Image captioning is the task of generating a natural language description of an image. The task requires techniques from two research areas, computer vision and natural language generation.
This thesis investigates the architectures of leading image captioning systems. The research question is: What components and architectures are used in state-of-the-art image captioning systems and how could image captioning systems be further improved by utilizing improved components and architectures? Five openly reported leading image captioning systems are investigated in detail: Attention on Attention, the Meshed-Memory Transformer, the X-Linear Attention Network, the Show, Edit and Tell method, and Prophet Attention.
The investigated leading image captioners all rely on the same object detector, the Faster R-CNN based Bottom-Up object detection network. Four out of five also rely on the same backbone convolutional neural network, ResNet-101. Both the backbone and the object detector could be improved by using newer approaches. Best choice in CNN-based object detectors is the EfficientDet with an EfficientNet backbone. A completely transformer-based approach with a Vision Transformer backbone and a Detection Transformer object detector is a fast-developing alternative. The main area of variation between the leading image captioners is in the types of attention blocks used in the high-level image encoder, the type of natural language decoder and the connections between these components. The best architectures and attention approaches to implement these components are currently the Meshed-Memory Transformer and the bilinear pooling approach of the X-Linear Attention Network. Implementing the Prophet Attention approach of using the future words available in the supervised training phase to guide the decoder attention further improves performance.
Pretraining the backbone using large image datasets is essential to reach semantically correct object detections and object features. The feature richness and dense annotation of data is equally important in training the object detector
