graphics: lines: more work on lines

graphics/gr/lines/lines_small.s

@@ -2,18 +2,20 @@
 
 ; by Vince `deater` Weaver <vince@deater.net>
 
-; based on code from https://gist.github.com/petrihakkinen/
+; based on pseudo-code from
-
+;	https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
-; note this code can break if X>32 or Y>32
-;	notice this if you try to plot 0,36 to 36,35
 
 
 
 .include "zp.inc"
 .include "hardware.inc"
 
-; 171 -- initial
+; 174 -- initial
-; 167 -- inline init
+; 170 -- inline init
+; 166 -- inline step
+; 161 -- merge absolute_value/negative code
+; 159 -- merging init X/Y paths
+; 156 -- more merging X/Y init
 
 B_X1	= $F0
 B_Y1	= $F1
@@ -40,8 +42,8 @@ lines_loop:
 
 	lda	#0
 	sta	B_X1
-;	lda	#36
+	lda	#36
-	lda	#35
+;	lda	#35
 	sta	B_Y2
 
 	lda	COUNT
@@ -72,75 +74,41 @@ draw_line:
 
 	; dx =  abs(x2 - x1)
 	; sx = x1 < x2 ? 1 : -1
-
 	; dy =  -abs(y2 - y1)
 	; sy = y1 < y2 ? 1 : -1
-
 	; err = dx+dy
 
-	; dx=dx*2
-	; dy=dy*2
-
 init_bresenham:
 
-	; dy = abs(y2-y1)
-	; sy = y1 < y2 ? 1 : -1
-
-	ldx	#$ff			; X = -1
-	lda	B_Y1
-	sec
-	sbc	B_Y2			; A = y1 - y2
-
-	bpl	yskip
-	inx
-	inx				; X = 1
-
-	jsr	neg			; A = y2 - y1
-yskip:
-	sta	B_DY
-	stx	B_SY
-
 	; dx = abs(x2-x1)
 	; sx = x1<x2 ? 1 : -1
 
-	ldx	#$ff			; X = -1
+	ldx	#0
-	lda	B_X1
+	jsr	do_abs
-	sec
+
-	sbc	B_X2			; A = x1 - x2
+	; dy = -abs(y2-y1)
+	; sy = y1 < y2 ? 1 : -1
 
-	bpl	xskip
 	inx
-	inx				; X = 1
+	jsr	do_abs
-	jsr	neg			; A = x2 - x1
+	jsr	neg			; dy = -abs(y2-y1)
-xskip:
-	sta	B_DX
-	stx	B_SX
 
-
+	; err = dx+dy
-	; err = dx > dy ? dx : -dy
+	; B_DY is in A already
-	; lda	B_DX
+	clc
-
+	adc	B_DX
-	cmp	B_DY	; dx - dy > 0
-	beq	noskiperr
-	bpl	skiperr
-noskiperr:
-	lda	B_DY
-	jsr	neg
-skiperr:
 	sta	B_ERR
 
-	; dx = dx * 2
-	; dy = dy * 2
-	asl	B_DX
-	asl	B_DY
-
 line_loop:
 
+	; plot X1,Y1
+
 	ldy	B_X1
 	lda	B_Y1
-
 	jsr	PLOT		; PLOT AT Y,A
 
+	; check if hit end
+
 	ldy	B_X1
 	cpy	B_X2
 	bne	line_no_end
@@ -150,62 +118,111 @@ line_loop:
 	beq	done_line
 line_no_end:
 
-	jsr	step_bresenham
+	;========================
+	; step
+	;========================
 
+step_bresenham:
+
+	; err2 = 2*err
+	lda	B_ERR
+	asl
+	pha			; push err2
+
+	; if err2 >= dy:
+	;   err = err + dy
+	;   x1 = x1 + sx
+				; signed compare
+	clc
+	cmp	B_DY
+	beq	do_x
+
+	sbc	B_DY
+	bvc	blah2
+	eor	#$80
+blah2:
+	bmi	skip_x		; ble
+
+do_x:
+	lda	B_ERR
+	clc
+	adc	B_DY
+	sta	B_ERR
+	lda	B_X1
+
+	clc
+	adc	B_SX
+	sta	B_X1
+skip_x:
+
+	; if err2 <= dx:
+	;   err = err + dx
+	;   y1 = y1 + sy
+	pla			; pop err2
+
+	clc			; signed compare
+	sbc	B_DX
+	bvc	blah
+	eor	#$80
+blah:
+	bpl	skip_y
+
+do_inc_y:
+	clc
+	lda	B_ERR
+	adc	B_DX
+	sta	B_ERR
+
+	clc
+	lda	B_Y1
+	adc	B_SY
+	sta	B_Y1
+
+skip_y:
 
 	jmp	line_loop
 
 done_line:
 	rts
 
-	;========================
-	; step
-	;========================
 
-step_bresenham:
+	; take just subtracted value
-	; err2 = err
+	; if negative, negate it, X=-1
-	lda	B_ERR
+	; if positive, fine, X=1
-	pha			; push err2
+do_abs:
-
-	; if err2 > -dx:
-	;   err = err - dy
-	;   x = x + sx
-	clc
-	adc	B_DX		; skip if err2 + dx <= 0
-	bmi	skip_x
-	beq	skip_x
-	lda	B_ERR
 	sec
-	sbc	B_DY
+	lda	B_X1,X
-	sta	B_ERR
+	sbc	B_X2,X			; A = x1 - x2
-	lda	B_X1
-	clc
-	adc	B_SX
-	sta	B_X1
-
-skip_x:
-	; if err2 < dy:
-	;   err = err + dx
-	;   y = y + sy
-	pla			; pop err2
-	cmp	B_DY		; skip if err2 - dy >= 0
-	bpl	skip_y
-	lda	B_ERR
-	clc
-	adc	B_DX
-	sta	B_ERR
-	lda	B_Y1
-	clc
-	adc	B_SY
-	sta	B_Y1
-
-skip_y:
-	rts
 
+	bmi	is_neg
 
+	ldy	#$ff
+	bmi	neg_done
 
+is_neg:
+	ldy	#$1
 neg:
 	eor	#$ff
 	clc
 	adc	#1
+
+neg_done:
+	sty	B_SX,X
+	sta	B_DX,X
 	rts
+
+
+
+;B_X1	= $F0	00
+;B_Y1	= $F1	00
+;B_X2	= $F2	00
+;B_Y2	= $F3	$23 = 35
+;B_DX	= $F4	$00
+;B_DY	= $F5	$DD = -35
+;B_SX	= $F6	$FF = -1
+;B_SY	= $F7	$01 = 1
+;B_ERR	= $F8	$DD = -35
+
+; E2=2*ERR = 1101 1101    1011 1010 = 0100 0110 = $46 = -70
+; if -70 >= -35 .... no
+; if -70 <= 0 ... yes, err=err+dx, -35+0=-35