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When a periodic signal is sampled at a rate below the Nyquist limit, it is considered undersampled because it is impossible to recover the original signal from the samples without some additional signal information. The authors have studied the visual effects of raster sampling upon the discriminability of simple monochrome test targets (edges and sinusoidal gratings) and found out that observers have good perceptual tolerance to the sinusoidal test gratings the low sampling rates produce illusory visual effects that disturb the recognition of the waveform. The occurrence of these illusions in sampled images points out two different sources of sampling noise in images: genuine spectral noise and purely visual spatial noise. The findings suggest that scenes containing abundant edge information tolerate low sampling rates better than scenes in which low and medium spatial frequencies are dominant.