Hi there!
Based on the repositioning technique of Battlezone, I
discovered a technique that can reposition 2 objects at
once(!) in a single(!) scannline - not table driven but
by calculation!!!
Here's the snippet straight out of my recent Star Fire
source (-> it works!)
This does a centered display of 16-Bit sprites with x
coordinates ranging from 0-127:
-------------------------------------------
LDA xpos,X ; A = desired X-position!
STA WSYNC ;
STA HMOVE ; Begin repositioning scannline
.wait
SBC #$0F ; Battlezone calculate/wait
BCS .wait ; 2 in 1 magic !
AND #$0F ; Manuels "forget about EOR/ASL
TAY ; nonsense" magic :-)
LDA hmovetab,Y ;
STA HMP1 ; Fine Position sprite 1
SBC #$0F ; Tada: carry is clear, so
;subtract $0F instead of $10
STA HMP0 ; Fine Position 1 pixel
; corrected sprite 2
STA RESP0 ;
STA RESP1 ;
JMP NextLine ; Immediate WSYNC/HMOVE follows!
hmovetab
.byte $80,$70,$60,$50,$40,$30,$20,$10,$00
.byte $F0,$E0,$D0,$C0,$B0,$A0,$90
-------------------------------------------
So, I hope this gives you some more inspiration about
ways of using the Battlezone magic :-)
For me it saved 2*160 bytes of tables, and even freed
lots of kernel cycles, since both HMPX are set directly
in the repositioning line now...
Greetings,
Manuel
----------------------------------------------------------------------------------------------
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/
