Canny Edge Detection is first applied to the image
to extract the edges from the input image. Then, Hough
Transform is applied to locate edges that are straight lines
and the midpoints of those straight lines are calculated.
Those straight lines and midpoints are of significant interest
to us because that the two dashed lines in the runway
hold-short lines would greatly contribute to those segmented
straight lines and midpoints. Another contributor to the
segmented straight lines and midpoints are the horizon in the
input image. Furthermore, the segmented straight lines and
midpoints from the hold-short line and the horizon exhibit a
clustering property, with the hold-short line position being 100
pixels below the peak in the y direction while the hold-short
line position being approximately the same as the peak in
histogram in the x direction. Since hold-short lines often
occur in a rectangular window of size 600 by 200 pixels, two
windows are searched, one with (PeakX, PeakY-100) as the
top right corner (left window), and the other with PeakX,
PeakY-100) as the top left corner (right window). The number
of midpoints in these two windows are computed respectively,
and if the count is larger than our threshold of 5 in any of
them, it is determined that hold-short lines are present.
Input images would typically come from consecutive video
frames, so the location of hold-short lines from one image to
the next one would only change by a relatively small amount.
We record PeakX and PeakY values from the previous image.
If no hold-short lines were detected in the previous image,
these values were both set to 0, and we proceed according
to the previously described algorithm. Otherwise, the two
windows near the previous PeakX and PeakY position also
need to be searched. The number of consecutive images that
have hold-short lines detected are also kept. If no hold-short
line has yet been detected, then a stricter criterion would be
imposed that requires the sum of midpoints in both left and
right windows to exceed a certain threshold of 12 to determine
that hold-short lines are present.