Next-generation image captioning: A survey of methodologies and emerging challenges from transformers to Multimodal Large Language Models

The widespread availability of visual data on the Internet has fueled a significant interest in image-to-text captioning systems. Automated image captioning remains a challenging multimodal analytics task, integrating advances in both Computer Vision (CV) and Natural Language Processing (NLP) to understand image content and generate semantically meaningful textual descriptions. Modern deep learning-based approaches have supplanted traditional approaches in image captioning, leading to more efficient and sophisticated models. The development of attention mechanisms and transformer-based architectures has further enhanced the modeling of both language and visual data. Despite these gains, challenges such as long-tailed object recognition, bias in training data, and shortcomings in evaluation metrics constrain the capabilities of current models. Furthermore, an important breakthrough has been made with the recent emergence of Multimodal Large Language Models (MLLMs). By incorporating textual and visual data, MLLMs provide improved captioning flexibility, generative capabilities, and reasoning. However, these models introduce new challenges, including faithfulness, grounding, and computational cost. Although relatively few studies have comprehensively surveyed these developments, this paper provides a thorough analysis of Transformer-based captioning approaches, investigates the shift to MLLMs, and discusses associated challenges and opportunities. We also present a performance comparison of the latest models on the MS-COCO benchmark and conclude with perspectives on potential future research directions.