Explicit Temporal-Semantic Modeling for Dense Video Captioning via Context-Aware Cross-Modal Interaction

Conference: AAAI 2026 arXiv: 2511.10134
Code: None
Area: Video Understanding Keywords: Dense Video Captioning, Cross-modal Retrieval, Temporal Clustering, Feature Enhancement, Retrieval-Augmented Generation

TL;DR

This paper proposes the CACMI framework, which addresses two fundamental limitations in dense video captioning (insufficient temporal modeling and modality gap) through explicit temporal-semantic modeling. It employs Cross-modal Frame Aggregation (CFA) to extract temporally coherent event semantics, and Context-aware Feature Enhancement (CFE) to bridge the visual-textual modality gap, achieving state-of-the-art performance on ActivityNet Captions and YouCook2.

Background & Motivation

Dense Video Captioning (DVC) requires simultaneously localizing and describing all salient events with precise temporal boundaries in untrimmed videos. Recent retrieval-augmented generation (RAG)-based methods (e.g., CM2) have begun incorporating external semantic knowledge to enhance understanding and generation capabilities.

Limitations of Prior Work: Existing memory-based methods rely on implicit RAG frameworks that use manually designed fixed windows for cross-modal retrieval, leading to two fundamental limitations:

Insufficient Temporal Modeling: Fixed-window visual features focus only on local segments, resulting in discontinuous semantic retrieval that fails to capture temporal coherence across event sequences.

Modality Gap: Retrieved semantic features are fused with visual representations via simple operations (concatenation or basic attention), which is insufficient to bridge the inherent gap between visual and textual modalities.

Key Challenge: Effective retrieval-augmented DVC requires exploiting the temporal structure and rich semantic information inherent in video data, yet current methods naively concatenate frame-level or fragmented textual information, neglecting temporal coherence.

Key Insight: Adjacent frames share similar visual and temporal contexts and typically represent the same semantic event. Based on this observation, the paper introduces explicit temporal-semantic modeling via pseudo-events, endowing retrieved textual semantics with temporal properties.

Method

Overall Architecture

CACMI follows a RAG paradigm: a CLIP image encoder extracts frame-level features → the CFA module aggregates temporally coherent frames and retrieves event-aligned text → the CFE module fuses visual and textual features → a Deformable Transformer with multi-task heads produces event localization and captioning outputs.

Key Designs

1. Cross-modal Frame Aggregation (CFA):

   • Event Context Clustering:
     • Function: Aggregates frame-level visual features into temporally coherent pseudo-event representations.
     • Mechanism: Applies agglomerative clustering (Euclidean distance + Ward linkage) to CLIP frame features, augmented with a temporal aggregation constraint (the temporal gap between any two frames within the same cluster must not exceed \(t_{\max}\)), ensuring that semantically similar and temporally contiguous frames are grouped together.
     • Design Motivation: Agglomerative clustering assumes no fixed cluster shape, making it well-suited for discovering flexible patterns in feature space; the temporal constraint enforces event-level temporal coherence.
     • Output: \(c\) cluster-level feature vectors \(F^c\), each representing a pseudo-event, computed via boundary-enhanced weighted averaging (inverted bell-shaped weights that assign higher weights to boundary frames).
   • Event Semantic Retrieval:
     • Function: Retrieves the most relevant textual descriptions from a sentence pool for each pseudo-event.
     • Mechanism: A CLIP text encoder preprocesses the sentence pool; cosine similarity matrices between pseudo-event features and all text features are computed; top-\(k\) candidates per pseudo-event are retrieved and average-pooled.
     • Design Motivation: The core innovation lies in performing retrieval at the event granularity rather than at the frame or fixed-window level, preserving the integrity of temporal structure.

2. Context-aware Feature Enhancement (CFE):

   • Function: Fine-grained cross-modal fusion that uses textual queries to guide visual feature enhancement.
   • Mechanism: Computes a similarity matrix \(M\) between frame-level visual features \(F^v\) and event-level textual queries \(F^q\); applies dual attention (column-wise and row-wise softmax) to obtain cross-attention features \(F^{v'}\) and \(F^{q'}\); concatenates these with the original features and projects them; finally incorporates a global text vector via 1D convolution for fusion.
   • Design Motivation: CM2 uses shared self-attention weights for feature enhancement, and this parameter-sharing scheme is insufficient to bridge the semantic gap between modalities. Query-guided fusion selectively suppresses irrelevant visual elements and enhances semantically aligned regions.

3. Multi-task Prediction Heads:

   • Localization Head: An MLP that regresses event centers and temporal spans.
   • Captioning Head: An LSTM with deformable soft attention for word-by-word caption generation.
   • Event Counter: Max-pooling + FC layers to predict the number of events in the video.
   • Hungarian algorithm is used for matching predictions to ground truth.

Loss & Training

• Matching loss: \(L_{\text{match}} = L_{\text{cls}} + \alpha \cdot L_{\text{loc}}\) (focal classification loss + generalized IoU loss)
• Total loss: \(L = \alpha_{\text{cls}} \cdot L_{\text{cls}} + \alpha_{\text{loc}} \cdot L_{\text{loc}} + \alpha_{\text{count}} \cdot L_{\text{count}} + \alpha_{\text{cap}} \cdot L_{\text{cap}}\)
• Frame sampling: 1 FPS; 100 frames fixed for ActivityNet, 200 for YouCook2.
• Event queries: 10 for ActivityNet, 100 for YouCook2.
• Cluster count: 10 for ActivityNet, 20 for YouCook2.
• Retrieval top-\(k\) = 40.

Key Experimental Results

Main Results (Captioning Performance)

ActivityNet Captions (comparison with non-pretrained methods):

Method	BLEU4↑	METEOR↑	CIDEr↑	SODA_c↑
PDVC (ICCV'21)	2.21	8.06	29.97	5.92
CM2 (CVPR'24)	2.38	8.55	33.01	6.18
E2DVC (CVPR'25)	2.43	8.57	33.63	6.13
CACMI (Ours)	2.44	8.68	33.80	6.39

YouCook2:

Method	BLEU4↑	METEOR↑	CIDEr↑	SODA_c↑
PDVC	1.40	5.56	29.69	4.92
CM2	1.63	6.08	31.66	5.34
CACMI (Ours)	1.70	6.21	34.83	5.57

Event Localization Performance

Method	ActivityNet F1↑	YouCook2 F1↑
PDVC	54.78	26.81
CM2	55.21	28.43
E2DVC	56.42	28.87
CACMI (Ours)	57.10	29.34

Ablation Study

CFA	CFE	CIDEr	SODA_c	F1
✗	✗	33.01	6.18	55.21
✓	✗	33.62	6.26	56.07
✗	✓	33.48	6.31	56.95
✓	✓	33.80	6.39	57.10

Cluster Count	CIDEr	F1	Note
3	32.84	54.91	Too coarse
10	33.80	57.10	Optimal
15	32.98	55.15	Over-segmented

Top-\(k\)	CIDEr	F1	Note
10	32.20	55.95	Insufficient semantic diversity
40	33.80	57.10	Optimal balance
80	32.57	56.15	Redundant information dilution

Key Findings

• CFE contributes more to localization: CFE alone improves F1 from 55.21 to 56.95 (+1.74), while CFA alone yields +0.86, indicating that cross-modal fusion is critical for temporal boundary prediction.
• CFA contributes more to captioning quality: CFA improves CIDEr by 0.61 vs. 0.47 for CFE alone, demonstrating that event-level semantic retrieval enriches caption content.
• Complementary effect of both modules: The combination achieves the best performance across all metrics.
• Most pronounced gains on SODA_c: The proposed method surpasses all baselines most notably on SODA_c, which evaluates narrative coherence, confirming that explicit temporal modeling effectively captures inter-event temporal dependencies.

Highlights & Insights

• Explicit vs. implicit temporal modeling: Discovering natural event boundaries via clustering, rather than manually designing fixed windows, better reflects the intrinsic structure of videos.
• Boundary-weighted event representation: The inverted bell-shaped weights assign higher importance to boundary frames, facilitating precise temporal localization.
• Effectiveness of query-guided fusion: More effectively bridges the modality gap compared to CM2's shared self-attention mechanism.
• No large-scale pretraining required: Surpasses certain pretrained methods without additional video pretraining data.

Limitations & Future Work

• The number of clusters is a hyperparameter that may require adaptive tuning for videos of varying lengths and complexity.
• The construction and quality of the sentence pool directly affect retrieval performance, yet this aspect is not thoroughly discussed in the paper.
• A performance gap relative to the pretrained Vid2Seq remains on YouCook2, likely due to limited domain coverage of the training videos.
• The captioning head still employs an LSTM; stronger generative models (e.g., LLM decoders) have not been explored.
• Euclidean distance used for clustering may not be the optimal metric in the high-dimensional CLIP feature space.
• Dynamic top-\(k\) or adaptive retrieval strategies have not been investigated.

Related Work & Insights

• CM2 as a pioneering work: CM2 first introduced a memory retrieval mechanism into DVC; the present work improves upon it in retrieval granularity and fusion design.
• Adapting the RAG paradigm to video: Extending text-retrieval-augmented generation from NLP to video understanding requires preserving temporal structure as a key consideration.
• Influence of PDVC's design: The parallel decoding structure allows localization and captioning subtasks to share intermediate representations.
• Insight: In any RAG system involving temporal data, performing retrieval at the granularity of semantically coherent segments—rather than fixed windows—may be a superior strategy.

Rating

• Novelty: ⭐⭐⭐⭐ (Clear motivation for explicit temporal-semantic modeling; theoretically grounded CFA+CFE design)
• Experimental Thoroughness: ⭐⭐⭐⭐ (Comprehensive evaluation on two datasets, thorough ablation studies, convincing visualizations)
• Writing Quality: ⭐⭐⭐⭐ (Well-structured, sufficiently motivated, mathematically rigorous)
• Value: ⭐⭐⭐⭐ (Provides new state-of-the-art results and a meaningful methodological contribution to DVC)

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