Re: [stella] skipDraw explained (I hope) (was: bipolar joustpong)

Subject: Re: [stella] skipDraw explained (I hope) (was: bipolar joustpong)
From: KirkIsrael@xxxxxxxxxxxxx
Date: 5 Sep 2003 23:16:30 -0000
> I'll try to explain it. If I miss something please correct it.
> First of all the original skipDraw posting can be found at: .
> The first method does...
>    tya                  ; y is used to keep track of which scan line
>                         ; you're on
>    sec                  ; this can be avoided if you know the carry state
>    sbc SpriteEnd        ; SpriteEnd would be the vertical position of your
>                         ; sprite
>    adc #SPRITEHEIGHT    ; SPRITEHEIGHT is the height of your sprite so...
> ;
> ; take the current scanline and subtract the sprite postion now add back in
> ; the sprite height. Now the value should be between the height of the
> ; sprite and it's position.
> ;
>    bcx .skipDraw         ; if not between then don't draw the sprite

Is SpriteEnd equal to which scanline the sprite should start
to be drawn on? And is that counting down from like 192 or 
up from 0?

And skipdraw itself doesn't say what to load into 
GRP0, you still have to figure out what value to put 
in there?

> > * does this sound like a reasonable implementation
> > scheme, with the byte for each player? The bricks
> > are a little tall, but not too too bad.
> I'll have to read through your description later. I'm busy counting to 76 at the moment ;) After reading this again I think I might have to use the illegal opcode method.

You can get a feel for what it looks like by grabbing 
the latest binary from

> Hmm...look at Thomas' post where he collaborated with Glenn Saunders on Death Derby. It may be a little different but it's still skipDraw. Just remember DD is a 2LK. I forgot that when I first looked at it. Also I think Gunfight uses the "legal" skipDraw.

Like ?

(man I wish stella's search worked, google doesn't put it in date order...)
> You could also look at: . This might help explain the difference in DD.

Cool, as one stupid non-atari tip, my webmail gives me the wrong 
URL when you attach the end of a sentence period on a URL...I inserted
spaces in the above :-)


"Power corrupts.  Absolute power is kind of neat."
        --John Lehman, secretary of the navy 1981-1987

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