Figure 3.

A vertical edge detector convolution kernel. Edges that transition from dark to light (as shown in the input image) will be light in the output image. The pixel values (representing light intensity) are shown as pink numbers. No values in the output image are <0. This is because all values <0 are turned into 0 by a function known as a rectified linear unit (ref. 95)—this is known as an activation function and is a common technique in deep learning. The rim of zeroes around the input image—known as “padding”—allows the output image to retain the same dimensions as the input image. Color figure can be viewed in the online issue, which is available at http://onlinelibrary.wiley.com/doi/10.1002/art.42296/abstract.