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https://github.com/deater/dos33fsprogs.git
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227 lines
2.8 KiB
ArmAsm
227 lines
2.8 KiB
ArmAsm
; Bresenham Lines
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; by Vince `deater` Weaver <vince@deater.net>
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; based on pseudo-code from
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; https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
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.include "zp.inc"
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.include "hardware.inc"
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; 174 -- initial
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; 170 -- inline init
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; 166 -- inline step
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; 161 -- merge absolute_value/negative code
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; 159 -- merging init X/Y paths
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; 156 -- more merging X/Y init
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; 154 -- note PLOT doesn't touch Y
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; 152 -- merge into common_inc
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; 151 -- share an RTS
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; 150 -- use X when plotting
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; 148 -- re-arrange do abs (thanks to 42BS)
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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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restart:
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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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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 restart
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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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; from wikipedia
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; dx = abs(x2 - x1)
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; sx = x1 < x2 ? 1 : -1
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; dy = -abs(y2 - y1)
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; sy = y1 < y2 ? 1 : -1
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; err = dx+dy
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init_bresenham:
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; dx = abs(x2-x1)
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; sx = x1<x2 ? 1 : -1
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ldx #0
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jsr do_abs
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; dy = -abs(y2-y1)
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; sy = y1 < y2 ? 1 : -1
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inx
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jsr do_abs
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jsr neg ; dy = -abs(y2-y1)
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; err = dx+dy
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; B_DY is in A already
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clc
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adc B_DX
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sta B_ERR
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;======================
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; iterative plot points
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line_loop:
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; plot X1,Y1
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ldy B_X1
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ldx B_Y1
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txa
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jsr PLOT ; PLOT AT Y,A (A colors output, Y preserved)
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; check if hit end
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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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cpx B_Y2
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beq done_line
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line_no_end:
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;========================
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; step bresenham
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;========================
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; err2 = 2*err
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lda B_ERR
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asl
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pha ; save err2 for later
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ldx #0 ; setup for common_inc
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; if err2 >= dy:
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; err = err + dy
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; x1 = x1 + sx
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; B_ERR already in A
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cmp B_DY ; check equal first
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beq do_x
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clc ; signed compare
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sbc B_DY
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bvc blah2
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eor #$80
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blah2:
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bmi skip_x ; ble
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do_x:
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lda B_DY
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jsr common_inc
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skip_x:
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inx ; setup common inc
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; if err2 <= dx:
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; err = err + dx
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; y1 = y1 + sy
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pla ; restore err2
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clc ; signed compare
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sbc B_DX
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bvc blah
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eor #$80
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blah:
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bpl skip_y
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do_y:
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lda B_DX
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jsr common_inc
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skip_y:
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jmp line_loop
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;=====================================
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; common increment
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;=====================================
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common_inc:
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clc
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adc B_ERR
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sta B_ERR
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lda B_X1,X
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clc
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adc B_SX,X
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sta B_X1,X
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done_line:
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rts
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;=====================================
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; init, do the abs and sx calculations
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; x=0, for X
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; x=1, for Y
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;=====================================
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do_abs:
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ldy #$ff
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sec
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lda B_X1,X
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sbc B_X2,X ; A = x1 - x2
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bpl is_pos
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ldy #$1
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neg:
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eor #$ff
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clc
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adc #1
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is_pos:
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sty B_SX,X
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sta B_DX,X
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rts
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