🌐 SpatialStack-Qwen3.5-4B

Layered Geometry-Language Fusion for 3D VLM Spatial Reasoning

CVPR 2026

         

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📋 Overview

SpatialStack-Qwen3.5-4B is a geometry-augmented vision-language model designed for 3D spatial reasoning. It extends Qwen3.5-4B with a parallel VGGT-1B geometry stream, using a novel layered geometry-language fusion mechanism that progressively aligns multi-level geometric and language features across model layers.

Geometry features from encoder layers [11, 17, 23] are projected and injected into decoder layers [0, 1, 2], preserving both fine local structure and higher-level spatial context.

🏗️ Architecture

Component	Detail
Base Model	Qwen/Qwen3.5-4B
Geometry Encoder	facebook/VGGT-1B
Encoder Layers	[11, 17, 23]
Fusion Layers	[0, 1, 2]
Fusion Method	DeepStack Language-Add
Geometry Merger	MLP
Precision	bfloat16

📊 Benchmark Results

Benchmark	Metric	Score
VSI-Bench	Average	67.5
CV-Bench	Average	85.5
CV-Bench	3D	92.2

Results from the SpatialStack project page and paper.

🚀 Quick Start

Installation

git clone https://github.com/jzh15/SpatialStack.git
cd SpatialStack
pip install -e . --no-deps

For full environment setup (PyTorch, flash_attn, Qwen3.5 dependencies), see the repo README.

Single-Image Inference

python scripts/inference/infer.py \
  --model-path Journey9ni/SpatialStack-Qwen3.5-4B \
  --image assets/sofas.jpg \
  --prompt "Describe this scene in a few complete sentences." \
  --disable-thinking \
  --max-new-tokens 128

VSI-Bench Evaluation

MODEL_PATH=Journey9ni/SpatialStack-Qwen3.5-4B \
MODEL_IMPL=qwen3_5 \
MODEL_ARGS_BASE="pretrained=Journey9ni/SpatialStack-Qwen3.5-4B,use_flash_attention_2=true,max_num_frames=32,max_length=12800,geometry_encoder_path=facebook/VGGT-1B,disable_thinking=true" \
OUTPUT_ROOT=logs/eval/spatialstack_qwen35_4b \
BENCHMARKS="vsibench" \
bash scripts/evaluation/eval.sh

⚠️ Limitations

• Requires a separate geometry encoder (VGGT-1B) alongside the vision-language backbone.
• Optimized for spatial reasoning benchmarks; not intended for general-purpose multimodal chat.
• Not validated for safety-critical use, robotics deployment, or real-world decision making.

📝 Citation

@article{zhang2026spatialstack,
  title={SpatialStack: Layered Geometry-Language Fusion for 3D VLM Spatial Reasoning},
  author={Zhang, Jiang and Zhou, Shijie and Liu, Bangya and Kadambi, Achuta and Fan, Zhiwen},
  journal={arXiv preprint arXiv:2603.27437},
  year={2026}
}