"Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing which in turn is often caused by non-linear mixing effects producing high-frequency components and/or missing or poor anti-aliasing filtering prior to sampling.
Jaggies are stairlike lines that appear where there should be smooth straight lines or curves. They can occur for a variety of reasons, the most common being that the output device (display monitor or printer) does not have enough resolution to portray a smooth line. In addition, jaggies often occur when a bit-mapped image is converted to a different resolution. This is one of the advantages that vector graphics has over bit-mapped graphics — the output looks the same regardless of the resolution of the output device. The effect of jaggies can be reduced somewhat by a graphics technique known as anti-aliasing. Anti-aliasing smooths out jagged lines by surrounding the jaggies with shaded pixels. This can be done in a computer or in a printer.
The origin of the term is believed to come from the Atari 8-bit game Rescue on Fractalus!, published by Lucasfilm Games in 1985. The graphics depicting the cockpit of the player's spacecraft contains two window struts, which are not anti-aliased and are therefore very "jagged". The developers made fun of this and named the in-game enemies "Jaggi", and the game itself in its prototype form bore the name Behind Jaggi Lines!. This is believed to be the first time the term "jaggies" was used to refer to jagged computer graphics.
In realtime computer graphics, especially gaming, full screen anti-aliasing (FSAA) is used to remove jaggies created by the edges of polygons and other lines entirely. Some newer game consoles, such as the Xbox 360 and Playstation 3, had publishing policies which mandated the use of FSAA in all games released for them. Jaggies in bitmaps, such as sprites and surface materials, are most often dealt with by separate texture filtering routines, which are far easier to perform than FSAA. Texture filtering became ubiquitous on PCs after the introduction of 3Dfx's Voodoo GPU, and on game consoles following the Nintendo 64's debut.
Note: jaggies should not be confused with most compression artifacts, which are a different phenomenon.
A dogleg occurs when a nominally straight, un-aliased line steps across one pixel. The human eye is very perceptive of small irregular changes.