Intermediate fusion is an approach to combining information from different modalities. It balances the direct merging of raw data (early fusion) and the combination of independent decisions (late fusion). With intermediate fusion, each data type, such as text or images, is first processed independently to extract meaningful features. These features serve as condensed summaries of each input. The summaries, or feature representations, are then merged after this initial processing.
The core idea of intermediate fusion is to let each modality undergo some initial, specialized processing before their information is combined. Here's how it generally works:
Individual Feature Extraction: Each modality (e.g., text, image, audio) is fed into its own dedicated model or set of operations designed to extract relevant features.
Merging Processed Features: Once we have these feature vectors, one for each modality, the next step is to combine them. This is where the actual "fusion" happens. There are several common ways to do this:
Concatenation: This is perhaps the most straightforward method. The feature vectors from different modalities are simply joined end-to-end to create a single, longer feature vector. If you have an image feature vector and a text feature vector , concatenating them would look like: If has a dimension of and has a dimension of , then will have a dimension of . This combined vector now contains information from both modalities.
Element-wise Operations: If the feature vectors from different modalities have the same dimension, you can combine them element by element.
More Complex Interactions: These simple operations have more advanced techniques. For instance, methods like bilinear pooling can capture more intricate, multiplicative interactions between all pairs of features from the two modalities. Attention mechanisms, which we'll touch upon later, can also play a role in intermediate fusion by allowing one modality to selectively focus on important parts of another modality's features before or during the merging process.
Further Processing: After the features are fused into a single representation, this combined representation is typically fed into subsequent layers of a neural network. These layers learn to make predictions or decisions based on the integrated information from all modalities.
The following diagram illustrates the flow of intermediate fusion:
Data from Modality A (like text) and Modality B (like an image) are first processed by their respective feature extractors. The resulting features are then combined in a fusion layer, producing a fused representation that passes through more layers for a final output.
Why choose intermediate fusion? It offers several benefits:
Intermediate fusion is particularly useful for tasks where the interaction between modalities at the feature level is important for the final outcome. Consider an application like Visual Question Answering (VQA). To answer a question about an image (e.g., "What color is the car?"), the system needs to:
Similarly, in multimodal sentiment analysis, features from text (what is said), audio (tone of voice), and video (facial expressions) can be extracted separately and then fused to get a more reliable sentiment prediction than any single modality could provide alone.
Intermediate fusion represents a powerful and flexible way to integrate information in multimodal systems. By processing each modality to an appropriate level of abstraction before combination, it strikes a balance that often leads to effective performance on complex tasks. As you continue learning, you'll see this strategy appear in many different multimodal architectures.
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