The comic production industry requires reference-based line art colorization
with high accuracy, efficiency, contextual consistency, and flexible control. A
comic page often involves diverse characters, objects, and backgrounds, which
complicates the coloring process. Despite advancements in diffusion models for
image generation, their application in line art colorization remains limited,
facing challenges related to handling extensive reference images,
time-consuming inference, and flexible control. We investigate the necessity of
extensive contextual image guidance on the quality of line art colorization. To
address these challenges, we introduce Cobra, an efficient and versatile method
that supports color hints and utilizes over 200 reference images while
maintaining low latency. Central to Cobra is a Causal Sparse DiT architecture,
which leverages specially designed positional encodings, causal sparse
attention, and Key-Value Cache to effectively manage long-context references
and ensure color identity consistency. Results demonstrate that Cobra achieves
accurate line art colorization through extensive contextual reference,
significantly enhancing inference speed and interactivity, thereby meeting
critical industrial demands. We release our codes and models on our project
page: https://zhuang2002.github.io/Cobra/.