The categorical distribution (figure 3.3) is a discrete distribution that determines the probability of observing one of K possible outcomes. Hence, the Bernoulli distribution is a special case of the categorical distribution when there are only two outcomes. In machine vision the intensity data at a pixel is usually quantized into discrete levels and so can be modeled with a categorical distribution. The state of the world may also take one of several discrete values. For example an image of a vehicle might be classified into {ca r,motorbike,van,truck} and our uncertainty over this state could be described by a categorical distribution. r,motorbike,van,truck} and our uncertainty over this state could be described by a categorical distribution.