Eric Ball wrote me the following:
> For the cookbook, here is the famous skipDraw routine:
>
> ; draw player sprite 0:
> lda #P0HEIGHT-1 ; 2
> dcp P0_Y ; 5 (DEC and CMP)
> bcs .doDraw0 ; 2/3
> lda #0 ; 2
> .byte $2c ;-1 (BIT ABS to skip next 2 bytes)
> .doDraw0:
> lda (P0_Ptr),y ; 5
> sta GRP0 ; 3 = 18 cycles (constant)
>
> P0_Y is initialized to the vertical position (0 = top) plus the height of
> the sprite minus 1 (P0HEIGHT-1). So when your sprite is 6 pixels tall and
> starts at line 45, then initialize P0_Y with 50 (45+6-1). With each line,
> P0_Y will be decreased.
>
> The result of the compare will be:
> P0_Y = 50..6: 5 (P0HEIGHT-1) is smaller than P0_Y -> carry = 0 -> skipDraw
> P0_Y = 5..0: 5 is bigger than/equal to P0_Y -> carry = 1 -> doDraw
> P0_Y = 255..: 5 is smaller than P0_Y -> carry = 0 -> skipDraw
I tried to put it in action at
http://alienbill.com/2600/cookbook/skipdraw.txt
The trouble is, he doesn't specify what Y, used in "lda (P0_Ptr),y",
in there--it looks like there should be "sty P0_Y" somewhere,
since judging by the results it only displays the correct graphic
when Y < POHEIGHT, though of course that would throw of the even count.
(in my code, I'm using Y as a dey/bne'ing countdown.)
Is Eric Ball on the right track, what would this need to be to be
a successful skipdraw?
In case people were wondering, I'm trying to make one miniprogram
(still easy to understand) that covers a few topics:
-subpixel movement
-skipDraw
-joystick reading/firebutton reading
-exact horizontal positioning
-indirect referencing for changing player graphics
then for each of those topics in 2600 Cookbook, the example
will be that miniprogram with the relevant section in red.
I think it should be a good learning tool.
--
KirkIsrael@xxxxxxxxxxxxx http://kisrael.com
"Sometimes your shallowness is so thorough, it's almost like depth."--Daria
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