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.
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.)
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.)