FINDPIXELS

Right. Let's start with this one... Any my first findpixel should be better than this. What does he do? He multiplies by adding 16 y times. One of the worst ideas thinkable (at least add y 16 times, but don't do that neither.) Close your eyes and scroll down.

;---------------------------------------------------------------------
;  Routine by James Yopp
;
;  134-2647 cycles - 34 bytes
;  (b,c) to hl:a - destroyes: bde
;---------------------------------------------------------------------

FindPixel:  ;©1997 James Yopp. If you use it, leave this comment in the source
	push bc
	ld h,$FC
	srl b
	srl b
	srl b
	ld l,b
	ld b,c
	ld de,16
	xor a
BLoop:
	add hl,de
	cp b
	jr z,XitLoop
	dec b
	jr BLoop
XitLoop:
	pop bc
	ld a,b
	and %00000111
	ld b,a
	inc b
	ld a,%00000001
ALoop:
	rrca
	djnz ALoop
	ret

Edited Eble-Yopp? Mainly rebuilt i'd say. A very nice routine because of a great idea. It uses the mainly unknown instuction rld. Neat. Too bad it somehow didn't work out though :( Unlike what the author may believe, it's actually slower and larger than the original (not much, but still.) Wouldn't use it, but it sure works cool ;)
Warning: hl will not point to the screen, but rather the screen offset! You'll have to add $FC00 to use it like any other findpixel.

;---------------------------------------------------------------------
;  An edited EBLE-YOPP Routine
;
;  127 cycles - 35 bytes
;  (b,c) to hl:a - destroyes: de
;---------------------------------------------------------------------

Find_Pixel:
	ld de,FP_Bits
	ld a,b
	and %00000111
	add a,e
	ld e,a
	adc a,d
	sub e
	ld d,a                ;(de)=bitmask

	ld hl,FP_RLD
	ld (hl),c             ;FP_RLD=y
	ld a,b                ;a=x
	rra
	rra
	rra                   ;  x/8
	rld                   ;a(hl)=16y+x/8
	and %00000011
	ld l,(hl)
	ld h,a
	ld a,(de)
	ret

FP_Bits:
	.db 128,64,32,16,8,4,2,1
FP_RLD:
	.db 0                 ;temp byte needed but doesnt have to be in program

ZAPO's long gone now, but they left us this great findpixel. Guess what? It's the fastest one yet! Only 105 cycles. Furthermore, the routine itself is also the smallest one I know of. One problem: it requires you to run some initiation code before use. This makes your program 19 bytes larger, and it'll also use a little memory at $8700. If you need a really fast findpixel, this one's the best. Otherwise, better not.

;---------------------------------------------------------------------
;  The ZAPO even faster findpixel
;  by ~Code7~
;
;  105 cycles - 21 bytes
;  (c,b) to hl:a - destroyes: de
;
;  101 cycles - 20 bytes
;  (c,a) to hl:a - destroyes: de
;---------------------------------------------------------------------

FindPixel:
	ld a,b
FindPixelShort:
	ld h,$3f
	add a,a
	add a,a
	ld l,a     ;hl=$3f00+4*y
	ld a,c
	rra
	add hl,hl
	rra
	add hl,hl  ;hl=$fc00+16*y
	rra        ;8*x
	add a,l
	ld l,a
	ld d,$87
	ld a,c
	and d      ;=7
	ld e,a
	ld a,(de)
	ret

;  the offset table program must be placed at $8700
;
;  163 cycles  19 bytes    destroyes: bcdehl

InitializeFP:
	ld de,$8700
	ld hl,Offset
	ld bc,8
	ldir
	ret
Offset:
	.db $80,$40,$20,$10,$8,$4,$2,$1

Nothing too special about this one. Good optimizing, nice routine, but there are better ones outthere.

;---------------------------------------------------------------------
;  CLEM's FINDPIXEL
;  by Clem
;
;  127 cycles - 35 bytes
;  (b,c) to hl:a - destroyes: bc
;---------------------------------------------------------------------

FindPixel:
	ld h,63
	ld a,c
	add a,a
	add a,a
	ld l,a    ;hl=$3f00+4*y
	ld a,b
	rra
	add hl,hl
	rra
	add hl,hl ;hl=$fc00+16*y
	rra       ;a=x/8
	or l
	ld l,a
	ld a,b
	and 7
	ld bc,FP_Bits
	add a,c
	ld c,a
	adc a,b
	sub c
	ld b,a
	ld a,(bc)
	ret
FP_Bits:
	.db $80,$40,$20,$10,$08,$04,$02,$01

And last but not least, the smallest findpixel ever! Look at it: only 24 bytes! And with an average of 158 cycles still quite usable. For most progs which don't need that speed anyhow, this one's your choice. And afterwards, b will be 0, so you can use that too to shave off another byte or so (think ld c,16 \ add hl,bc.)