Multisprite positioning bug?

[SeaGtGruff]

I've been working on a game for someone, but I've run into problems with positioning the multisprite players near the bottom of the screen. The following test program demonstrates what's happening. The sprite (which is just a single line for this little test) should drop down the screen, keeping the same x position all the way down-- but as it nears the bottom of the screen, it shoots over toward the middle of the screen (presumably to where player1 is positioned for the score?).

 

   includesfile multisprite.inc
  set kernel multisprite

  player1:
  %11111111
end

  player1x = 8
  player1y = 85
  _COLUP1 = $1A

loop
  drawscreen
  player1y = player1y - 1
  if player1y = 3 then player1x = 8 : player1y = 85
  goto loop

Is this a bug in the multisptite kernel?

 

Michael

[+batari]

Is this a bug in the multisptite kernel?

I'm afraid so, but it only occurs under very specific circumstances, i.e. when the top of a sprite is more than a few lines below the top of the lowest playfield block. In most games, you will never see this bug.

 

If you pfheight=3 or less, you will never see the bug. Or if you use pfheight=7 and all of your sprites are more than a 4 or 5 lines tall.

 

Of course I'll look into fixing it, but for now, it's easy to avoid and most games probably won't encounter it.

[+batari]

I think this might fix it. The problem seems to have been a page-wrapping issue. The fix adds 4 bytes to the kernel.

msk_5_25_07.zip

[SeaGtGruff]

I think this might fix it. The problem seems to have been a page-wrapping issue. The fix adds 4 bytes to the kernel.

Yes, that seems to have fixed it. Thank you, batari! Now I can get back to working on that game.

 

Michael