Lens effect
A demo effect in which a round, magnifying lens slides across an image and warps whatever lies beneath it, like a magnifying glass over a photograph.
The trick is that nothing is calculated in real time. Before the effect runs, the program builds a lookup table for a single lens: for each pixel inside the circle, it records which background pixel to show there, shifted towards the centre to give the impression of magnification. At runtime, pixels are simply copied through the table to wherever the lens currently is. It's the same idea behind the tunnel effect and plenty of other precalculated effects.
The lens was a staple of early-1990s PC demos, once VGA's 256-colour mode made pixel copying cheap. It usually bounces around the screen, preferably over the group's logo, in a motion that was novel the first time and predictable by the fiftieth.