is skipped. Ensure ego-vehicle is at origin with road on xy-plane.
• Object data: Set normalize_coord=True to enable NormalizeCoord transform (centers and normalizes to unit sphere).
• Missing modalities: When color or normal is unavailable, pass np.zeros_like(coord). Do NOT omit the key from the dict. The model was trained with Causal Modality Blinding and handles missing inputs gracefully.
• FlashAttention required by default: The checkpoint config has enable_flash=True. If flash_attn is not installed, pass custom_config=dict(enable_flash=False) to load().
• Memory: Large scenes can OOM. Reduce enc_patch_size (e.g., [512]*5 or [256]*5) to lower memory usage at the cost of attention window size.
• Unpooling depth matters: Use range(2) concat + remaining copy for visualization. Use range(4) concat for quantitative evaluation (all skip features concatenated). The demo/2_sem_seg.py uses full concat (while loop with concat at every level).
• Feature dims after unpool: Concat over K=2 stages gives 48+96+192=336 dims. Full K=4 gives 48+96+192+384+512=1232 dims.
• Grid size: Fixed at 0.01m in the default transform. Changing this is not recommended without retraining.
• Batch collation: Use utonia.data.collate_fn() for batching multiple transformed point clouds. It handles offset-based batching correctly (concatenates coords, builds cumulative offset). Do NOT use PyTorch default collation.
• Weights license: CC-BY-NC 4.0 (non-commercial due to HM3D/ArkitScenes dataset restrictions). Code is Apache 2.0.
• Fine-tuning: This repo is inference-only. For training/fine-tuning, use the full Pointcept codebase with Utonia weights as initialization.
• enc_mode=True: The Utonia checkpoint is encoder-only. The decoder path in PointTransformerV3 exists in code but is not used. All feature recovery is done via manual unpooling with pooling_parent/pooling_inverse.
• 3D RoPE constraint: head_dim = channels // num_heads must be divisible by 3. This is enforced by assertion in Point3DRoPE.__init__.
• Serialization orders: Default is ("z", "z-trans"). Blocks alternate between orders via order_index = i % len(self.order).
• drop_path: Default 0.3 stochastic depth, linearly increasing across encoder depth.
• rope_base: Default 10 (not the typical 10000 used in NLP RoPE).