Pixel Recurrent Neural Networks Pixel Recurrent Neural Networks
Paper summary *Note*: This paper felt rather hard to read. The summary might not have hit exactly what the authors tried to explain. * The authors describe multiple architectures that can model the distributions of images. * These networks can be used to generate new images or to complete existing ones. * The networks are mostly based on RNNs. ### How * They define three architectures: * Row LSTM: * Predicts a pixel value based on all previous pixels in the image. * It applies 1D convolutions (with kernel size 3) to the current and previous rows of the image. * It uses the convolution results as features to predict a pixel value. * Diagonal BiLSTM: * Predicts a pixel value based on all previous pixels in the image. * Instead of applying convolutions in a row-wise fashion, they apply them to the diagonals towards the top left and top right of the pixel. * Diagonal convolutions can be applied by padding the n-th row with `n-1` pixels from the left (diagonal towards top left) or from the right (diagonal towards the top right), then apply a 3x1 column convolution. * PixelCNN: * Applies convolutions to the region around a pixel to predict its values. * Uses masks to zero out pixels that follow after the target pixel. * They use no pooling layers. * While for the LSTMs each pixel is conditioned on all previous pixels, the dependency range of the CNN is bounded. * They use up to 12 LSTM layers. * They use residual connections between their LSTM layers. * All architectures predict pixel values as a softmax over 255 distinct values (per channel). According to the authors that leads to better results than just using one continuous output (i.e. sigmoid) per channel. * They also try a multi-scale approach: First, one network generates a small image. Then a second networks generates the full scale image while being conditioned on the small image. ### Results * The softmax layers learn reasonable distributions. E.g. neighboring colors end up with similar probabilities. Values 0 and 255 tend to have higher probabilities than others, especially for the very first pixel. * In the 12-layer LSTM row model, residual and skip connections seem to have roughly the same effect on the network's results. Using both yields a tiny improvement over just using one of the techniques alone. * They achieve a slightly better result on MNIST than DRAW did. * Their negative log likelihood results for CIFAR-10 improve upon previous models. The diagonal BiLSTM model performs best, followed by the row LSTM model, followed by PixelCNN. * Their generated images for CIFAR-10 and Imagenet capture real local spatial dependencies. The multi-scale model produces better looking results. The images do not appear blurry. Overall they still look very unreal. ![Generated ImageNet images](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Pixel_Recurrent_Neural_Networks__imagenet_multiscale.png?raw=true "Generated ImageNet images") *Generated ImageNet 64x64 images.* ![Image completion](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Pixel_Recurrent_Neural_Networks__occlusion.png?raw=true "Image completion") *Completing partially occluded images.*

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