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Make Your Training Flexible: Towards Deployment-Efficient Video Models

Conference: ICCV 2025 arXiv: 2503.14237 Code: https://github.com/OpenGVLab/FluxViT Area: LLM Pretraining Keywords: Flexible Training, Token Optimization, Video Pretraining, Deployment Efficiency, Data Augmentation

TL;DR

This paper proposes Flux — a data augmentation tool that enables flexible video model training through flexible sampling grids and group-dynamic token selection, allowing a single model to operate efficiently across varying computational budgets. The paper further introduces a Token Optimization test-time paradigm that matches previous SOTA performance using only 1/4 of the tokens, saving approximately 90% of computation.

Background & Motivation

Background: Video representation learning is a foundational task in computer vision, critical for multimodal LLMs and embodied AI. Current mainstream methods operate on fixed spatiotemporal sampling grids (e.g., 8 frames × 224²) with a fixed number of tokens, leading to substantial redundancy during both training and deployment.

Limitations of Prior Work: - Redundancy from fixed sampling: Videos contain abundant spatiotemporal redundancy; many tokens extracted via fixed sampling carry low information content. - Inflexible deployment: Models are trained at 8 frames × 224², yet real deployment may require adaptation to varying computational budgets. Directly reducing frames or resolution causes significant performance degradation. - Limited token reduction: Existing token pruning/merging methods perform poorly at high reduction ratios, and the strategies themselves introduce computational overhead. - Incomplete flexible training methods: ResFormer and FFN address spatial or temporal flexibility independently, without jointly handling both dimensions, and neither has been validated in large-scale pretraining.

Key Challenge: How can a single model simultaneously satisfy deployment requirements under diverse computational budgets? Simply reducing frames or resolution is suboptimal — for a given token budget, the goal should be to select the most information-maximizing token set.

Goal: Propose the Token Optimization paradigm — under a given token budget, select the optimal token set from better-sampled video to maximize information.

Key Insight: Combine flexible sampling and token selection as training-time data augmentation, enabling the model to naturally adapt to various resolutions and token counts. A test-time Token Optimization strategy is also introduced to identify the optimal sampling-selection combination.

Core Idea: Use flexible sampling and group-dynamic token selection as zero-cost training augmentation, enabling video models to identify the optimal token set via Token Optimization across all computational budgets.

Method

Overall Architecture

As illustrated in Figs. 2–3, Flux comprises three levels of design: (1) Flexi-Sampling: randomly selecting different frame counts and resolutions during training; (2) Group-Dynamic Token Selector: selecting a high-information token subset from the flexibly sampled token pool; (3) FluxViT architectural enhancements: GLPE (Global-Local Positional Embedding) and DPN (Dual Patch Normalization) to adapt ViT to variable token counts. At test time, Token Optimization searches for the optimal sampling-selection configuration.

Key Designs

  1. Flexi-Sampling:

    • Function: Each video during training adopts a randomly chosen spatiotemporal resolution.
    • Mechanism: For each video in the batch, a frame count \([F_{min}, F_{max}]\) (stride \(t_s\)) and spatial resolution \([R_{min}, R_{max}]\) (stride \(r_s\)) are randomly selected, with a token count threshold \(T_{thres}\) to maintain a reasonable pool size. Default settings: 4–24 frames, 168–252 resolution.
    • Design Motivation: Models trained with fixed sampling have only observed a single resolution and generalize poorly to others. Flexible sampling exposes the model to diverse resolution combinations, naturally inducing cross-resolution robustness.

  2. Group-Dynamic Token Selector:

    • Function: Selects the most informative token subset from the token pool for the teacher model.
    • Mechanism: The frame sequence is evenly divided into \(N\) sparse groups \(B_i\). Within each group, the dynamic value of each token is computed via inter-frame difference: \(D(F_{t+1,i}) = \|F_{t+1,i} - F_{t,i}\|_p\). The top \(K/N\) tokens with the highest dynamic values are selected. This ensures: (a) tokens with the greatest variation (highest information) are selected; (b) grouping guarantees full-video temporal coverage.
    • Design Motivation: A large proportion of video tokens represent static background (low information). Tokens with high inter-frame variation are more semantically meaningful. Grouping prevents tokens from localized rapid motion from dominating while neglecting other temporal segments.

  3. Double Mask Module:

    • Function: Simultaneously augments both teacher and student within the UMT (Unmasked Teacher) framework.
    • Mechanism: On the teacher side, Flexi-Sampling combined with the Group-Dynamic Selector yields informative tokens; on the student side, masking is based on attention scores from the teacher's CLS token. The two masks are complementary — the teacher provides high-quality representations filtered from richly sampled video, while the student learns to understand video from a sparser perspective.
    • Design Motivation: Richer information is extracted from higher-resolution sampling without increasing the teacher's computational cost (the number of tokens after selection remains unchanged).

  4. Global-Local Positional Embedding (GLPE):

    • Function: Provides positional encoding for a flexible number and arrangement of tokens.
    • Mechanism: Globally, a learnable positional encoding (sine-cosine initialized) combined with a Depth-Wise Conv enhances local relationships. A linear projection encodes local position over Value vectors in the attention mechanism: \(Z = (\text{Softmax}(\frac{QK^T}{\sqrt{D}}) + LPE) \cdot V\). The LPE is value-dependent and is not affected by the number of input tokens.
    • Design Motivation: Standard positional encodings assume a fixed token count and arrangement. When tokens are selected or masked, they originate from discrete spatiotemporal positions that require explicit positional encoding.

  5. Dual Patch Normalization (DPN):

    • Function: Stabilizes training under flexible sampling.
    • Mechanism: A LayerNorm is added after the standard Patch Embedding layer (to assist dynamic estimation) and another before the Patch Embedding layer (to stabilize gradients).
    • Design Motivation: Flexible sampling causes large distributional shifts in input tokens, potentially leading to excessively large gradients in Patch Embedding. Dual normalization stabilizes training.

Loss & Training

  • Flux-PT (Pretraining): Teacher-student alignment loss under the UMT framework, using InternVideo2-1B as the teacher.
  • Flux-FT (Fine-tuning): Standard supervised training with self-distillation, where aggregated features from larger token counts guide training with smaller token counts.
  • Multi-number co-training: Three different token counts are used within a single batch to train the student, maximizing the utilization of teacher computation.

Key Experimental Results

Main Results

Model K400 Top-1 SSv2 Top-1 MSRVTT R@1 COIN Scale
InternVideo2-S 87.8 - - - Small
FluxViT-S 90.0 - - - Small
InternVideo2-B 89.0 73.5 48.2 92.5 Base
FluxViT-B 90.0 75.8 49.9 94.1 Base

Token Optimization Results

Configuration Token Count K400 Relative to Full Compute Saved
FluxViT-B Full 3072 90.0 100% 0%
FluxViT-B TO (1/4) 768 ~89.0 ~99% ~90%
InternVideo2-B Full 3072 89.0 - 0%

Ablation Study

Configuration K400 SSv2 Note
Baseline (InternVideo2 UMT) 87.5 71.8 Original pipeline
+ Flexi-Sampling 88.2 73.0 Flexible sampling improves robustness
+ Group-Dynamic Selector 89.0 74.2 Informative token selection is effective
+ GLPE + DPN 89.5 75.0 Architectural enhancements are critical
+ Multi-number training 90.0 75.8 Co-training with multiple token counts further improves performance

Key Findings

  • Remarkable Token Optimization efficiency: 1/4 of the tokens suffice to match the performance of the previous SOTA (InternVideo2), saving approximately 90% of computation — demonstrating substantial redundancy in fixed sampling.
  • Generality of Flux as an augmentation tool: Effective in both pretraining (UMT) and supervised fine-tuning without increasing training cost.
  • Joint spatiotemporal flexibility outperforms single-dimensional flexibility: ResFormer (spatial) and FFN (temporal) each address only one dimension; Flux handles both simultaneously and achieves superior results.
  • FluxViT-B surpasses larger models across multiple tasks: K400 90.0%, SSv2 75.8%, MSRVTT 49.9%, COIN 94.1% — new SOTA at the same scale.
  • Improvements on chat-centric tasks: When used as a visual encoder integrated with an LLM, FluxViT outperforms SigLIP/CLIP on MVBench and Dream-1k.

Highlights & Insights

  • Token Optimization as a new paradigm: The shift from "fixed sampling + all tokens" to "flexible sampling + optimal token selection" represents a paradigm shift in video model deployment — not "fewer frames or lower resolution," but "finding the optimal token set under a given budget."
  • Zero-cost augmentation: Token selection in Flux keeps the number of tokens processed by the teacher unchanged, incurring no additional training cost. Sampling from higher resolutions thus becomes a "free lunch."
  • Elegant design of the Group-Dynamic Selector: Grouping ensures temporal coverage; only tokens with high inter-frame variation are selected. Simple yet highly effective — it guarantees informativeness while avoiding overfitting to rapid motion.
  • Validation with LLM integration: Validation in chat-centric settings opens the door to applying Flux in multimodal LLMs.

Limitations & Future Work

  • The optimal configuration search in Token Optimization incurs non-trivial overhead (requiring evaluation of multiple configurations on a validation set).
  • Flexible sampling increases data preprocessing complexity due to multi-resolution support.
  • InternVideo2-1B is used as the teacher, and teacher quality constitutes an upper bound for the student model.
  • GLPE and DPN introduce a small number of additional parameters and computations.
  • A learned token selector (rather than the heuristic inter-frame-difference-based approach) could be explored.
  • vs. ResFormer/FFN: ResFormer addresses only spatial flexibility and FFN only temporal flexibility; neither has been validated in large-scale pretraining. Flux handles both spatiotemporal dimensions and is validated at the InternVideo2 scale.
  • vs. MAR/MCM (token reduction): Traditional masking reduces tokens at fine-tuning or inference time but lacks training-time flexibility. Flux introduces flexibility during training, enabling the model to naturally adapt.
  • vs. InternVideo2: FluxViT comprehensively outperforms the InternVideo2 series at the same scale, demonstrating the effectiveness of Flux augmentation.

Rating

  • Novelty: ⭐⭐⭐⭐ The Token Optimization paradigm is innovative, though the core components (flexible sampling, dynamic selection) are relatively standard.
  • Experimental Thoroughness: ⭐⭐⭐⭐⭐ Extensive evaluation across multiple tasks (action recognition + retrieval + chat), scales, and settings.
  • Writing Quality: ⭐⭐⭐⭐ Clear structure and systematic ablation.
  • Value: ⭐⭐⭐⭐⭐ Significant practical value for efficient video model deployment; matching SOTA with 1/4 tokens is a strong result.