Compute!'s First Book of Atari Graphics

The Basics Of Atari Graphics

Tom R. Halfhill

If you are new to the Atari and have acquired a bit of familiarity with BASIC, but have not yet taken the plunge into graphics, this article will introduce you to the fundamentals.

For some reason, many people are intimidated by the programming steps required to create computer graphics. Probably this is because creating computer graphics is not as easy as it looks. The typical buyer of a personal computer is dazzled in the store by all the fantastic arcade games and impressive graphics demos with which the sales people are armed. It all looks so simple. Then the buyer eagerly unpacks the computer at home and quickly discovers that even crude pictures cannot be created without screenfuls of cryptic programming that seemingly have more in common with Sanskrit than English.

But there is hope. It's not really that hard--honest. Nobody is promising that you'll be able to duplicate Star Raidersor PacManany time soon, but the basics of computer graphics are quite easy to grasp for anyone who has some knowledge of BASIC programming. You don't need to be a math wizard, either. The most valuable attributes are a willingness to learn and to experiment. And, of course, to be creative.

Choosing A Graphics Mode
Atari graphics are particularly challenging to learn, mainly because the Atari computers have extremely versatile graphics. Luckily, Atari made it easier for us by including many special keywords in Atari BASIC that are dedicated to graphics. The first step, then, is to learn those keywords. And by the way, if you don't already have your Atari BASIC Reference Manualhandy, take a second to grab it. This book and the Manualshould help to explain each other.

The most basic of the keywords is the GRAPHICS command. This tells the computer which graphics mode you want, which in turn determines how the screen will look. The format is GRAPHICS (aexp), where (aexp) is any arithmetic expression that results in a positive integer (in other words, not a negative number or a fraction). For example, GRAPHICS 6 is a valid command which tells the computer you want graphics mode six. GRAPHICS 3 + 3 or GRAPHICS 3*2 would do the same thing.

Depending upon how old your Atari is, the GRAPHICS command gives you access to either nine or twelve different graphics modes. The reason for the difference is that earlier Ataris (generally, those shipped before late 1981) came with a TV controller chip called the CTIA. Later Ataris have a GTIA chip instead. The chips are fully compatible--programs written on CTIA Ataris will run on GTIA machines and vice versa--but the GTIA adds three new graphics modes. Users with CTIA chips can have their computers upgraded if they wish. (See "Atari Video Graphics And The New GTIA" in Chapter 6.)

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Atari Graphics & Arcade Game Design

By Jeffrey Stanton with Dan Pinal

Atari computers are wonderful graphics machines capable of extraordinary visual effects. Unfortunately, few of these can be implemented directly from Atari BASIC without a thorough knowledge of Machine language and the architecture of the machine. Those who understand the techniques and have mastered them are mostly too busy writing programs to share their knowledge.

This book will allow you to enter the world of Atari graphics in which your most imaginative ideas can be animated. The various chapters will present a comprehensive course in both Atari graphics and high-speed arcade animation techniques. While at least half of the book requires the ability to program in Assembly language, we were careful to bigin the book with the simplest graphics concepts 'in Atari BASIC. The book alms to increase the novice programmer's skill. It assumes no prior knowledge of either Atari graphics or Assembly language. Since we know that many of our readers will be young teenagers, we made every attempt to include BASIC program examples, some with Machine language subroutines, in most of the chapters. We felt that concepts like custom display lists, color indirection, scrolling, character set animation, and player-missile graphics can be learned by beginners, but we didn't neglect the advanced programmer either. We cover the most advanced topics possible on a Machine language level. We discuss vertical blank and display list interrupts, kernals, bit-mapped graphics, sound, scrolling, and player-missile graphics, and use these techniques to develop four complete Assembly language games.

The only requirements for this book are an inquisitive mind, perseverance, and a good Assembler. Although prior Assembly language programming experience isn't necessary, you won't be able to write code without an Assembler.

We will attempt to explain the ideas in this book through a combination of text, drawings, flow charts, and working code. The concepts in this book may seem easy at times, and somewhat difficult at other times. The Atari is a complex machine with many idiosyncrasies. The hardware sometimes makes game design relatively easy, yet the concept of an interrupt-driven machine with its timing problems can make advanced programming frustrating. Out advice is to read the book in stages and try the examples. Learn how they work.

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