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, k_x and k_y). 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.