A propshaft assembly includes an axle flange, a propshaft, and a damper. A plurality of flags each move into and out of a field of view of each of the cameras. If the current pixel is a boundary pixel, then the 1-D central variance is computed for horizontal, vertical and diagonal directions that cross the block boundary by using three pixels along a line centered at the current pixel. When the number of consecutive surrounding pixels having the same signal value is equal to or greater than a threshold, the determining section determines the target pixel as the feature point. The apparatus can also be used for determining a degree of contamination of a surface of the tape, the surface being contaminated by particles, by detecting illumination radiation scattered at the tape.