Simply put, a distance transform applied to an image will generate an output image whose pixel values will be the closest distance to a zero-valued pixel in the input image. Basically, they will have the closest distance to the background, given a specified distance measure. The following screenshot gives you an idea of what happens to the silhouette of a human body:

Human silhouette by J E Theriot

This transform can be very useful in the process of getting the topological skeleton of a given segmented image as well as to produce blurring effects. Another interesting application of this transform is in the segmentation of overlapping objects, along with a watershed.

Generally, the distance transform is applied to an edge image, which results from a Canny filter. We are going to make use of Imgproc's distanceTransform method, which can be seen in action in the distance project, which you can find in this chapter's source code. Here are the most important lines of this example...