Writer Retrieval using Compact Convolutional Transformers and NetMVLAD

This paper presents a method for writer retrieval where embeddings of patches extracted at SIFT keypoint locations are learned by a Compact Convolutional Transformer (CCT), a modified attention-based transformer architecture including convolutions, followed by a NetMVLAD layer and Generalized Max Pooling (GMP) to obtain global page descriptors. We introduce the application of CCTs for writer retrieval and show that they outperform Convolutional Neural Networks (CNNs) used in current State-of-the-Art methods for writer retrieval, namely ResNet18, while at the same time only have one-third of the number of parameters. Additionally, we propose NetMVLAD, an extension of NetVLAD with multiple vocabularies, to encode information with different vocabulary sizes improving the original NetVLAD. An evaluation of the performance of CCTs compared to ResNet18 is provided on the ICDAR2013 Competition on Writer Identification dataset (ICDAR2013) and CVL dataset. The effect of multiple vocabularies applied within the NetVLAD layer is shown. CCT7 pretrained on CIFAR100 combined with NetMVLAD achieves 89.3% Mean Average Precision (mAP) on the ICDAR2013 dataset and 96.5% on the CVL dataset.