If you lay a sheet of white paper on an old gravestone, and sweep the tip of a pencil across it, you get one horizontal line, dark in some places and faint in others, and not very meaningful. If you move downwards on the page by a small distance, a single pencil-line-width, and repeat, an image begins to emerge. The process of working your way down the page in a series of horizontal sweeps is what a nerd would call raster-scanning, or just rastering. With a conventional video monitor—a cathode-ray tube—the electron beam physically rasters down the glass something like sixty to eighty times a second. In the case of a laptop screen like Randy’s, there is no physical scanning; the individual pixels are turned on or off directly. But still a scanning process is taking place; what’s being scanned and made manifest on the screen is a region of the computer’s memory called the screen buffer. The contents of the screen buffer have to be slapped up onto the screen sixty to eighty times every second or else (1) the screen flickers and (2) the images move jerkily.