This paper presents GridNet, a new Convolutional Neural Network (CNN) architecture for semantic image segmentation (full scene labelling). Classical neural networks are implemented as one stream from the input to the output with subsampling operators applied in the stream in order to reduce the feature maps size and to increase the receptive field for the final prediction. However, for semantic image segmentation, where the task consists in providing a semantic class to each pixel of an image, feature maps reduction is harmful because it leads to a resolution loss in the output prediction. To tackle this problem, our GridNet follows a grid pattern allowing multiple interconnected streams to work at different resolutions. We show that our network generalizes many well known networks such as conv-deconv, residual or U-Net networks. GridNet is trained from scratch and achieves competitive results on the Cityscapes dataset.
@cite_21: Despite the success of CNNs, selecting the optimal architecture for a given task remains an open problem. Instead of aiming to select a single optimal architecture, we propose a "fabric" that embeds an exponentially large number of architectures. The fabric consists of a 3D trellis that connects response maps at different layers, scales, and channels with a sparse homogeneous local connectivity pattern. The only hyper-parameters of a fabric are the number of channels and layers. While individual architectures can be recovered as paths, the fabric can in addition ensemble all embedded architectures together, sharing their weights where their paths overlap. Parameters can be learned using standard methods based on back-propagation, at a cost that scales linearly in the fabric size. We present benchmark results competitive with the state of the art for image classification on MNIST and CIFAR10, and for semantic segmentation on the Part Labels dataset.
@cite_4: 
@cite_17: Face parsing is a basic task in face image analysis. It amounts to labeling each pixel with appropriate facial parts such as eyes and nose. In the paper, we present a interlinked convolutional neural network iCNN for solving this problem in an end-to-end fashion. It consists of multiple convolutional neural networks CNNs taking input in different scales. A special interlinking layer is designed to allow the CNNs to exchange information, enabling them to integrate local and contextual information efficiently. The hallmark of iCNN is the extensive use of downsampling and upsampling in the interlinking layers, while traditional CNNs usually uses downsampling only. A two-stage pipeline is proposed for face parsing and both stages use iCNN. The first stage localizes facial parts in the size-reduced image and the second stage labels the pixels in the identified facial parts in the original image. On a benchmark dataset we have obtained better results than the state-of-the-art methods.
The idea of networks with multiple paths is not new @cite_17 @cite_4 @cite_21 . Zhou al studied a face parsing task with interlinked convolutional neural networks @cite_17 . An input image is used at different resolutions by multiple CNN whose feature maps are interconnected. Huand al @cite_4 use the same architecture but make it dynamically adaptable to computational resource limits at test time. Recently, Saxena al have presented Convolutional Neural Fabrics @cite_21 which structure forming a grid is very similar to ours and which also use the full multi-scale grid to make predictions. However, to better scale to full resolution images and large size datasets, we make use of residual units and we introduce a new dropout technique to better train our grids. Besides, we constrain our network, similarly to conv-deconv ones, to have down-sampling layers, followed by upsampling blocks, where @cite_21 use up and down sampling across all network layers.