There are a number of different types of graphics file formats. Each type stores graphics data in a different way. Bitmap, vector, and metafile formats are by far the most commonly used formats, and we focus on these. However, there are other types of formats as well--scene, animation, multimedia, hybrid, hypertext, hypermedia, 3D, virtual modeling reality language (VRML), audio, font, and page description language (PDL). The increasing popularity of the World Wide Web has made some of these formats more popular, and we anticipate increased interest in them in the future. Although most of these file types are outside the scope of this book, we do introduce them in this section.
Bitmap formats are used to store bitmap data. Files of this type are particularly well-suited for the storage of real-world images such as photographs and video images. Bitmap files, sometimes called raster files, essentially contain an exact pixel-by-pixel map of an image. A rendering application can subsequently reconstruct this image on the display surface of an output device.
Microsoft BMP, PCX, TIFF, and TGA are examples of commonly used bitmap formats. Chapter 3 describes the construction of bitmap files in some detail.
Vector format files are particularly useful for storing line-based elements, such as lines and polygons, or those that can be decomposed into simple geometric objects, such as text. Vector files contain mathematical descriptions of image elements, rather than pixel values. A rendering application uses these mathematical descriptions of graphical shapes (e.g., lines, curves, and splines) to construct a final image.
In general, vector files are structurally simpler than most bitmap files and are typically organized as data streams.
AutoCAD DXF and Microsoft SYLK are examples of commonly used vector formats. Chapter 4, Vector Files, describes the construction of vector files in some detail.
Metafiles can contain both bitmap and vector data in a single file. The simplest metafiles resemble vector format files; they provide a language or grammar that may be used to define vector data elements, but they may also store a bitmap representation of an image. Metafiles are frequently used to transport bitmap or vector data between hardware platforms, or to move image data between software platforms.
WPG, Macintosh PICT, and CGM are examples of commonly used metafile formats. Chapter 5, Metafiles, describes the construction of metafiles in some detail.
Scene format files (sometimes called scene description files) are designed to store a condensed representation of an image or scene, which is used by a program to reconstruct the actual image. What's the difference between a vector format file and a scene format file? Just that vector files contain descriptions of portions of the image, and scene files contain instructions that the rendering program uses to construct the image. In practice it's sometimes hard to decide whether a particular format is scene or vector; it's more a matter of degree than anything absolute.
Animation formats have been around for some time. The basic idea is that of the flip-books you played with as a kid; with those books, you rapidly displayed one image superimposed over another to make it appear as if the objects in the image are moving. Very primitive animation formats store entire images that are displayed in sequence, usually in a loop. Slightly more advanced formats store only a single image but multiple color maps for the image. By loading in a new color map, the colors in the image change, and the objects appear to move. Advanced animation formats store only the differences between two adjacent images (called frames) and update only the pixels that have actually changed as each frame is displayed. A display rate of 10-15 frames per second is typical for cartoon-like animations. Video animations usually require a display rate of 20 frames per second or better to produce a smoother motion.
TDDD and TTDDD are examples of animation formats.
Multimedia formats are relatively new but are becoming more and more important. They are designed to allow the storage of data of different types in the same file. Multimedia formats usually allow the inclusion of graphics, audio, and video information. Microsoft's RIFF, Apple's QuickTime, MPEG, and Autodesk's FLI are well-known examples, and others are likely to emerge in the near future. Chapter 10, Multimedia, describes various issues concerning multimedia formats.
Currently, there is a good deal of research being conducted on the integration of unstructured text and bitmap data ("hybrid text") and the integration of record-based information and bitmap data ("hybrid database"). As this work bears fruit, we expect that hybrid formats capable of efficiently storing graphics data will emerge and will steadily become more important.
Hypertext is a strategy for allowing nonlinear access to information. In contrast, most books are linear, having a beginning, an end, and a definite pattern of progression through the text. Hypertext, however, enables documents to be constructed with one or more beginnings, with one, none, or multiple ends, and with many hypertext links that allow users to jump to any available place in the document they wish to go.
Hypertext languages are not graphics file formats, like the GIF or DXF formats. Instead, they are programming languages, like PostScript or C. As such, they are specifically designed for serial data stream transmission. That is, you can start decoding a stream of hypertext information as you receive the data. You need not wait for the entire hypertext document to be downloaded before viewing it.
The term hypermedia refers to the marriage of hypertext and multimedia. Modern hypertext languages and network protocols support a wide variety of media, including text and fonts, still and animated graphics, audio, video, and 3D data. Hypertext allows the creation of a structure that enables multimedia data to be organized, displayed, and interactively navigated through by a computer user.
Hypertext and hypermedia systems, such as the World Wide Web, contain millions of information resources stored in the form of GIF, JPEG, PostScript, MPEG, and AVI files. Many other formats are used as well.
Three-dimensional data files store descriptions of the shape and color of 3D models of imaginary and real-world objects. 3D models are typically constructed of polygons and smooth surfaces, combined with descriptions of related elements, such as color, texture, reflections, and so on, that a rendering application can use to reconstruct the object. Models are placed in scenes with lights and cameras, so objects in 3D files are often called scene elements.
Rendering applications that can use 3D data are generally modeling and animation programs, such as NewTek's Lightwave and Autodesk's 3D Studio. They provide the ability to adjust the appearance of the rendered image through changes and additions to the lighting, textures applied to scene elements, and the relative positions of scene elements. In addition, they allow the user to animate, or assign motions to, scene elements. The application then creates a series of bitmap files, or frames, that taken in sequence can be assembled into a movie.
It's important to understand that vector data historically has been 2D in nature. That is, the creator application with which the data originated made no attempt to simulate 3D display through the application of perspective. Examples of vector data include CAD drawings and most clip art designed to be used in desktop publishing applications. There is a certain amount of confusion in the market about what constitutes 3D rendering. This is complicated by the fact that 3D data is now supported by a number of formats that previously stored only 2D vector data. An example of this is Autodesk's DXF format. Formats like DXF are sometimes referred to as extended vector formats.
VRML (pronounced "vermel") may be thought of as a hybrid of 3D graphics and HTML. VRML v1.0 is essentially a subset of the Silicon Graphics Inventor file format and adds to it support for linking to Uniform Resource Locators URLs in the World Wide Web.
VRML encodes 3D data in a format suitable for exchange across the Internet using the Hypertext Transfer Protocol (HTTP). VRML data received from a Web server is displayed on a Web browser that supports VRML language interpretation. We expect that VRML-based 3D graphics will soon be very common on the World Wide Web.
This book does not contain an in-depth discussion of VRML for some of the same reasons that we do not provide detailed descriptions of hypertext, hypermedia, and 3D formats. The VRML specification is a moving target, but you can keep up with it by looking at the following resources on the Internet:
VRML information repositories
Audio is typically stored on magnetic tape as analog data. For audio data to be stored on media such as a CD-ROM or hard disk, it must first be encoded using a digital sampling process similar to that used to store digital video data. Once encoded, the audio data can then be written to disk as a raw digital audio data stream, or, more commonly, stored using an audio file format.
Audio file formats are identical in concept to graphics file formats, except that the data they store is rendered for your ears and not for your eyes. Most formats contain a simple header that describes the audio data they contain. Information commonly stored in audio file format headers includes samples per second, number of channels, and number of bits per sample. This information roughly corresponds to the number of samples per pixel, number of color planes, and number of bits per sample information commonly found in graphics file headers.
Where audio file formats differ greatly is in the methods of data compression they use. Huffman encoding is commonly used for both 8-bit graphical and audio data. 16-bit audio data, however, requires algorithms specially adapted to the problems of compressing audio data. Such compression schemes include the CCITT (International Telegraph and Telephone Consultative Committee) recommendations G.711 (uLAW), G.721 (ADPCM 32) and G.723 (ADPCM 24), and the U.S. federal standards FIPS-1016 (CELP) and FIPS-1015 (LPC-10E).
Because audio data is very different from graphics data, this book does not attempt to cover audio file formats. If you need more information on audio file formats, we recommend that you check out the following information resources on the Internet:
Guide to audio file formats
Audio file formats FAQ
Another class of formats not covered in this book are font files. Font files contain the descriptions of sets of alphanumeric characters and symbols in a compact, easy-to-access format. They are generally designed to facilitate random access of the data associated with individual characters. In this sense, they are databases of character or symbol information, and for this reason font files are sometimes used to store graphics data that is not alphanumeric or symbolic in nature. Font files may or may not have a global header, and some files support sub-headers for each character. In any case, it is necessary to know the start of the actual character data, the size of each character's data, and the order in which the characters are stored in order to retrieve individual characters without having to read and analyze the entire file. Character data in the file may be indexed alphanumerically, by ASCII code, or by some other scheme. Some font files support arbitrary additions and editing, and thus have an index somewhere in the file to help you find the character data.
Some font files support compression, and many support encryption of the character data. The creation of character sets by hand has always been a difficult and time-consuming process, and typically a font designer spent a year or more on a single character set. Consequently, companies that market fonts (called foundries for reasons dating back to the origins of printing using mechanical type) often seek to protect their investments through legal means or through encryption. In the United States, for instance, the names of fonts are considered proprietary, but the outlines described by the character data are not. It is not uncommon to see pirated data embedded in font files under names different from the original.
Historically there have been three main types of font files: bitmap, stroke, and spline-based outlines, described in the following sections.
We choose not to cover font files in this book because font technology is a world to itself, with different terminology and concerns. Many of the font file formats are still proprietary and encrypted and, in fact, are not available to the general public. Although there are a few older spline-based font formats still in use, font data in the TrueType and Adobe Type 1 formats is readily available on all the major platforms and is well-documented elsewhere in publications readily available to developers. We recommend that you check out the following resources on the Internet:
Fonts information repositories
Bitmap fonts consist of a series of character images rendered to small rectangular bitmaps and stored sequentially in a single file. The file may or may not have a header. Most bitmap font files are monochrome, and most store fonts in uniformly sized rectangles to facilitate speed of access. Characters stored in bitmap format may be quite elaborate, but the size of the file increases, and, consequently, speed and ease of use decline with increasingly complex images.
The advantages of bitmap files are speed of access and ease of use--reading and displaying a character from a bitmap file usually involve little more than reading the rectangle containing the data into memory and displaying it on the display surface of the output device. Sometimes, however, the data is analyzed and used as a template for display of the character by the rendering application. The chief disadvantages of bitmap fonts are that they are not easily scaled, and that rotated bitmap fonts look good only on screens with square pixels.
Most character-based systems, such as MS-DOS, character-mode UNIX, and character terminal-based systems use bitmap fonts stored in ROM or on disk. However, bitmap fonts are seldom used today when sufficient processing power is available to enable the use of other types of font data.
Stroke fonts are databases of characters stored in vector form. Characters can consist of single strokes or may be hollow outlines. Stroke character data usually consists of a list of line endpoints meant to be drawn sequentially, reflecting the origin of many stroke fonts in applications supporting pen plotters. Some stroke fonts may be more elaborate, however, and may include instructions for arcs and other curves. Perhaps the best-known and most widely used stroke fonts were the Hershey character sets, which are still available online.
The advantages of stroke fonts are that they can be scaled and rotated easily, and that they are composed of primitives, such as lines and arcs, which are well-supported by most GUI operating environments and rendering applications. The main disadvantage of stroke fonts is that they generally have a mechanical look at variance with what we've come to expect from reading high-quality printed text all our lives.
Stroke fonts are seldom used today. Most pen plotters support them, however. You also may need to know more about them if you have a specialized industrial application using a vector display or something similar.
Character descriptions in spline-based fonts are composed of control points allowing the reconstruction of geometric primitives known as splines. There are a number of types of splines, but they all enable the drawing of the subtle, eye-pleasing curves we've come to associate with high-quality characters that make up printed text. The actual outline data is usually accompanied by information used in the reconstruction of the characters, which can include information about kerning, and information useful when scaling characters that are very large or very small ("hints").
The advantages of spline-based fonts are that they can be used to create high-quality character representations, in some cases indistinguishable from text made with metal type. Most traditional fonts, in fact, have been converted to spline-based outlines. In addition, characters can be scaled, rotated, and otherwise manipulated in ways only dreamed about even a generation ago.
Unfortunatly, the reconstruction of characters from spline outline data is no trivial task, and the higher quality afforded by spline outlines comes at a price in rendering time and program development costs.
Page description languages (PDLs) are actual computer languages used for describing the layout, font information, and graphics of printed and displayed pages. PDLs are used as the interpreted languages used to communicate information to printing devices, such as hardcopy printers, or to display devices, such as graphical user interface (GUI) displays. The greatest difference is that PDL code is very device-dependent. A typical PostScript file contains detailed information on the output device, font metrics, color palettes, and so on. A PostScript file containing code for a 4-color, A4-sized document can only be printed or displayed on a device that can handle these metrics.
Markup languages, on the other hand, contain no information specific to the output device. Instead, they rely on the fact that the device that is rendering the markup language code can adapt to the formatting instructions that are sent to it. The rendering program chooses the fonts, colors, and method of displaying the graphical data. The markup language provides only the information and how it is structured.
Although PDL files can contain graphical information, we do not consider PDLs to be graphics file formats any more than we would consider a module of C code that contains an array of graphical information to be a graphics file format. PDLs are complete programming languages, requiring the use of sophisticated interpreters to read their data; they are quite different from the much simpler parsers used to read graphics file formats.
Copyright © 1996, 1994 O'Reilly & Associates, Inc. All Rights Reserved.