  We focus on the word-level visual lipreading, which requires recognizing the
word being spoken, given only the video but not the audio. State-of-the-art
methods explore the use of end-to-end neural networks, including a shallow (up
to three layers) 3D convolutional neural network (CNN) + a deep 2D CNN (e.g.,
ResNet) as the front-end to extract visual features, and a recurrent neural
network (e.g., bidirectional LSTM) as the back-end for classification. In this
work, we propose to replace the shallow 3D CNNs + deep 2D CNNs front-end with
recent successful deep 3D CNNs --- two-stream (i.e., grayscale video and
optical flow streams) I3D. We evaluate different combinations of front-end and
back-end modules with the grayscale video and optical flow inputs on the LRW
dataset. The experiments show that, compared to the shallow 3D CNNs + deep 2D
CNNs front-end, the deep 3D CNNs front-end with pre-training on the large-scale
image and video datasets (e.g., ImageNet and Kinetics) can improve the
classification accuracy. Also, we demonstrate that using the optical flow input
alone can achieve comparable performance as using the grayscale video as input.
Moreover, the two-stream network using both the grayscale video and optical
flow inputs can further improve the performance. Overall, our two-stream I3D
front-end with a Bi-LSTM back-end results in an absolute improvement of 5.3%
over the previous art on the LRW dataset.
