This is b ecause the line through f 0 (s) and f 1 (s) is linearly parametrized as f 0 (s) + t[f 1 (s) − f 0 (s)] = (1 − t)f 0 (s) + tf 1 (s), with the segment from f 0 (s) to f 1 (s) covered by t values in the interval from 0 to 1 . If f 1 (s) happens to equal f 0 (s)