I am looking for a method to classify pixels into three groups: (1) Pixels which does not change their intensity much in general (the slight changes may be due to noise only), (2) Pixels which change their intensity very slowly, in general monotonically and gradually, and (3) Pixels which change their intensity drastically over a certain threshold.
I tried using the Running Gaussian Average Method, such as checking how many standard deviations the current pixel is away from the running mean. The pixel is classified as group (1) when the result is very small, as group (3) when the result is very large, and (2) otherwise. But it does not seem to work very well, especially for group (2) because by definition, this group contains pixels which change their intensity moderately instead.
I expect I may need to build 2 or more models, because background subtraction methods normally result in classifying the pixels into 2 groups only, but here I need to classify pixels into 3 groups. Any ideas will be welcome. Thanks!