... The user has the opportunity both to supply a variance image of the input and to receive a variance image of the smoothed output. ... If the task is required to make use of the variance (either because the user has set writevarianceset =`yes' or smoothstyle =`adaptive'), but none is supplied by the user, the task assumes that the input image inset is Poissonian - that is, that the image itself is a reasonable estimate of its own variance. ...