In this Chapter you will learn how to take full advantage of the Amiga's "Blitter" chip, which can copy large sections of a screen almost instantaneously.
At its fastest, the Blitter can move a million screen points per second, which is the equivalent of a dozen graphic screens. AMOS Professional exploits this facility for the incredible speed achieved in commands like SCREEN COPY, but the Blitter is capable of far more than simple graphics.
Professional animations are readily available, using special "Blitter Objects" known as "Bobs". Bobs can be displayed at any point on the screen and freely moved over the entire screen area, without disturbing any existing graphics. They may be guided, tested for collisions and even animated with AMAL, exactly like Sprites.
The main advantage of Bobs over Sprites is that they are far easier to use. There is no imit to the size or number of Bobs, and they are stored as part of the current screen, so all positions are measured in simple screen coordinates. As has been explained in the previous Chapter, Sprites only work in certain combinations, but Bobs may be displayed with no restrictions at all, at any position, and in vast numbers. The only limit is the amount of available memory! The other main advantage over Sprites is that Bobs can have up to 64 colours.
Naturally, all this power carries a price tag, and although Bobs are more flexible than Sprites, they are also slightly slower and consume additional memory. So the ideal solution is to use both Sprites and Bobs to their full advantage in the same program. They make a superb team, just like your Amiga, AMOS Professional and you!
Displaying a Bob
Images to be used as Bobs are stored in memory Bank 1, and are each referred to by a simple
number, which ranges from 1 up to the maximum number of objects in the bank. Load up some
images now, like this:
E> Load "AMOSPro Tutorial:Objects/Bobs.abk"
To find how many objects are in memory bank 1, use the LENGTH function for an instant read- out. Type this next line from Direct Mode:
D> Print Length(1)
This Object Bank is also used for any Sprite images, so the same objects can be displayed as Bobs or Sprites with great ease. To create a Bob, the image of an object is taken from the bank, and allocated for display as follows.
BOB
instruction: display a Bob on screen
Bob number,image
Bob number,x,y,image
Each Bob must be given an identification number from 0 to 63. As a default, only 64 Bobs may be displayed on screen at once, but this limit can be increased if necessary.