Pixel-Imperfect Collisions

[ LiB ]

One thing you may have noticed is that we have been testing the ship only against single pixels. What if we want to test an image against another image? Say, a bullet against a ship, or a missile crash? Blitz Basic provides an excellent way to do this.

There is a function provided by Blitz Basic called ImagesOverlap() . It is defined like this:

 ImagesOverlap (image1,x1,y1,image2,x2,y2)

Table 9.3 explains all the parameters.

Table 9.3. ImageOverlap()'s Parameters

Parameter	Description
`image1`	The handle to the first image you want to test for collision.
`x1`	The x coordinate of the first image.
`y1`	The y coordinate of the first image.
`image2`	The handle to the second image you want to test for collision.
`x2`	The x coordinate of the second image.
`y2`	The y coordinate of the second image.

Now, let's write a program that uses this. The following, demo09-05.bb, allows you to control a ship. If you hit the randomly moving ship that is also onscreen, a collision occurs.

The program is pretty easy to follow, so I am just going to list some important parts . Following are the types from the program, and the initial values for these types.

 ;TYPES ;The enemy ship is a randomly moving object on the screen Type enemyship         Field x,y    ;the x and y coordinates         Field xv,yv    ;The velocity          Field image   ;the image End Type ;The playership type defines the player Type playership         Field x,y   ;The x and y coordinate position         Field collisions    ;How many collisions have occurred         Field image    ;The player's image End Type ;Create the enemy and assign it default variables Global enemy.enemyship = New enemyship enemy\x = 400 enemy\y = 200 enemy\xv = Rand(-5,5) enemy\yv = Rand(-5,5) enemy\image = LoadImage("enemyship.bmp") ;Create player and assign it default and random variables Global player.playership = New playership player\x = 400 player\y = 400 player\collisions = 0 player\image = LoadImage("ship.bmp")

The only major difference between the player and the enemy is that the player has velocity fields, xv and yv . These velocity values are added to the x and y coordinates of the enemy each frame and serve as movement values.

Following is the main loop. Notice how little it actually does.

 ;MAIN LOOP While Not KeyDown(1) ;Clear the screen Cls ;Make sure all text appears in top left corner Locate 0,0 Print "Collisions: " + player\collisions ;Find out if enemy hit a wall TestEnemyCollisions() ;Test keyboard TestKeys()  ;If player and enemy overlap, increment collisions and reset player and enemy If (ImagesOverlap(player\image,player\x,player\y,enemy\image,enemy\x,enemy\y))         player\collisions = player\collisions + 1         player\x = 400         player\y = 400         enemy\x = 400         enemy\y = 200 EndIf ;Move the enemy enemy\x = enemy\x + enemy\xv enemy\y = enemy\y + enemy\yv ;Draw the player and the enemy DrawImage enemy\image,enemy\x,enemy\y DrawImage player\image,player\x,player\y Flip Wend ;END OF MAIN LOOP

The main loop does this: it prints out how many collisions have occurred, it calls TestEnemyKeys() and TestKeys() , it checks if any collisions have occurred, it moves the enemy, and it draws the two ships. The only major line of interest is the test.

 If (ImagesOverlap(player\image,player\x,player\y,enemy\image,enemy\x,enemy\y))

This tests if the player's image and the enemy's image have overlapped .

That's just about it for pixel-imperfect collisions. Now, let's learn how to find out if collisions have actually occurred, using pixel-perfect collisions.

[ LiB ]