base:sprite_projectiles
Sprite projectiles
by Achim
Here's a small piece of code to make one sprite fly directly to another one. If the player is aiming at an enemy (or vice versa), this routine will make sure the projectile hits the target.
It's using the bresenham line algorithm for all four quadrants. It only works with deltaX < $ff.
- Call “projecileslope” to prepare the line algorithm.
- Call “projectileflying” to move the projectile.
; calculate slope ; projectileY, -X and -MSB = sprite coordinates ; targetY, -X and -MSB = sprite coordinates ; error, deltay, deltax, deltaxhi, tmp0, tmp1 = variables, locate them wherever you want : projectileslope lda projectileY sec sbc targetY sta deltay lda projectileX ; figure out x-position sec sbc targetX sta deltax lda projectileMSB sbc targetMSB bcc skip3 sta deltaxhi lda #0 sta xdirectionflag+1 jmp setupslope skip3 lda targetX sec sbc projectileX sta deltax lda targetMSB sbc projectileMSB sta deltaxhi lda #1 sta xdirectionflag+1 setupslope lda deltaxhi ; quit if deltaX>$ff beq skip4 rts skip4 ldx #$fc ; y=y-4 lda projectileY ; figure out y-position cmp targetY bcc skip5 ldx #$04 ; y=y+4 skip5 stx mirrory1+1 ; set y direction (up or down) stx mirrory2+1 bpl skip6 lda deltay ; invert deltaY if moving up eor #$ff clc adc #1 sta deltay skip6 lda deltay cmp deltax bcs setyfast ; prepare y as fast direction if deltaY>deltaX lda #1 ; set flag for x=fast sta fastxoryflag+1 lda deltax ; x=fast -> error=deltaX/2 lsr sta error lda projectileMSB ; fly the the left or to the right? lsr ; projectile(x+msb)/2 = tmp0 lda projectileX ror sta tmp0 lda targetMSB ; target(x+msb)/2 = tmp1 lsr lda targetX ror sta tmp1 lda tmp0 ; tmp0<tmp1 -> projectile on the left -> fly to the right cmp tmp1 bcc fastx2right1 lda #$fc ; -> x=x-4 sta xsteps2+1 lda #$e9 ; 'sbc' msb value sta msbcorr2 rts fastx2right1 lda #$04 ; -> x=x+4 sta xsteps2+1 lda #$69 ; 'adc' msb value sta msbcorr2 rts setyfast lda #0 ; set flag for y=fast sta fastxoryflag+1 lda deltay ; y=fast -> error=deltaY/2 lsr sta error xdirectionflag lda #0 ; 0=fly to the left, 1=fly to the right bne fastx2right2 lda #$fc ; -> x=x-4 sta xsteps1+1 lda #$e9 ; 'sbc' msb value sta msbcorr1 rts fastx2right2 lda #$04 ; -> x=x+4 sta xsteps1+1 lda #$69 ; 'adc' msb value sta msbcorr1 rts ; ; 1) move projectile ; 2) check if projectile hits or leaves the screen ; projectileflying lda $d01e ; hardware hit detection bne projectiledecomm ; target hit nocollision lda projectileY cmp #$32 ; if y<$32 -> decommission bcc projectiledecom cmp #$f8 ; if y>$f8 -> decommission bcs projectiledecom lda projectileMSB beq skip0 lda projectileX cmp #$60 ; if x>$0160 -> decommission bcc fastxoryflag ; keep on moving projectile bcs projectiledecom skip0 lda projectileX cmp #$10 ; if x<$0010 -> decommission bcs fastxoryflag ; keep on moving sprite projectiledecom lda #$00 ; x=$0000 sta projectileX sta projectileMSB lda #$ff ; y=$ff or do whatever is necessary here sta projectileY ; to switch off sprite/projectile for your main program rts fastxoryflag lda #0 ; selfmod: 0 -> y is fast, 1 -> x is fast bne xfast lda projectileY sec mirrory1 sbc #4 ; = px/frame, +$04 or +$fc (selfmodifying code) sta projectileY lda error sec sbc deltax sta error bcs skip1 lda projectileX clc xsteps1 adc #4 ; =px/frame, +$04 or +$fc (selfmodifying code) sta projectileX lda projectileMSB msbcorr1 adc #0 sta projectileMSB lda error clc adc deltay sta error skip1 rts xfast lda projectileX clc xsteps2 adc #4 ; =px/frame, +$04 or +$fc (selfmodifying code) sta projectileX lda projectileMSB msbcorr2 adc #0 sta projectileMSB lda error sec sbc deltay sta error bcs skip2 lda projectileY sec mirrory2 sbc #4 ; =px/frame, +$04 or +$fc (selfmodifying code) sta projectileY lda error clc adc deltax sta error skip2 rts
base/sprite_projectiles.txt · Last modified: 2016-06-26 17:47 by achim