collision detection

[tomek]

hello, after a long and unwanted break, I returned to amos. With the help of books, I created something that works. title page, menu, level 1 art. I even transferred photos from pc to amiga . Unfortunately i have problem with 2 things:
1. i can't understand the collision system, i think something is wrong with bob and sprite numbers but i tried all ways and nothing.
2. as long as the background works fine, other objects like planes except bullets are flashing. The colours palette is the same in bobs and in background tiles.
Can someone look at the code and correct it and explain to me what is wrong there. please

[adrazar]

Hi, I have some suspicions as to what the problems might be.
1. The Spritebob Col (and Bobsprite Col) command doesn't take Screen Offset into account, so to make it work you need to reposition the missile (to compensate the screen offset) before using Spritebob Col, and then move it back right after. (It is probably a good idea to turn off automatic sprite updates if you're doing this, using Sprite Update Off.)
2. Could it be that you're not in Double Buffer mode?

[tomek]

I can't use double buffer because it breaks the scrolling of the screen. how to add screen offset to sprite bob col? sprite update off disables bullets.

[adrazar]

Yes, with Sprite Update Off you need to use the command Sprite Update to show and move the sprites. To adjust for Screen Offset, I think you just add the screen offset coordinates, like

XOFFSET=SCRPOINTER+LICZNIK
Sprite 0,X Sprite(0)+XOFFSET,
Spritebob Col(0,0 To LICZBAOBCYCH-1)
Sprite 0,X Sprite(0)-XOFFSET,

Yes, the scrolling will need some (minor?) rewriting to work in Double Buffer mode. But it is generally required to use double buffer when using bobs, so I recommend looking into it.

[tomek]

hello, because i had a big problem with screen scrolling code replacement and with bobs (empty tiles sometimes, flashing bobs) i decided to use amos tome program and as usual almost everything works. works: scrolling, bullets, aliens, player-alien collisions, and bullet-player collisions. only bullet-alien collisions don't work. Please help as I still don't understand this system.

[adrazar]

From the code it seems like you have used the same bob numbers both for bullets and enemies? I suppose what you want is to have Bob 0-1 for players, Bob 2-7 for bullets and Bob 8- for enemies.

[tomek]

I changed bob numbers:
1: player
4: alien
8: projectile
and everything went to hell, even the shooting

I don't understand it, I'm too stupid for that, I'm good at graphics but not at programming


Dim EX(5),EY(5),EP(5),EH(5),ESX(5)
Dim BX(5),BY(5)
Global EX(),EY()
Data 1,64,128,1,128
Load "ikonki.abk"
Load "samoloty.abk"
Reserve As Work 6,404
Bload "mapa.map",Start(6)
Tile Size 32,32
Map View 0,0 To 256,288
Hide On
No Icon Mask
Auto View Off
Screen Open 1,320,288,16,Lowres
Curs Off : Flash Off : Cls 0
Wait Vbl
Screen Hide 1
Screen Open 0,320,256,16,Lowres
Curs Off : Flash Off : Cls 0
Get Icon Palette : Wait Vbl
Double Buffer
Screen To Front 0
Auto View On
HYO=-1
MXY=Map Y*32
MY=MXY-256
SCR=1
X=128 : Y=128 : T=0
LIVES=3
Bob Update Off
While LIVES>0
DEAD=0
X=128 : Y=128 : T=0
ENEMY=0
BL=0
While DEAD<4
HY=MY/32
Screen Copy 1,0,0,256,288 To 1,0,SCR
If HYO<>HY : Gosub VERTMAP : End If
Screen 0
Bob Clear
Screen Copy 1,0,32,256,288 To Logic(0),0,0
HYO=HY
Add MY,-SCR
MY=(MY+MXY) mod MXY
Bob 1,X,Y,1
NB=0
BADDIES
If BL>0
Gosub BULLETS
End If
If NB<NBO
For A=NB To NBO
Bob Off A+2
Next A
End If
NBO=NB
C=Bob Col(1,2 To NB+1)
If C and DEAD=0 : DEAD=1 : Boom : End If
Bob Draw
Screen Swap
Wait Vbl
T=1-T
X=Max(16,Min(240,X+(Jleft(1)-Jright(1))*8))
Y=Max(128,Min(240,Y+(Jup(1)-Jdown(1))*8))
If Fire(1)
Gosub GUN
End If
Wend
Wend
End
GUN:
If BL<6
BX(BL)=X : BY(BL)=Y-8
Inc BL
Shoot
End If
Return
BULLETS:
For A=0 To BL-1
If BX(A)>0 and BX(A)<256 and BY(A)>0 and BY(A)<256
Bob 8+NB,BX(A),BY(A),8
C=Bob Col(4+NB,3 To 8+NB)
If C and ENEMY>0
For B=0 To ENEMY-1
If Col(B+2)
EH(B)=1
Boom
SCRE=SCRE+100
BX(A)=-1 : BY(A)=-1 : Bob Off 4+NB : B=999
End If
Next B
End If
Inc NB
BY(A)=BY(A)-16
Else
BX(A)=0
End If
Next A
For A=BL-1 To 0 Step -1
If BX(A)=0
Swap BX(BL-1),BX(A) : Swap BY(BL-1),BY(A)
Dec BL : BL=Max(0,BL)
End If
Next A
Return
Procedure BADDIES
Bob 4+NB,EX(A),EY(A),4
Add EY(A),4
If EY(A)>300
EX(A)=Rnd(250)+30
EY(A)=30
End If
End Proc
VERTMAP:
Screen 1
If HYO=-1
Map Do 0,HY
Screen Copy 1,0,0,256,288 To 1,0,32-SCR
Else
Map Top 0,HY
End If
Return

[adrazar]

Well, I think you just need to work on your debugging skills. The most common way to figure out these things is not by thinking, most of the time you simply start a debugger from a suitable place in the code. In AMOS you can do this with the command Monitor. The debugger in AMOS works like most other debuggers: it allows you to execute one line of code at a time and look directly into the values held by your variables.

When learning how to use a new system, another thing to consider is creating smaller programs to test its features. In this case for instance, if you're uncertain about whether or not you've understood the bob system correctly, then writing a small program could help you figure that out.

I'm unable to take in exactly what's going on in the code you posted, but I think the values you pass to Bob Col looks a bit odd. I challenge you to try the Monitor from the beginning of BULLETS: and see if you discover anything out of order.