Jaggies is the slang term for the blurry jagged lines that can appear in computer graphics. They typically occur in raster, also known as bitmap, images with decreased resolution. The lines resemble stairsteps and appear in place of straight lines or curves at the edges of the image.
Bitmap images often have jaggies because they are made of separate square pixels of varying colors and shades. When their size is changed, a process called aliasing adds or removes pixels in such a way that an error of representation, also known as an artifact, emerges. The artifacts often have a stairstep-like appearance because of this addition or subtraction of pixels around the image's edges. Non-raster graphics, known as vector graphics, rely on a mathematical formula instead of pixels to represent images. This is why vector graphics do not produce jaggies.
Animated computer graphics or videos may also have jaggies. Temporal posterization is responsible for producing these artifacts in moving graphics. This process describes what happens visually when the number of frames in a video are decreased, but its speed remains the same.
Jaggies can be reduced in computer graphics through anti-aliasing. Anti-aliasing adds shaded pixels to the areas around the edges of the image to create a smoother look. Temporal anti-aliasing, also known as motion blur, can fix jaggies in animated graphics or video. In video, this process accounts for the scene or image being captured over a span of time, not just in one instance. Temporal anti-aliasing makes the image appear as if it is darting across the screen.
Full screen anti-aliasing (FSAA) is used in three-dimensional graphics cards to reduce jaggies in video game systems. The method applies anti-aliasing to all three-dimensional images on the screen. The most commonly used anti-aliasing technique in FSAA is over-sampling. Over-sampling involves doubling or quadrupling the resolution of each frame. The frames are then down-sampled to coincide with the display resolution.
The term "jaggies" is believed to have come from the Atari video game Rescue on Fractalus!, which was released in 1985. One of the computer graphics in the game had not been anti-aliased. Due to the low-resolution on the Atari system, the artifacts were never eliminated from the image, and the developers described the effect as "jagged." They later named the enemies in the game "Jaggi" and even reportedly wanted to call the video game Behind Jaggi Lines.