  In the quest for greater computer lip-reading performance there are a number
of tacit assumptions which are either present in the datasets (high resolution
for example) or in the methods (recognition of spoken visual units called
visemes for example). Here we review these and other assumptions and show the
surprising result that computer lip-reading is not heavily constrained by video
resolution, pose, lighting and other practical factors. However, the working
assumption that visemes, which are the visual equivalent of phonemes, are the
best unit for recognition does need further examination. We conclude that
visemes, which were defined over a century ago, are unlikely to be optimal for
a modern computer lip-reading system.
