====== 8-bit atan2 ======

might be more precise to add a clc adc #$01 after each eor #$ff, you have to modify all the preceding bcs *+4/ bcc *+4  to *+7 to get the branches correct. also you can omit the clc where bcs is used. adding a SEC before all sbc's might increase the accuracy even further. :)
/Oswald
 
<code>
;; Calculate the angle, in a 256-degree circle, between two points.
;; The trick is to use logarithmic division to get the y/x ratio and
;; integrate the power function into the atan table. Some branching is
;; avoided by using a table to adjust for the octants.
;; In otherwords nothing new or particularily clever but nevertheless
;; quite useful.
;;
;; by Johan Forslöf (doynax)

octant		= $fb			;; temporary zeropage variable

atan2		lda x1
		sbc x2
		bcs *+4
		eor #$ff
		tax
		rol octant

		lda y1
		sbc y2
		bcs *+4
		eor #$ff
		tay
		rol octant

		lda log2_tab,x
		sbc log2_tab,y
		bcc *+4
		eor #$ff
		tax

		lda octant
		rol
		and #%111
		tay

		lda atan_tab,x
		eor octant_adjust,y
		rts

octant_adjust	.byte %00111111		;; x+,y+,|x|>|y|
		.byte %00000000		;; x+,y+,|x|<|y|
		.byte %11000000		;; x+,y-,|x|>|y|
		.byte %11111111		;; x+,y-,|x|<|y|
		.byte %01000000		;; x-,y+,|x|>|y|
		.byte %01111111		;; x-,y+,|x|<|y|
		.byte %10111111		;; x-,y-,|x|>|y|
		.byte %10000000		;; x-,y-,|x|<|y|


		;;;;;;;; atan(2^(x/32))*128/pi ;;;;;;;;

atan_tab	.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$00,$00,$00
		.byte $00,$00,$00,$00,$00,$01,$01,$01
		.byte $01,$01,$01,$01,$01,$01,$01,$01
		.byte $01,$01,$01,$01,$01,$01,$01,$01
		.byte $01,$01,$01,$01,$01,$01,$01,$01
		.byte $01,$01,$01,$01,$01,$02,$02,$02
		.byte $02,$02,$02,$02,$02,$02,$02,$02
		.byte $02,$02,$02,$02,$02,$02,$02,$02
		.byte $03,$03,$03,$03,$03,$03,$03,$03
		.byte $03,$03,$03,$03,$03,$04,$04,$04
		.byte $04,$04,$04,$04,$04,$04,$04,$04
		.byte $05,$05,$05,$05,$05,$05,$05,$05
		.byte $06,$06,$06,$06,$06,$06,$06,$06
		.byte $07,$07,$07,$07,$07,$07,$08,$08
		.byte $08,$08,$08,$08,$09,$09,$09,$09
		.byte $09,$0a,$0a,$0a,$0a,$0b,$0b,$0b
		.byte $0b,$0c,$0c,$0c,$0c,$0d,$0d,$0d
		.byte $0d,$0e,$0e,$0e,$0e,$0f,$0f,$0f
		.byte $10,$10,$10,$11,$11,$11,$12,$12
		.byte $12,$13,$13,$13,$14,$14,$15,$15
		.byte $15,$16,$16,$17,$17,$17,$18,$18
		.byte $19,$19,$19,$1a,$1a,$1b,$1b,$1c
		.byte $1c,$1c,$1d,$1d,$1e,$1e,$1f,$1f


		;;;;;;;; log2(x)*32 ;;;;;;;;

log2_tab	.byte $00,$00,$20,$32,$40,$4a,$52,$59
		.byte $60,$65,$6a,$6e,$72,$76,$79,$7d
		.byte $80,$82,$85,$87,$8a,$8c,$8e,$90
		.byte $92,$94,$96,$98,$99,$9b,$9d,$9e
		.byte $a0,$a1,$a2,$a4,$a5,$a6,$a7,$a9
		.byte $aa,$ab,$ac,$ad,$ae,$af,$b0,$b1
		.byte $b2,$b3,$b4,$b5,$b6,$b7,$b8,$b9
		.byte $b9,$ba,$bb,$bc,$bd,$bd,$be,$bf
		.byte $c0,$c0,$c1,$c2,$c2,$c3,$c4,$c4
		.byte $c5,$c6,$c6,$c7,$c7,$c8,$c9,$c9
		.byte $ca,$ca,$cb,$cc,$cc,$cd,$cd,$ce
		.byte $ce,$cf,$cf,$d0,$d0,$d1,$d1,$d2
		.byte $d2,$d3,$d3,$d4,$d4,$d5,$d5,$d5
		.byte $d6,$d6,$d7,$d7,$d8,$d8,$d9,$d9
		.byte $d9,$da,$da,$db,$db,$db,$dc,$dc
		.byte $dd,$dd,$dd,$de,$de,$de,$df,$df
		.byte $df,$e0,$e0,$e1,$e1,$e1,$e2,$e2
		.byte $e2,$e3,$e3,$e3,$e4,$e4,$e4,$e5
		.byte $e5,$e5,$e6,$e6,$e6,$e7,$e7,$e7
		.byte $e7,$e8,$e8,$e8,$e9,$e9,$e9,$ea
		.byte $ea,$ea,$ea,$eb,$eb,$eb,$ec,$ec
		.byte $ec,$ec,$ed,$ed,$ed,$ed,$ee,$ee
		.byte $ee,$ee,$ef,$ef,$ef,$ef,$f0,$f0
		.byte $f0,$f1,$f1,$f1,$f1,$f1,$f2,$f2
		.byte $f2,$f2,$f3,$f3,$f3,$f3,$f4,$f4
		.byte $f4,$f4,$f5,$f5,$f5,$f5,$f5,$f6
		.byte $f6,$f6,$f6,$f7,$f7,$f7,$f7,$f7
		.byte $f8,$f8,$f8,$f8,$f9,$f9,$f9,$f9
		.byte $f9,$fa,$fa,$fa,$fa,$fa,$fb,$fb
		.byte $fb,$fb,$fb,$fc,$fc,$fc,$fc,$fc
		.byte $fd,$fd,$fd,$fd,$fd,$fd,$fe,$fe
		.byte $fe,$fe,$fe,$ff,$ff,$ff,$ff,$ff
</code>