Rivers are especially hard to detect because to model what people see the program would have to look at actual shapes of the letters. Two similar configurations

Rivers are especially hard to detect because to model what people see the program would have to look at actual shapes of the letters. Two similar configurations of character boxes may or may not give the appearance of a river depending on which characters are in these boxes. E.g. if a diagonal river passes along the long stem of "y", the shape of the letter contributes to the illusion, but if the river goes along the short stem, them the longer stem will visually work against it.