magnitude spectrum

A grid showing how strongly an image repeats at each possible speed and direction.

The magnitude spectrum $|F(u,v)|$ of the 2D discrete Fourier transform is a second $H\times W$ grid, indexed by row-frequency $u$ and column-frequency $v$ instead of by pixel position, that records how much of the image's brightness pattern repeats at that speed and direction. A sharp edge spreads energy across many frequencies; a repeating texture concentrates energy at one frequency; a perfectly periodic ripple produces an isolated spike.

Examples

A woven fabric's repeating weave shows up as a tight, bright cluster in the magnitude spectrum, away from the DC term at the center.