 Fast Line Draw |
Programming/Assembly
Draw a line (Bresenham-algorithm)
The original C64 source is from codebase64.
Restrictions
x= 0-255 (128 is middle of the screen)
For drawing only a point:
set x_1=v and x2=v+1
;Start this routine at $1100
;zp
plot_lo = $fe
plot_hi = $ff
;coords
x_1 = 155
x_2 = 0
y_1 = 0
y_2 = 100
;init screen
lda $ff06
ora #$20 ; enable bitmap mode
sta $ff06
lda #$d8 ; bitmap at $6000
sta $ff12
lda #$40 ; color/luma screen at $4000
sta $ff14
; Set up colors
ldx #0
loop1 lda #$70
sta $4000,x
sta $4100,x
sta $4200,x
sta $4300,x
lda #$01
sta $4400,x
sta $4500,x
sta $4600,x
sta $4700,x
inx
bne loop1
; Clear bitmap area. X is 0 here.
lda #$00
loop2 sta $6000,x
sta $6100,x
sta $6200,x
sta $6300,x
sta $6400,x
sta $6500,x
sta $6600,x
sta $6700,x
sta $6800,x
sta $6900,x
sta $6a00,x
sta $6b00,x
sta $6c00,x
sta $6d00,x
sta $6e00,x
sta $6f00,x
sta $7000,x
sta $7100,x
sta $7200,x
sta $7300,x
sta $7400,x
sta $7500,x
sta $7600,x
sta $7700,x
sta $7800,x
sta $7900,x
sta $7a00,x
sta $7b00,x
sta $7c00,x
sta $7d00,x
sta $7e00,x
sta $7f00,x
inx
bne loop2
; Draw a single line in infinite loop.
sei
loop3 lda #$cb
cmp $ff1d
bne *-3
dec $ff19
jsr draw_line
inc $ff19
jmp loop3
; ----------------------------------------------------------------------
draw_line
;init
ldx #$e8 ;inx
lda #y_2
sta to_y+1
sec
sbc #y_1
bcs skip1
eor #$ff
adc #1
ldx #$ca ;dex - change direction
skip1
sta d_y+1
sta t_y_1+1
sta t_y_2+1
stx incx1
stx incx2
ldx #$c8 ;iny
lda #x_2
sta to_x+1
sec
sbc #x_1
bcs skip2
eor #$ff
adc #1
ldx #$88 ;dey - change direction
skip2
stx incy1
stx incy2
ldy #x_1
ldx #y_1
;loop
;start y in x-register
;start x in y-register
;delta x in a-register
d_y cmp #0
bcc steep
sta t_x_1+1
lsr
sta errx+1
loopx
clc ;needed, as previous cmp could set carry. could be saved if we always count up and branch with bcc;
lda x_char,y
adc y_char_lo,x
sta plot_lo
lda y_char_hi,x
sta plot_hi
lda x_pixel_char,y
ora (plot_lo),y
sta (plot_lo),y ;Remember that the y_char_lo table in this example starts at $20 (which center hires mode plotting). If you lower the start of table to below $08 (say for multicolor purposes where x steps are in doubles), you will get high-byte issues when you $FE in the adc x_char with the sta (),y
errx lda #$00
sec
t_y_1 sbc #0
bcs skip3
;one might also swap cases (bcc here) and duplicate the loopend. saves more or less cycles as the subtract-case occurs more often than the add-case. Copying the whole loop to zeropage also save cycles as sta errx+1 is only 3 cycles then. (Bitbreaker)
t_x_1 adc #0
incx1 inx
skip3 sta errx+1
incy1 iny
to_x cpy #0
bne loopx
rts
steep
sta t_x_2+1
lsr
sta erry+1
loopy
clc ;needed, as previous cmp could set carry. could be saved if we always count up and branch with bcc;
lda x_char,y
adc y_char_lo,x
sta plot_lo
lda y_char_hi,x
sta plot_hi
lda x_pixel_char,y
ora (plot_lo),y
sta (plot_lo),y
erry lda #$00
sec
t_x_2 sbc #0
bcs skip4
t_y_2 adc #0
incy2 iny
skip4 sta erry+1
incx2 inx
to_y cpx #0
bne loopy
rts
y_char_lo
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $a0,$a1,$a2,$a3,$a4,$a5,$a6,$a7,$e0,$e1,$e2,$e3,$e4,$e5,$e6,$e7
.byte $20,$21,$22,$23,$24,$25,$26,$27,$60,$61,$62,$63,$64,$65,$66,$67
.byte $60,$61,$62,$63,$64,$65,$66,$67,$60,$61,$62,$63,$64,$65,$66,$67
.byte $60,$61,$62,$63,$64,$65,$66,$67,$60,$61,$62,$63,$64,$65,$66,$67
.byte $60,$61,$62,$63,$64,$65,$66,$67,$60,$61,$62,$63,$64,$65,$66,$67
y_char_hi
.byte $60,$60,$60,$60,$60,$60,$60,$60,$61,$61,$61,$61,$61,$61,$61,$61
.byte $62,$62,$62,$62,$62,$62,$62,$62,$63,$63,$63,$63,$63,$63,$63,$63
.byte $65,$65,$65,$65,$65,$65,$65,$65,$66,$66,$66,$66,$66,$66,$66,$66
.byte $67,$67,$67,$67,$67,$67,$67,$67,$68,$68,$68,$68,$68,$68,$68,$68
.byte $6a,$6a,$6a,$6a,$6a,$6a,$6a,$6a,$6b,$6b,$6b,$6b,$6b,$6b,$6b,$6b
.byte $6c,$6c,$6c,$6c,$6c,$6c,$6c,$6c,$6d,$6d,$6d,$6d,$6d,$6d,$6d,$6d
.byte $6f,$6f,$6f,$6f,$6f,$6f,$6f,$6f,$70,$70,$70,$70,$70,$70,$70,$70
.byte $71,$71,$71,$71,$71,$71,$71,$71,$72,$72,$72,$72,$72,$72,$72,$72
.byte $74,$74,$74,$74,$74,$74,$74,$74,$75,$75,$75,$75,$75,$75,$75,$75
.byte $76,$76,$76,$76,$76,$76,$76,$76,$77,$77,$77,$77,$77,$77,$77,$77
.byte $79,$79,$79,$79,$79,$79,$79,$79,$7a,$7a,$7a,$7a,$7a,$7a,$7a,$7a
.byte $7b,$7b,$7b,$7b,$7b,$7b,$7b,$7b,$7c,$7c,$7c,$7c,$7c,$7c,$7c,$7c
.byte $7e,$7e,$7e,$7e,$7e,$7e,$7e,$7e,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f
.byte $7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f
.byte $7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f
.byte $7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f,$7f
x_char
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
.byte $00,$ff,$fe,$fd,$fc,$fb,$fa,$f9,$00,$ff,$fe,$fd,$fc,$fb,$fa,$f9
x_pixel_char
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
.byte $80,$40,$20,$10,$08,$04,$02,$01,$80,$40,$20,$10,$08,$04,$02,$01
KiCHY |
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