HoPE: Hybrid of Position Embedding for Long Context Vision-Language Models

Conference: NeurIPS 2025 arXiv: 2505.20444 Code: GitHub Area: Multimodal VLM / Position Encoding Keywords: Rotary Position Embedding, Long Context, Frequency Allocation, Vision-Language Models, Video Understanding

TL;DR

This work presents the first theoretical analysis of frequency allocation strategies in multimodal RoPE for long-context VLMs. It proposes HoPE, which sets the lowest frequency to zero for temporal modeling to guarantee the semantic preference property, coupled with a dynamic temporal scaling mechanism, achieving gains of 8.35% on long video understanding and 22.23% on retrieval tasks.

Background & Motivation

State of the Field

Background: VLMs suffer significant performance degradation in long-context scenarios, particularly struggling with object counting and temporal localization in long video tasks.

Root Cause

Key Challenge: While RoPE has successfully enabled length generalization in text-only LLMs, directly applying 1D RoPE fails to capture the spatiotemporal structure of video.

Limitations of Prior Work

Limitations of Prior Work: Existing multimodal RoPE extensions exhibit the following limitations:

Starting Point

Key Insight: M-RoPE (Qwen2-VL) assigns the highest frequencies to the temporal dimension—a heuristic design lacking theoretical justification.

Supplementary Notes

Supplementary Notes: VideoRoPE assigns the lowest frequencies to the temporal dimension and shows strong empirical performance, yet remains unreliable over long distances.

Supplementary Notes

Supplementary Notes: Fixed and unidirectional temporal scaling factors cannot adapt to videos of varying speeds and information densities.

Supplementary Notes

Supplementary Notes: Core question: How do different frequency allocation strategies affect long-range semantic modeling? Can theoretical guarantees be obtained?

Method

Overall Architecture

HoPE comprises two components: (1) a Hybrid Frequency Allocation (HFA) strategy—high frequencies are interleaved to encode spatial information \((x, y)\), while the lowest frequency is set to zero for temporal modeling, guaranteeing an upper bound on the semantic preference property; and (2) a Dynamic Temporal Scaling (DTS) mechanism—scaling factors are randomly sampled during training (including both compression and stretching), enabling flexible adaptation to varying sequence lengths at inference.

Key Designs

1. Hybrid Frequency Allocation (HFA):

   • Function: Partitions the 128-dimensional rotary encoding into 96 dimensions for spatial encoding and 32 dimensions for temporal encoding, with the temporal frequency set to zero (degenerating to NoPE).
   • Mechanism: Exploits the identity \(\cos(0 \cdot \Delta t) = 1\) to eliminate the adverse effect of temporal distance on attention scores.
   • Design Motivation: Theorem 3.1 proves that any nonzero frequency will eventually violate the semantic preference property given sufficiently long contexts; zero frequency provides the strongest possible guarantee.
   • Key Theorem: \(\sum_{i \in i_t} 2\sigma^2 \cdot 1 \geq \sum_{i \in i_t} 2\sigma^2 \cos(\Delta t \cdot \theta_i)\), establishing that the zero-frequency scheme dominates all other frequency choices.

2. Dynamic Temporal Scaling (DTS):

   • Function: During training, scaling factors are randomly sampled from the set \(\Gamma = \{0.5, 0.75, 1, 1.25, 1.5\}\).
   • Mechanism: Stretching (\(\gamma > 1\)) preserves spatial detail, while compression (\(\gamma < 1\)) reinforces semantic preference; bidirectional scaling enables the model to learn multi-scale temporal relations.
   • Design Motivation: Real-world videos vary in speed, making fixed scaling insufficient; at inference, the scaling factor can be flexibly selected based on task requirements.

Loss & Training

• Built upon Qwen2-1.5B/7B backbones; trained on a subset of LLaVA-Video-178k (approximately 30k short videos and 3k medium-to-long videos).
• Training context length: 8k; maximum video frames: 128; sampling rate: 2.
• Learning rate: 1e-5 (2B) / 2e-5 (7B); cosine scheduler; batch size: 128.
• Training required approximately 304 H800 GPU hours.

Key Experimental Results

Main Results

Qwen2-7B-Video model, 32k context length:

Method	MLVU	LongVideoBench	Video-MME
Vanilla RoPE	61.03	51.29	57.99
M-RoPE	62.46	53.49	58.37
VideoRoPE	62.51	53.82	59.13
HoPE	63.85	55.34	59.44

Long video retrieval (V-NIAH): HoPE achieves a 22.23% improvement over the best baseline.

Ablation Study

• 3D Structure: Introducing 3D structure from 1D RoPE alone yields consistent gains, validating Proposition 3.1.
• HFA Strategy: Adding HFA on top of the 3D structure yields an average improvement of 1.69.
• DTS Mechanism: DTS provides additional gains over HFA, enhancing robustness to varying video speeds.
• Inference Scaling Factor: Retrieval tasks favor smaller factors (\(\gamma=0.75\)), while understanding tasks favor larger factors (\(\gamma=1.5\)).

Key Findings

• At 64k extrapolation, all methods degrade substantially, but HoPE remains the most robust (Video-MME: 27.34 vs. Vanilla 26.13).
• Scaling from 2B to 7B amplifies HoPE's advantage (LongVideoBench 32k gain increases from 0.66 to 4.05).
• Retrieval and understanding tasks exhibit opposite preferences for the scaling factor: retrieval requires preserving semantic preference, while understanding requires retaining spatial detail.

Highlights & Insights

• This is the first work to provide rigorous theoretical analysis of frequency allocation in multimodal RoPE, rather than relying on purely empirical comparisons.
• The insight that "zero frequency = NoPE for the temporal dimension" is both elegant and profound—the lowest frequency is not low enough; it must be exactly zero to provide guarantees.
• The semantic preference property (Definition 3.1) offers a general analytical framework applicable to broader position encoding designs.
• Bidirectional scaling is a natural approach for learning multi-scale temporal relations, and affords strong flexibility at inference time.

Limitations & Future Work

• Experiments are limited to the 7B scale; larger models and more training data may further amplify the observed advantages.
• Performance at 64k extrapolation still degrades substantially, and extreme-length generalization remains an open problem.
• The zero-frequency strategy entirely forgoes explicit temporal encoding, which may be detrimental for tasks requiring precise temporal localization.
• The interaction between HoPE and implicit positional learning in causal attention within decoder-only architectures is not discussed.

Related Work & Insights

• HoPE is complementary to LLM length extension methods such as LongRoPE and YaRN, focusing specifically on multimodal extension.
• The success of NoPE in decoder-only LLMs provides prior support for the zero-frequency strategy.
• The dynamic scaling idea is generalizable to position encoding design for other multimodal inputs, such as audio and point clouds.

Rating

• ⭐⭐⭐⭐⭐ — Theoretically rigorous, elegantly designed, experimentally comprehensive, and highly insightful for the long-context VLM community.

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