Adding a missile

[nmoog]

Ok, I've followed Andrews great tutorials and am up to the point where I have a cool lil' guy running around the screen.

 

I am now attempting to add a "missile". I was reading the Stella programmers guide I gathered that to get a missile to display I just had to enable it with

lda #1

sta ENAM0

 

Then at the correct x position (which I just did after the xpositioning code of the player):

sta RESM1

 

I do that straight after the player positioning and I thought i would see a missile that was as tall as my screen a couple of pixels right of my player. But I saw nothing. Is there anything else I need to set?

 

Thanks!

[+SpiceWare]

Need to set bit 1, not bit 0. . 2^0 = 1, 2^1 =2 so

 

lda #2

sta enam0

Edited June 1, 2006 by SpiceWare

[nmoog]

Damn... yep. Thanks for that Spiceware (or is it Darrell?)

[nmoog]

Actually, I have another question for you (well I have thousands of questions, but I'll stick to the one on the top of the heap...)

 

I have 3 objects on screen now, and in the vertical blank time I do a WSYNC, then run the XPosition code for player 1, then another WSYNC then x positiong for player 2 and another WSYNC then Missile 1 postioning all using the "Battlezone variant"

 

That works lovely. but the Y positioning - how do I fit in all the ypositioning code for all the objects into a scanline? I am using the "skipdraw" method (i think!) like so:

	sec
tya
sbc SpriteEnd
adc #SpriteHeight
bcs .MBDraw3

nop; Make y drawing always
nop; use same amount of cycles
sec

bcs .skipMBDraw3
			
.MBDraw3
lda (SpriteTablePtr),y
sta GRP0
sta GRP1
			
.skipMBDraw3

 

Do I just run exactly the same code for each object on screen? Is there a standard method to achieve the vertical positioning checks?

[+SpiceWare]

Darrell's my name, SpiceWare's my alias. I've been doing software under SpiceWare name since the early 80s. here's an example of my logo from 88.

 

It's all tradeoffs when programming the Atari as there's a limited amount of time to do everything you have to do. In Medieval Mayhem I'm controlling 5 objects, the playfield, changing the playfield color and reading the paddles. There's no way I could do that on every scanline so I'm using a 2-line-kernel where most things are updated every other scanline(the playfield has to be updated every scanline).

 

One of the tricks I'm using is my sprites are limited in the Y range they can be drawn at, so I've chosen to pad the graphics with 0s:

TopShieldFacingDown
.byte zz________
.byte zz__XXXX__
.byte zz_XXXXXX_
.byte zzXXXXXXXX
.byte zzXXXXXXXX
.byte zzXX____XX
.byte zzX______X
.byte zz________

.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows

TopShieldFacingSideways
.byte zz________
.byte zzXXXXX___
.byte zz_XXXXX__
.byte zz__XXXXX_
.byte zz__XXXXX_
.byte zz__XXXXX_
.byte zz_XXXXX__
.byte zzXXXXX___

.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows

TopShieldFacingDiagonal
.byte zz___XXX__
.byte zz__XXXXX_
.byte zz_XXXXXXX
.byte zzXXXXXXXX
.byte zz___XXXXX
.byte zz____XXX_
.byte zz____XX__
.byte zz____X___
TopShieldDead
.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; 2 brick rows
.byte 0,0,0,0,0,0,0,0; for "dead" shape

 

and just draw the shields on every line w/out using any skipdraw logic

lda (P1Shield),y
sta GRP0
lda (P3Shield),y
sta GRP1

[nmoog]

Hmm, I'm not sure I understand how that differs from the "skipdraw" method - don't you still have to say somewhere "dont start drawing the shields until you get to scanline #xx?

[+SpiceWare]

It's different in that it's faster during the display kernel. I had too many things to display and could not spare SkipDraw's 17-18 cycles per player. The method I use only takes 8 cycles per player.

 

Outside of the kernel I have to add zeros to either side of the sprite image(taking up extra ROM space) and adjust the pointer to the sprite (the value in P1Shield) up/down based on the Y value.

[nmoog]

Is that the kind of technique they would have used to make a frogger style game? I was thinking of doing something like this as a project to bring together all the stuff I've been reading here - I've got it kinda working, but I've just been ignoring all the line-changes that the sprites do mid-frame

 

What would be the best way to store multiple sprites on fixed y coordinates a-la frogger?