Figure 3. Why a donut? For those in the know, a mathematical operation called the Fourier transform breaks down Lisa's face into a sum of sine waves with different wavelengths or frequencies. These frequencies are often plotted on a rectangular grid, where high frequencies reside near the edges of the rectangle and low frequencies near the center. Excluding the low spatial frequencies leaves a hole at the center of the grid, resulting in a rectangular donut. If you can't visualize a rectangular donut, don't worry; this kind of imagining is best left to professional mathematicians! Credit: Gerry Harp
