The ‘ideal’ viseme acts as a centre-of-mass
within the distribution pulling the speech trajectory
towards it – as we have already mentioned, coarticulation
effects and speed of talking will usually mean that we do
not make the ideal viseme shape with our mouth. Each
target parameter is weighted in accordance with its
relative dominance for a particular viseme. An ideal
closed mouth shape is more likely to be met than an open
mouth shape, and is something that we would notice
visually