Fig. 6.

a Hough transform of pixels in a space–time diagram (see Fig. 7b) having the same gray level. The Hough count (H) is the number of pixels on a line with orientation angle φ at distance ρ from the origin. The orientation (φ) represents the angle between the vector normal to the line and the positive x-axis (Fig. 7b). b Result of thresholding, which accepts long lines exceeding 90% of the maximum Hough count in (a). c is obtained by adding the responses, as in (b), for all gray levels. The obtained result is less sensitive to noise or artifacts in the space–time diagram and preserves long lines out of the space–time diagram