Detecting Collisions

a little time, depending on the number of objects involved. If an optional number is given, then a mask is created for that specified image only.

BOB COL
function: detect for collision between Blitter Objects
c=Bob Col(number)
c=Bob Col(number, first To last)

The BOB COL function checks the screen for collisions between Blitter Objects. It is invaluable in the type of arcade games that rely on hitting or avoiding moving objects. To test for a collision with BOB COL, simply specify the number of the Bob you are interested in (in brackets) and a value of -1 (true) will be returned if a collision occurs. Otherwise zero (false) is generated.

Note that the AMOS Professional collision system uses "masks", and so it is sensitive to the physical shape of your objects. This means that when different objects have extremely varied appearances, the collision will only be detected when the objects happen to overlap on screen.

Normally, BOB COL will check for collisions between the specified Bob and any other Blitter Object, but you can also monitor the movements of a particular range of Bobs using this As optional parameters, after the specified Bob number, you may set the range of Bobs to be checked for collision from the first to the last in your Bob list.

BOB COL is very similar to the BC function used by the AMAL animation system. AMAL is detailed at the end of this section, in Chapter 7.6. For a rapid status test of an individual Bob, after a collision detection routine, the COL function can be used to determine precisely which pair of objects have collided amongst a whole range of similar objects. The COL function is explained later.

SPRITE COL
function: test for collision between Sprites
c=Sprite Col(number)
c=Sprite Col(number,start To finish)

SPRITE COL provides an easy method of checking to see if two or more Sprites have collided on screen. If the test is successful, SPRITE COL returns a value of -1 (true), and if not 0 (false) is returned instead. As you would expect, the brackets contain the number of any active Sprite on screen. This can be a standard Amiga hardware Sprite, or an AMOS Professional computed Sprite, but the image it displays must carry a mask. As a default, masks are created for Bobs only, so you must deliberately create a mask for each Sprite image at the start of your program, using MAKE MASK.

If you want to check for a selected group of Sprites, include the optional first to last parameters to set the range of the Sprite numbers you are interested in.

Note that any mixture of hardware Sprites and computed Sprites can be tested in the same SPRITE COL instruction. Also that the equivalent AMAL function is SC.