Learning OpenCV, 2nd Edition

The Hough transform

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is a method for finding lines, circles, or other simple forms in an image. The

original Hough transform was a line transform, which is a relatively fast way of searching a binary image

for straight lines. The transform can be further generalized to cases other than just simple lines (we will

return to this in the next section).

In addition to the Hough Line Transform, another more recent algorithm called LSD (for Line Segment

Detector) provides a fast technique which is, in general, more robust than the Hough Line Transform.

Hough Line Transform

The basic theory of the Hough line transform is that any point in a binary image could be part of some set

of possible lines. If we parameterize each line by, for example, a slope 𝑎 and an intercept 𝑏, then a point in

the original image is transformed to a locus of points in the (𝑎, 𝑏) plane corresponding to all of the lines

passing through that point (see

Figure 6-12). If we convert every nonzero pixel in the input image into such a set of points in the output

image and sum over all such contributions, then lines that appear in the input (i.e., (𝑥, 𝑦) plane) image will

appear as local maxima in the output (i.e., (𝑎, 𝑏) plane) image. Because we are summing the contributions

from each point, the (𝑎, 𝑏) plane is commonly called the accumulator plane.

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Hough developed the ...