  Visual-only speech recognition is dependent upon a number of factors that can
be difficult to control, such as: lighting; identity; motion; emotion and
expression. But some factors, such as video resolution are controllable, so it
is surprising that there is not yet a systematic study of the effect of
resolution on lip-reading. Here we use a new data set, the Rosetta Raven data,
to train and test recognizers so we can measure the affect of video resolution
on recognition accuracy. We conclude that, contrary to common practice,
resolution need not be that great for automatic lip-reading. However it is
highly unlikely that automatic lip-reading can work reliably when the distance
between the bottom of the lower lip and the top of the upper lip is less than
four pixels at rest.
