Aliasing in Images
Aliasing in images can have some very disturbing psychovisual effects. Consider the following applet:
The bottom slider varies the tilt of the sinusoidal pattern. The right slider varies the frequency. If you set the tilt to slight, and then increase the frequency, then near the Nyquist frequency (halfway up), very strange things happen to the image. New patterns appear. These are called Moire patterns.
Digital images are always susceptible to these artifacts. It turns out, however, that even analog television is susceptible. Analog television is generated using a raster scan pattern, where the image is constructed by drawing horizontal lines and varying the intensity and color along the horizontal line. This means that in the vertical direction, the image is sampled. For this reason, television performers will not wear striped clothing, particularly not with horizontal stripes. As you can see from the above applet, horizontal stripes are particularly susceptible to aliasing artifacts.
The print industry is also susceptible. Even simple black-and-white images are made by placing dots of ink on the page. These dots translate into samples. The following applet illustrates the emergence of artifacts in black-and-white images. Move the slider until you see spurious patterns appearing at the upper left.