Triangle filling routine using doublemask HLINE machine code
Written for the Acorn Atom

Lines 10 - 70 belong to triangle filling routine

10	DIM XX(3), YY(3)	co-ordinates
20	DIM DD(3), GG(3)	divisors and gradients
30	S = 256
40	XX1 = 10,  YY1 = 15
50	XX2 = 250, YY2 = 100
60	XX3 = 120, YY3 = 190

Common to both hline routines:

rem S	0000 0001
	0000 0011
	0000 0111
	0000 1111
	0001 1111
	0011 1111
	0111 1111
	1111 1111
rem E	1111 1110
	1111 1100
	1111 1000
	1111 0000
	1110 0000
	1100 0000
	1000 0000

70	S =#87: E=#80			base addresses of two overlapping tables
80	E!4=#FFFEFCF8: E!E=#F0E0C080	end-byte mask table
90	S!4=#0103070F: S!S=#1F3F7FFF	start-byte mask table contents
100	L=#91:V=#93:M=#95:T=#97
110	R=#92:W=#94:G=#96:B=#98
120	DIM LL9: P.$21
130	FOR N = 1 TO 2: DIM P(-1)	or P=#2800

This is the generic HLINE, does any operation

140	[
150	:LL1	LDA L
170		LSR A: LSR A: LSR A	Left X/8 = no of bytes
180		TAY: STA G		from left of screen in G, and Y reg
190		LDA L			Left X%7 = no of bits in X reg
200		AND @7: TAX
210		LDA S,X: STA M
220		LDA R
230		LSR A: LSR A: LSR A
240		SEC: SBC G: BEQ LL4	If == 0, then start and end pixel in same byte

250		STA G			Get start_byte
260		LDA M: AND W
270		ORA (B),Y: STA T
280		LDA M: AND V
290		EOR T: STA (B),Y	Ex-or mask from table T into display RAM
300		INY: DEC G: BEQ LL3	Next Y byte. If byte-count == 0 then LL3
					Handle midbyte
310	:LL2	LDA (B),Y		do { Fetch display Byte Y
320		ORA W: EOR V		
330		STA (B),Y			Store display Byte
340		INY: DEC G: BNE LL2		Y--;G-- } while ( G );
350	:LL3				Endbyte:
360		LDA R			Get right-X
370		AND @7: TAX		number of bits in X
380		LDA E,X			and E
390		STA M: AND W		
400		ORA (B),Y: STA T
410		LDA M: AND V
420		EOR T: STA (B),Y
430		RTS			Return

440	:LL4	LDA R
450		AND @7: TAX: LDA E,X
460		AND M
470		STA M: AND W
480		ORA (B),Y: STA T
490		LDA M: AND V
500		EOR T: STA (B),Y
510		RTS
520	]
530	NEXT
540	P.$6:P.$7: CLEAR 4
550	?W = 
560	?V = 

610	!B=38880 - 32 * YY1		Triangle fill
620	DD3 = YY3-YY1
630	GG3 = S*(XX3-XX1)/DD3
640	F = S * XX1: H=F
650	DD1 = YY2 - YY1
660	GG1 = S*(XX2-XX1)/DD1


Horizontal Line-drawing routine, with HLINE dedicated to XOR
This is the version dedicated to XOR

150	:LL1				Entry to HLINE_XOR (37* us)
		LDA L			Get Left-X
170		LSR A: LSR A: LSR A	divide by 8 = no of bytes
180		TAY: STA G		from left of screen in G, and Y reg
190		LDA L			
200		AND @7: TAX		modulo 8 = no of bits in X reg
					rem LDA S,X: STA M snipped out
220		LDA R
230		LSR A: LSR A: LSR A
240		SEC: SBC G		If right and left in same byte
		BEQ LL4			then skip to Same_byte()

250		STA G			Start_byte ( 27** us)
					rem LDA M: AND W snipped
270		LDA (B),Y: STA T
					rem LDA M: AND V snipped
290		EOR T: STA (B),Y	Ex-or mask from table T into display RAM
300		INY			Next Y byte
		DEC G: BEQ LL3		If byte-count == 0 then LL3
					Handle midbyte (22** us)
310	:LL2	LDA (B),Y		do { Fetch display Byte Y
					rem ORA W: EOR V snipped
		EOR @#FF			flip whole byte of pixels (new)
330		STA (B),Y			Store display Byte
340		INY				Y++
		DEC G: BNE LL2		} while ( --G );
350	:LL3				Endbyte: (28* us)
360		LDA R			Get right-X
370		AND @7: TAX		number of bits in X
		LDA (B),Y		Fetch display_byte[Y]
		EOR E,X			operate mask from table E
		STA (B),Y		replace
430		RTS			Return

440	:LL4				Single_byte case (48us)
		LDA S,X: STA G		Store left mask in G
		LDA R			right-bits
450		AND @7: TAX: LDA E,X
460		AND M			A = rightbit mask AND leftbitmask
500		EOR (B),Y		Operate
		STA (B),Y		Replace
510		RTS
520	]

Execution time

85** us if on same byte = 10.6 us per pixel
92+22*midbytes otherwise
eg 256x192 = 49152 pixels
192*(92+22*30) = 144384 us = 2.9375 us per pixel

Hex dump from location #2800

2800 A5 91
2802 4A 4A 4A
2805 85 93
2807 A8
2808 A5 91
280A 29 07
280C AA
280D A5 A2
280F 4A 4A 4A
2812 38
2813 E5 93
2815 F0 24
2817 85 93
2819 B1 94
281B 55 87
281D 91 94
281F C8
2820 C6 93
2822 F0 0B
2824 B1 94
2826 49 FF
2828 91 94
282A C8
282B C6 93
282D D0 F5
282F A5 92
2831 29 07
2833 AA
2834 B1 94
2836 55 80
2838 91 94
283A 60
283B B5 87
283D 85 93
283F A5 92
2841 29 07
2843 AA
2844 B5 80
2845 25 93
2848 51 94
284A 91 94
284C 60

4D = a minute 76 bytes!