Towards Robust Full Low-bit Quantization of Super Resolution Networks

Quantization is among the most common strategies to accelerate neural networks (NNs) on terminal devices. We are interested in increasing the robustness of Super Resolution (SR) networks to low-bit quantization considering mathematical model of natural images. Natural images contain partially smooth areas with edges between them. The number of pixels corresponding to edges is significantly smaller than the overall number of pixels. As SR task could be considered as ill-posed restoration of edges and texture, we propose to manually focus quantized CNNs on high-frequency part of the input image thus hiding quantization error in edges and texture providing visually appealing results. We extract edges and texture using well-known edge detectors based on finite-difference approximations of differential operators. To perform inverse transformation we propose to use solver for partial differential equations with regularization term that significantly increase solution robustness to errors in operator domain. The proposed approach significantly outperforms regular quantization counterpart in the case of full 4-bit quantization, for example, we achieved +3.76 dB for EDSR x2 and +3.67 dB for RFDN x2 on test part of DIV2K.