sprite collision questions (DPC)

[+Muddyfunster]

Hi all,

 

I've been working on a new project and I've run into a problem that I'm struggling to work around.

 

I'm using the DPC+ Kernel and have an issue with sprite collision. I understand the constraints of working with virtual sprites in that they are all basically copies of p1.

 

My problem is to do with checking for collisions between these virtual sprites and player 0. I've drawn a little picture to illustrate :

 

 

Playfield is represented by the red box,

Green is "off screen"

Blue circles are "player8" virtual sprite and are used to show lives left

 

Player 0 enters from the top of the screen and exits the bottom and can move left and right. when player 0 re-enters at the top he inherits the X position where he exited. My problem occurs because player0 can hit player 8, triggering a collision and a loss of a life.

 

Player0 starts off with his Y variable set to -16 so that he is offscreen and uses an 8.8 variable for movement for both X and Y axis

 

I tried a simple : if player0y < 17 then goto skip_collision_check

 

My logic is this check should cover everything below 0 and everything up to y becoming = 17 (0 + the size of the sprite)

 

This works with the sprite is fully visible on the playfield but not when part of the sprite is offscreen.

 

i've tried gating the collision check every way I can think of but it looks like the IF statement isn't seeing a negative y co-ordinate as being less than 0.

 

Any ideas or suggestions would be appreciated.

 

 

 

[+Lewis2907]

This may help. I was using DPC for this, I came close to your want your asking I think. Hope this helps.

 

http://atariage.com/forums/index.php?app=core&module=attach&section=attach&attach_id=472376

[+Random Terrain]

Isn't negative 16 above 200? If so, try something like this:

---

   if player0y < 17 || player0y > 200 then goto __skip_collision_check

[bogax]

Hi all,

 

I've been working on a new project and I've run into a problem that I'm struggling to work around.

 

I'm using the DPC+ Kernel and have an issue with sprite collision. I understand the constraints of working with virtual sprites in that they are all basically copies of p1.

 

My problem is to do with checking for collisions between these virtual sprites and player 0. I've drawn a little picture to illustrate :

 

sprite explanation.png

 

Playfield is represented by the red box,

Green is "off screen"

Blue circles are "player8" virtual sprite and are used to show lives left

 

Player 0 enters from the top of the screen and exits the bottom and can move left and right. when player 0 re-enters at the top he inherits the X position where he exited. My problem occurs because player0 can hit player 8, triggering a collision and a loss of a life.

 

Player0 starts off with his Y variable set to -16 so that he is offscreen and uses an 8.8 variable for movement for both X and Y axis

 

I tried a simple : if player0y < 17 then goto skip_collision_check

 

My logic is this check should cover everything below 0 and everything up to y becoming = 17 (0 + the size of the sprite)

 

This works with the sprite is fully visible on the playfield but not when part of the sprite is offscreen.

 

i've tried gating the collision check every way I can think of but it looks like the IF statement isn't seeing a negative y co-ordinate as being less than 0.

 

Any ideas or suggestions would be appreciated.

 

 

 

 

The processor (and therfore bB) doesn't really know about negative numbers

You can subtract and there is a flag for negative numbers. The flag reflects the state of of bit 7

(There's also an overflow flag which is (the carry from bit 6) ^ (the carry from bit 7))

(It's a pity that bB doesn't give you access to the flags)

But it's really how you interpret the numbers (go read about 2's complement (Wikipedia))

Your bytes go from 0..255

255 = -1, n + 255 = n - 1 (this is all MOD 256)

if you want a range of say -17 to +17, your if statement would be something like

If x >= 239 && x <= 17 then

and in bB you can write that as

if x >= -17 && x <= 17 then

239 = 256 - 17

Using 239 takes less time and code

Edited December 26, 2018 by bogax

[+Muddyfunster]

Lewis, thanks for the code example,

 

RT, Your code tweak was actually perfect and with Bogax's additional guidance I understand more clearly now where I was going wrong.

 

Thanks a lot !