Genned-up on sprites

How many times have you had a good idea for a game but found that moving large numbers of objects around the screen has slowed everything down so much that the whole thing is pointless? Well, what you obviously need are some sprites.

Sprites are picture elements that can be made to move around the screen. They are normally generated with the aid of hardware but can also be formed and moved by software. The Sprite-gen program allows you to do just that in a painless manner.

The cassette contains the sprite generator program and two example games that use ready-defined sprites. Both games are listed in the 13-page manual and I was impressed that such effective games can be written in so few statements.

The sprite generator itself is easy to use. Sprites are defined on a 7 x 8 grid and because the program uses mode 2 any of the BBC Micro's 16 colours may be used. Larger objects can be defined by linking a number of sprites in the user program.

There are 32 sprites available although numbers 0 to 7 are the main ones. The rest are known as clones. A clone normally takes on the design of its associated sprite although this can be altered.

An added degree of animation can be included by careful use of the flashing colours. The example called Chopper Chase shows this admirably. Any sprite can be simply placed in any position by a line of Basic. It is positioned by use of integer variables assigned to that particular sprite. For example, sprite 0 is allocated A% and B%. Thus to position sprite 0 at (640,512) you just key in:

100 A%=640:B%=512: Z%=0:CALL&10A3

Z% contains the sprite number and the CALL merely invokes the machine code that displays the sprite. If A% or B% is updated and sprite 0 displayed again then the old image is deleted automatically. A nice addition is that by adding 256 to the sprite number that sprite is removed from the screen.

On defining a sprite two images are created, allowing smooth movement. The second image can, however, be defined to differ from the first allowing another level of animation.

Sprite-gen is a very useful utility. The examples are good to look at and fun to play. DACC have even more impressive demonstrations so it's very versatile. A minor omission is that it doesn't include a facility to detect when one sprite hits another. But that is a minor flaw in an otherwise impressive package.

Alan Plume