Enhanced Performance of Pre-Trained Networks by Matched Augmentation Distributions
This provides a simple, computationally free method to boost accuracy for users of pre-trained models without retraining, though it's an incremental improvement over existing multi-crop techniques.
The paper tackles the performance degradation of pre-trained CNNs due to distribution mismatch between training (with random crops) and testing (with center crops), and shows that averaging predictions from multiple random test crops improves accuracy without retraining. Softmax averaging achieved the best results across various network families.
There exists a distribution discrepancy between training and testing, in the way images are fed to modern CNNs. Recent work tried to bridge this gap either by fine-tuning or re-training the network at different resolutions. However re-training a network is rarely cheap and not always viable. To this end, we propose a simple solution to address the train-test distributional shift and enhance the performance of pre-trained models -- which commonly ship as a package with deep learning platforms \eg, PyTorch. Specifically, we demonstrate that running inference on the center crop of an image is not always the best as important discriminatory information may be cropped-off. Instead we propose to combine results for multiple random crops for a test image. This not only matches the train time augmentation but also provides the full coverage of the input image. We explore combining representation of random crops through averaging at different levels \ie, deep feature level, logit level, and softmax level. We demonstrate that, for various families of modern deep networks, such averaging results in better validation accuracy compared to using a single central crop per image. The softmax averaging results in the best performance for various pre-trained networks without requiring any re-training or fine-tuning whatsoever. On modern GPUs with batch processing, the paper's approach to inference of pre-trained networks, is essentially free as all images in a batch can all be processed at once.