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169 lines
1.8 KiB
ArmAsm
169 lines
1.8 KiB
ArmAsm
; Bresenham Lines
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; by Vince `deater` Weaver <vince@deater.net>
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; based on the HGR line code from Applesoft ROM
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.include "zp.inc"
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.include "hardware.inc"
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; 171 -- initial
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; 167 -- inline init
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B_X1 = $F0
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B_Y1 = $F1
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B_X2 = $F2
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B_Y2 = $F3
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B_DX = $F4
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B_DY = $F5
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B_SX = $F6
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B_SY = $F7
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B_ERR = $F8
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COUNT = $F9
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lines:
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jsr SETGR ; set lo-res 40x40 mode
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; A=$D0 afterward
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lda #0
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sta COUNT
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lines_loop:
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jsr NEXTCOL
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lda #0
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sta B_X1
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lda #36
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; lda #35
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sta B_Y2
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lda COUNT
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cmp #10
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end:
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beq end
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asl
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asl
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sta B_Y1
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sta B_X2
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jsr draw_line
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inc COUNT
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jmp lines_loop
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;============================
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; draw line
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; from x1,y1 to x2,y2
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;============================
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draw_line:
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init_bresenham:
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; compute DX = X2-X1
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sec
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lda B_X2
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sbc B_X1
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sta B_SX ; dx sign, +=right -=left
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bcs no_invert_dx ; get ABS
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eor #$ff
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sec
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adc #1
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no_invert_dx:
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sta B_DX
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sta B_ERR
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lda B_Y2 ; get DY=Y2-Y1
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clc
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sbc B_Y1 ; actually do DY= -ABS(Y2-Y1)-1
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bcc no_invert_dy
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eor #$ff
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adc #$fe
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no_invert_dy:
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sta B_DY
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ror B_SX ; get y direction into quadrant
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sec ; count = dx - (-dy)
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sbc B_DX
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line_loop:
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ldy B_X1
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lda B_Y1
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jsr PLOT ; PLOT AT Y,A
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ldy B_X1
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cpy B_X2
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bne line_no_end
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lda B_Y1
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cmp B_Y2
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beq done_line
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line_no_end:
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jsr step_bresenham
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jmp line_loop
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done_line:
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rts
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;========================
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; step
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;========================
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step_bresenham:
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; err2 = err
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lda B_ERR
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pha ; push err2
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; if err2 > -dx:
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; err = err - dy
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; x = x + sx
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clc
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adc B_DX ; skip if err2 + dx <= 0
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bmi skip_x
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beq skip_x
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lda B_ERR
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sec
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sbc B_DY
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sta B_ERR
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lda B_X1
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clc
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adc B_SX
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sta B_X1
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skip_x:
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; if err2 < dy:
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; err = err + dx
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; y = y + sy
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pla ; pop err2
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cmp B_DY ; skip if err2 - dy >= 0
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bpl skip_y
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lda B_ERR
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clc
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adc B_DX
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sta B_ERR
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lda B_Y1
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clc
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adc B_SY
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sta B_Y1
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skip_y:
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rts
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neg:
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eor #$ff
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clc
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adc #1
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rts
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