Computer Laboratory

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Teaser figure
Graphical illustration of the two phases of our loss. Phase 1 denotes the adversarial training of the discriminators. The generated image is produced by the scale-specific generator Gk , which takes as input the upscaled output of the previous level added with the task specific distortions zk . For SISR, no distortions are added (zk = 0). The levels are sequentially trained from the coarsest scale to the finest. In Phase 2, the discriminators are frozen and used as feature extractors over whose outputs an L2 distance is measured between the ground-truth training image xi and the restoration output x̃ i . The distance is measured between the two images at every scale k and intermediate layers of the discriminator.

Abstract

The choice of a loss function is an important factor when training neural networks for image restoration problems, such as single image super resolution. The loss function should encourage natural and perceptually pleasing results. A popular choice for a loss is a pre-trained network, such as VGG, which is used as a feature extractor for computing the difference between restored and reference images. However, such an approach has multiple drawbacks: it is computationally expensive, requires regularization and hyperparameter tuning, and involves a large network trained on an unrelated task. Furthermore, it has been observed that there is no single loss function that works best across all applications and across different datasets. In this work, we instead propose to train a set of loss functions that are application specific in nature. Our loss function comprises a series of discriminators that are trained to detect and penalize the presence of application-specific artifacts. We show that a single natural image and corresponding distortions are sufficient to train our feature extractor that outperforms state-of-the-art loss functions in applications like single image super resolution, denoising, and JPEG artifact removal. Finally, we conclude that an effective loss function does not have to be a good predictor of perceived image quality, but instead needs to be specialized in identifying the distortions for a given restoration method.

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Applications

We provide a comprehensive comparison of qualitative results for different loss functions across different applications. To begin with, we show results for two Single Image Super-Resolution (SISR) networks, namely, Enhanced Deep Super-Resolution (EDSR) and Super-Resolution ResNet (SR-ResNet). Further, we show the results for the applications of image denoising and JPEG artefact removal.

Single Image Super-Resolution (SISR)

Image denoising

JPEG artefact removal

We compare the performance of different losses for two codec compression qualities.

Hyper-parameter tuning for VGG and LPIPS

To find the best weightage, we conduct a hyper-parameter search over controlling the weightage sum of of VGG/LPIPS and MSE feature-wise loss fucntions: MSE + weight * VGG/LPIPS.

Contact

Please contact Aamir Mustafa with any questions regarding the method.

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement N◦ 725253–EyeCode).