Glossary

= 2.5D =

"Two and a half-D" is an optimization trick for RT3D that fakes the viewer into thinking they are seeing true 3D graphics. A whole scene (or an object in a scene) is made of 2-dimensional graphics that are scaled and drawn in perspective to look like polygonal graphics, but there are no polygons involved. This is done by projecting displacement information onto a single plane. Accordingly, this technique is limited to 2 point perspective, not allowing any Y-axis rotation. The groundbreaking game Doom is built entirely on this concept. Billboards are 2.5D, but voxels can be either 2.5D or 3D.

Other effects classified as 2.5D can include parallax scrolling effects or orthographic 3/4 perspective (popularized by many real-time strategy and role-playing games).

= Additive Blending =

A texture blending method that uses the Additive Color Model. The pixels of a base map and a light map are blended together to make a brighter texture.

See also additive color model, additive transparency, average blending, invert blending, subtractive blending.

= Additive Color Model =

In the additive color model, red, green, and blue (RGB) are the primary colors, and mixing them together creates white. This is the way light blends together-- shine a red, a green, and a blue spotlight in the same place, and it will make white light. You add to get white. To get a lighter color use more of each color, or to get a darker color use less of each color. Additive is the color model used to display graphics on your computer screen, where all the colors are just combinations of the colors red, green and blue. Also called RGB space. See also subtractive color model, texture blending.

= Additive Transparency =

A way to calculate the color behind a transparent object, using the Additive Color Model. In general, wherever the object is more opaque, the brighter the background. If you use an alpha channel to change the transparency, the white areas get brighter, and the black areas have no effect on the background. This works well for flames, explosions, lens flares, etc., because it makes them look cleaner and hotter. If you use material opacity, it makes the whole background behind the object the same brightness level: the more opaque the object, the brighter the background, and the lower the opacity, the less bright the background. This works well for things like water or prisms or holograms. See also average transparency, subtractive transparency.

= AI =

Artificial Intelligence is a set of computer instructions or algorithms designed to simulate the actions of an intelligent being to the extent necessary to meet design requirements of the game. Unlike the AI computer science field, AI in games is much less dependent on accuracy and uses a variety of tricks and hacks to reduce memory and better serve the design of the game.

= AL =

Artificial Life. In a nutshell, AL is the antithesis of AI. While AI seeks to simulate real-world behaviour by following a complex series of rules, AL starts with very simple rules for a system and enables complex behaviour to emerge from them. Galapagos from Anark is the first commercial game to use AL.

= Aliasing =

When edges look jagged instead of smooth, and moiré patterns develop in fine parallel lines. The problem is most prevalent in diagonal lines. Aliasing happens when the engine tries to display an image on a portion of the screen where the resolution is too low to display its details correctly. This is solved with anti-aliasing, mip mapping, or texture filtering.

= Alpha Channel =

An optional channel in the texture file that usually defines the transparency of the texture’s pixels.

It can also be used for other things like a height map or grey-scale.

The alpha is usually anywhere from 1bit up to 8bits, depending on the amount of detail you need. Generally, the lower the bit depth you use, the more memory you save, but the less image quality you get. See Transparency map for more information, and some visual examples can be found on the Glossary page Transparency.

Different image channels can be extracted by a to use them for particular effects, like alpha for physics info, red channel for glow and green channel for specular and blue channel for sound types, etc. In this way, a bitmap can store more information than just RGB color.

= Ancestor =

Any node in a hierarchy that is above the current node.

See also child, parent, node

= Animatic =

An animated storyboard. This is used to refine timing, cameras, composition, etc., when a static storyboard just isn't enough. The animatic is kind of like a slide-show, with zooms and pans added to the storyboard panels to flesh out timing and composition. Where needed, low-resolution animated 3d scenes are used, intermixed with the remaining 2D storyboard panels. Sometimes called a "story reel." Also called a "Leica reel," (pronounced LIKE-uh) a term sometimes still used by gray-haired animators.

= Anisotropic Filtering =

A texture filtering method, specifically for non-square filtering, usually of textures shown in radical perspective (such as a pavement texture as seen from a camera lying on the road). More generally, anisotropic improves clarity of images with severely unequal aspect ratios. Anisotropic filtering is an improvement of isotropic mip-mapping, but because it must take many samples, it can be very memory intensive.

= Anti-Aliasing =

Anti-aliasing removes the stair-stepping or jaggies which occur at the edges of polygons or between the texels on the polygons. It works by interpolating the pixels at the edges to make the difference between two color areas less dramatic. See also aliasing, MIP mapping, texture filtering.

= ASCII =

The American Standard Code for Information Interchange (pronounced "ASS-key") is a encoding type for text, based on the English alphabet. A more universal encoding type is Unicode which allows for display of many different characters.

= Aspect Ratio =

A number that describes the shape of a rectangular texture, whether it's tall or wide. To get the aspect ratio, either divide the width by the height, or write it out as width:height. Aspect ratio helps you decide what kinds of changes need to be done to an image to get it to display correctly, like when you have to scale the image. Aspect ratio gets kind of complex when you have to deal with non-square pixels and other oddities-- not very important to the artist. See also anisotropic filtering.

= Average Blending =

When using texture blending, average is when the colors of the base map and the light map are blended together evenly. This helps when you don't want the light map to brighten or darken the base map, like when you are placing a decal of a crack on a wall to make it looked cracked. See also additive blending, average transparency, invert blending, subtractive blending.

= Average Transparency =

Colors are mixed together evenly to create a new color. Sometimes called filter transparency. See also additive transparency, average blending, subtractive transparency.

= Azimuth =

One of the two rotational axes used by the astronomers, and also by RT3D programmers. Azimuth is similar to yaw-- if you shake your head "no," you are rotating in azimuth. Most engines count from 0 to 360 degrees, starting out pointed straight forward, turning clockwise all the way around, and ending pointed straight forward again. Straight forward is both 0 and 360 degrees, and straight backwards is 180 degrees. Sometimes mathematicians measure it in radians, a unit of measure that comes from the magical number Pi. A radian equals Pi/180, but most people use degrees instead of radians.

Astronomers use only two axes (the other axis is declination), because they do not use roll. This is similar to the way most FPS games work. It is preferred in some RT3D engines because it keeps the viewer level no matter where he points. But this system has a drawback because it suffers from gimbal lock whenever the viewer points either straight up or straight down. The Quake games, for instance, avoid gimble lock by not allowing the viewer to point all the way up or down.