Aliasing results in reflection of the wave frequency about the Nyquist frequency, a.k.a. "folding". The app depicts frequency folding in 2D (two frequency components, kx and ky). The true frequency (black vector) is folded about the Nyquist limit (dashed box) until a frequency vector within the Nyquist limit is obtained (blue). This is the observed frequency. The fact that the Nyquist limit is a square in this case implies that the wave function is sampled with equal spacing in both x and y dimensions.