Sentence:  Characteristics of <WSD>blotches</WSD> are that they seldom appear at the same spatial location in consecutive frames, they tend to be smooth (little texture), and they usually have intensity values that are very different from the original contents they cover. 
Meaning of the word marked with <WSD> tags:  A bright or dark spot on old film caused by dirt and  loss of the gelatin covering the film, due to age and poor film quality.