Infinite Collision Detection tutorial

Missing hits with sprites that are half outside the screen isn't that horrible; it's quite an errm.. edge case.

I would be more concerned about false positives with EC and non-EC sprites having the 'same' position. Does that fail as well?

in case positions of objects were stored on 16 bit Edwin's code could become

;-----------------------------------------------------

.check16.
        xor  a            ; HL=x1,DE=x2,B=size2,C=size1
        sbc    hl,de
        jr    nc,.other16
        xor     a
        ex    de,hl
        sbc    hl,de            ; Carry is reset
   
        ld    d,a
        ld    e,b
        sbc    hl,de             ; compare with size 2
        ret

.other16:
        ld    d,a
        ld    e,c
        sbc    hl,de             ; compare with size 1         
        ret

it is the case of objects in a virtual map where only a window is displaied

my contribution (from MOAM project): it works on SAT data stored in ram and with offsets and sizes passed on the stack

    psect   npcs

    global  _check_collision
				; check_collision
				;  (byte *sprite1,       DE - SAT pointer
				;	byte *sprite2,       BC - SAT pointer
				;	unsigned sprite1_size_hor,   	ix+4
				;	unsigned sprite1_size_vert,  	ix+6
				;	unsigned sprite2_size_hor,   	ix+8
				;	unsigned sprite2_size_vert);	ix+10
				;	sizes decode as follows:
				;		lobyte - first pixel set
				;		hibyte - number of pixels set
				;	it returns:
				; 		HL = 0 for no collision
				; 		HL = 1  collision from right
				; 		HL = -1 collision from left
_check_collision:
				push    ix
				ld  	ix,0
				add 	ix,sp	; use ix as poiter to parameters on the stack

				push 	bc		; store the two pointerst to the SAT
				push 	de

				; test y
				ld  	a,(de)	; a = vertical pos. sprite 1 (y1)   

				add a,32		; needed to deal with sprites partially under the top border
				add a,(ix+6)    ; y offeset within the sprite 1
				ld  e,a
				ld  d,0     	; de = vertical pos. sprite 1 + 32 (i.e. y1+yoffset1+32)

				ld  	a,(bc)	; a = vertical pos. sprite 2 (y2)

				add a,32		; needed to deal with sprites partially under the top border
				add a,(ix+10)   ; y offeset within the sprite 2
				ld  l,a
				ld  h,0     	; hl = vertical pos. sprite 2 + 32 (i.e. y2+yoffset2+32)

				ld  b,(ix+11)   ; b = number of pixels, sprite 2 (i.e. ysize2)
				ex  de,hl   	; swap de and hl
				or  a
				sbc hl,de
				
				jr  nc,1f   	; if hly1+yoffset1) ? ysize1 : ysize2;
				cp  b
				jr  nc,nocollosion  ; test on y failed

				; test x
				
				pop hl      	; pointer to SAT for sprite 1

				inc hl
				ld  e,(hl)
				inc hl
				inc hl
				ld  a,(hl)		; retrive the EC bit of sprite 1
				and 128
				rrca
				rrca
				xor 32
				add a,(ix+4)
				add a,e
				ld  e,a
				ld  a,0
				adc a,a
				ld  d,a     	; de = horizontal pos. sprite 1 + 32*EC

				pop hl			; pointer to SAT for sprite 2

				inc hl
				ld  c,(hl)
				inc hl
				inc hl
				ld  a,(hl)		; retrive the EC bit of sprite 2
				and 128
				rrca
				rrca
				xor 32
				add a,(ix+8)
				add a,c
				ld  l,a
				ld  a,0
				adc a,a
				ld  h,a     	; hl = horizontal pos. sprite 2 + 32*EC

				ld  b,(ix+9)    ; b = number of pixels, sprite 2
				ex  de,hl
				or  a
				sbc hl,de

				push af
				push af

				jr  nc,1f
				
				ld  b,(ix+5)    ; swap sprites
				add hl,de
				ex  de,hl
				or  a
				sbc hl,de
				
1:				ld  a,h
				or  a
				jr  nz,collision
				
				ld  a,l
				cp  b
				jp  nc,nocollision

collision:		
				pop af
				pop af

				ld  hl,-1		; return -1 if sprt1 is hit by sprt2 from left
				jp  nc,sprt1_hit_by_left

				ld  hl,1		; return 1 if sprt1 is hit by sprt2 from right

sprt1_hit_by_left:
exit:
				pop ix

				pop bc			; return address

				pop af			; remove sprite 1 and 2 sizes
				pop af
				pop af
				pop af

				push bc
				ret


nocollision:
				ld  hl,0

				pop af
				pop af

				jp  exit    ; return 0 if no collision

here there is the code above integrated in a small demo RON project (use space and cursors)
https://docs.google.com/leaf?id=0Bx4kWAc-fapqMWQ0NGM2N2EtN2U1My00MWI2LWIzZWYtYjk3MjM4MGJlODcw&hl=en&authkey=COTshr8O