[CV] Scaling Vision Models by Image Scales

Scaling up model size has been a significant factor driving recent advances in various domains of AI, and computer vision is no exception. Larger models and datasets have consistently demonstrated improvements across a broad range of downstream task. However, against such trend, Shi et al., 2024 explored whether larger vision models are not necessary; by scaling vision models on multiple image scales.

S²: Scaling on Scales

Instead of scaling up model size, the authors rather considered scaling on the dimension of image scales, proposing Scaling on Sales (S²). Surprisingly, they observed a smaller vision model scaled by S² often gives comparable or better performance than larger models.

Regular vision models are normally pre-trained at a single image scale (e.g., $224^2$). S²-Wrapper extends a pre-trained model to multiple image scales (e.g. $448^2$, interpolated from $224^2$).

$\mathbf{Fig\ 1.}$ Overview of S²-Wrapper (Shi et al., 2024)

S²-Wrapper is designed as a parameter-free, model-agnostic mechanism that enables multi-scale feature extraction on any pre-trained vision model. It consists of 3 key components:

1. Splitting
   To avoid quadratic computation complexity in self-attention and prevents performance degradation caused by position embedding interpolation, it splits $448^2$ image into 4 $224^2$ images.
2. Individual processing of sub-images
   All $224^2$ images are individually processed by vision model to avoid training additional parameters or the necessity of special designs of vision model.
3. Average Pooling
   The features of $4$ sub-images are merged back to the large feature map of the $448^2$ image, which is then average-pooled to the same size as the feature map of $224^2$ image. This ensures the number of output tokens stays the same, preventing computational overhead in downstream applications.

Superiority Beyond Model Size Scaling

On image classification, semantic segmentation, and depth estimation tasks, in most cases, S² from base models gives a better scaling curve than model size scaling, outperforming large or giant models with similar GFLOPs and much fewer parameters.

• It has more advantages on dense prediction tasks such as segmentation and depth estimation;
  • This matches the intuition that multi-scale features can offer better detailed understanding which is especially required by these tasks
• S² is sometimes worse than model size scaling in image classification;
  • Due to the weak generalizability of the base model feature

$\mathbf{Fig\ 2.}$ Comparison of S² and model size scaling (Shi et al., 2024)

S² is a preferable scaling approach for vision understanding in Multimodal LLMs as well. On all benchmarks, larger image scales consistently improve performance while bigger models sometimes fail to improve or even hurt performance.

$\mathbf{Fig\ 3.}$ S² scaling significantly improves the detailed understanding capability (Shi et al., 2024)

Sweet Spot Between Model Size Scaling and S²

The appropriate model size that synergizes with S² varies for different pre-trained models.

• ViT, OpenCLIP: S² from large models has no significant benefit than from base models
• DINOv2: S² from large models beats S² from base models

$\mathbf{Fig\ 4.}$ Which model size should we scale up image scales on? (Shi et al., 2024)

When Do We Not Need Larger Vision Models?

Although S² demonstrates competitive performance in various downstream tasks, larger vision models still possess advantages, such as better generalization.

Better Generalizability of Larger Models

As an example, the authors explored two types of images in the image classification task where larger models have advantages:

• (a) Rare samples: larger models have greater capacity to learn to classify these rare examples during pre-training;
• (b) Ambiguous labels: despite multiple correct labels, the large model is able to remember the label presented in the dataset during pre-training;

$\mathbf{Fig\ 5.}$ Types of samples that ViT-L improves the most but ViT-B-S² does not. (Shi et al., 2024)

Smaller Models Can Learn What Larger Models Learn

Then, when can smaller models with S² achieve the same capability as larger models? They found that most features of larger models can be well approximated by features of multi-scale smaller models. For evaluation, they trained a linear transform to reconstruct the representation of a larger model from that of a multi-scale smaller model and utilized the following measurements:

• reconstruction loss $l$
• the mutual information $I = - \log (l / l_0)$
  • $l_0$ is the reconstruction loss when the large model representation is reconstructed by a dummy vector;
  • quantify how much information in the larger model representation is also contained in the multi-scale smaller model;
• ratio $r = I / I^*$
  • $I^* = - \log (l^* / l_0)$
  • $l^*$ is the reconstruction loss when the larger model representation is reconstructed by giant models;
  • due to randomness in weight initialization and optimization dynamics, they observed that $l^*$ is never zero;
  • therefore, $I^*$ serves as upper bound of $I$;

$\mathbf{Fig\ 6.}$ Reconstructing representation of larger models from representation of regular or multi-scale smaller models. (Shi et al., 2024)

1. Compared to base models, multi-scale base models consistently exhibit lower loss and reconstruct more information from the large model representation;
2. In most cases, except for OpenCLIP-Huge features, the amount of information reconstructed from a multi-scale base model is usually close to that of a huge or giant model. Although sometimes slightly lower, it never falls short by a large margin;

Therefore, huge/giant models serve as a valid upper bound for feature reconstruction, and most part of the feature of larger models can also be learned by multi-scale smaller models. But smaller models with S² should have at least a similar capacity to learn what larger models learn.

Pre-Training With S² Makes Smaller Models Better

Given that multi-scale smaller models can learn most of the representations that larger models have, we can deduce that smaller models with S² scaling have at least comparable capacity to larger models. The authors have demonstrated that multi-scale smaller models indeed exhibit similar capacity to larger models.

To quantify model capacity, they assessed the memorization capability by treating each image in the dataset as a separate category and training the model to classify individual images, which necessitates the model to memorize each image. Additionally, classification loss on the training set of ImageNet-1k for DINOv2 and OpenCLIP is also reported.

$\mathbf{Fig\ 7.}$ Training loss on instance memorization and image classification. (Shi et al., 2024)

The results above indicate that multi-scale smaller models have comparable model capacity to larger models. Therefore, smaller models can achieve similar or even better generalizability than larger models if they are pre-trained with S² scaling.

In the figure below, it is evident that when base models are trained with a single image scale and scaled to multiple image scales only after pre-training, their performance is suboptimal compared to larger models. However, with S² during pre-training, the multi-scale base model can outperform the large model in the case of ViT. Similarly, in DINOv2, the base model pre-trained with S² achieves significantly improved performance and approached the performance level of the larger model.

$\mathbf{Fig\ 8.}$ Pre-training with S². (Shi et al., 2024)

Reference

[1] Shi et al., “When Do We Not Need Larger Vision Models?” arXiv 2024