Game Engine Architecture

286 6. Resources and the File System

Unreal Engine 3 takes a similar approach, with a few important diff er-

ences. In Unreal, all resources must be contained within large composite

fi les known as packages (a.k.a. “pak fi les”). No loose disk fi les are permitt ed.

The format of a package fi le is proprietary. The Unreal Engine’s game editor,

UnrealEd, allows developers to create and manage packages and the resourc-

es they contain.

6.2.2.3. Resource File Formats

Each type of resource fi le potentially has a diff erent format. For example, a

mesh fi le is always stored in a diff erent format than that of a texture bitmap.

Some kinds of assets are stored in standardized, open formats. For example,

textures are typically stored as Targa fi les (TGA), Portable Network Graph-

ics fi les (PNG), Tagged Image File Format fi les (TIFF), Joint Photographic Ex-

perts Group fi les (JPEG), or Windows Bitmap fi les (BMP)—or in a standard-

ized compressed format such as DirectX’s S3 Texture Compression family of

formats (S3TC, also known as DXTn or DXTC). Likewise, 3D mesh data is

oft en exported out of a modeling tool like Maya or Lightwave into a stan-

dardized format such as OBJ or COLLADA for consumption by the game

engine.

Sometimes a single fi le format can be used to house many diff erent types

of assets. For example, the Granny SDK by Rad Game Tools (htt p://www.rad-

gametools.com) implements a fl exible open fi le format that can be used to

store 3D mesh data, skeletal hierarchies, and skeletal animation data. (In fact

the Granny fi le format can be easily repurposed to store virtually any kind of

data imaginable.)

Many game engine programmers roll their own fi le formats for various

reasons. This might be necessary if no standardized format provides all of

the information needed by the engine. Also, many game engines endeavor to

do as much off -line processing as possible in order to minimize the amount

of time needed to load and process resource data at runtime. If the data

needs to conform to a particular layout in memory, for example, a raw binary

format might be chosen so that the data can be laid out by an off -line tool

(rather than att empting to format it at runtime aft er the resource has been

loaded).

6.2.2.4. Resource GUIDs

Every resource in a game must have some kind of globally unique identifi er

(GUID). The most common choice of GUID is the resource’s fi le system path

(stored either as a string or a 32-bit hash). This kind of GUID is intuitive, be-

cause it clearly maps each resource to a physical fi le on-disk. And it’s guar-