  Designing better machine translation systems by considering auxiliary inputs
such as images has attracted much attention in recent years. While existing
methods show promising performance over the conventional text-only translation
systems, they typically require paired text and image as input during
inference, which limits their applicability to real-world scenarios. In this
paper, we introduce a visual hallucination framework, called VALHALLA, which
requires only source sentences at inference time and instead uses hallucinated
visual representations for multimodal machine translation. In particular, given
a source sentence an autoregressive hallucination transformer is used to
predict a discrete visual representation from the input text, and the combined
text and hallucinated representations are utilized to obtain the target
translation. We train the hallucination transformer jointly with the
translation transformer using standard backpropagation with cross-entropy
losses while being guided by an additional loss that encourages consistency
between predictions using either ground-truth or hallucinated visual
representations. Extensive experiments on three standard translation datasets
with a diverse set of language pairs demonstrate the effectiveness of our
approach over both text-only baselines and state-of-the-art methods. Project
page: http://www.svcl.ucsd.edu/projects/valhalla.
