
        A Pixel-Precision Method For Detecting Sprite Collision

                                v 1.0

                           by Mark Mackey


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Introduction

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A common query on the USENET newsgroup rec.games.programmer over the 

years has been how to do fast pixel-precision collision detection. A 

number of people have presented some excellent algorithms for extending 

and speeding up bounding-box collision detection between large numbers of 

objects (for instance, dividing the play area into subfields and sorting 

the sprites into these subfields, thus greatly reducing the number of 

sprite-sprite collisions that have to be checked in the first place). 


However, there have been little information available on how to detect 

collisions efficiently at the pixel level. I thus present here some code 

from my game XQuest (blatant plug) which checks for collisions at the 

pixel level. The routine assumes that the bounding boxes for the two 

sprites overlap, and hence should be relatively easy to add to an 

existing bounding-box collision detection routine. This code is fast, 

relatively efficient in terms of space (requiring 4 bytes per row of each 

sprite), and even works! The one limitation is that sprites are assumed 

to be 32 pixels or less in width. For larger sprites, you could either 

treat them as 2 or more 32-pixel wide objects, or with a bit of creative 

thought this routine could be rewritten to allow for 64-pixel wide 

objects on the 386 (left as an exercise for the reader :). 


The code in the enclosed file COLLIDE.PAS is organised as a Turbo Pascal 

unit, but all of the essential code is in assembly language and could be 

easily ported to work under C or in a pure asm program. If any of the 

Pascalisations confuse you just let me know and I'll explain :). Also, if 

you need a version of this code to work on a 286 then let me know and 

I'll send it to you (but be warned: it's not nearly as nice :).


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The Algorithm 

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The first step is to create a 'transparency' mask for each sprite. The 

mask consists of a dword for each row of the sprite, with each bit being 

a 0 if the corresponding pixel is 'empty', and a 1 otherwise. For 

example, if a row of your sprite looked like (colour indexes, 0 being 

transparent) 


0  0  0  1  23  42  0  1  56  0  0  0  0  0 


the the corresponging mask bytes would be 


00011101 10000000 00000000 00000000 =  1D 80 00 00 


The MakeMask procedure given will produce such a mask from a sprite 

supplied in the XLib linear bitmap format (with pixels of colour zero 

being transparent), and is easily adaptable to your own sprite format. 


OK, now the hard part. We have found in our general-purpose bounding-box 

collision detection routine that two sprites' bounding boxes have 

collided (ie their masks overlap). 


Let the leftmost sprite have (x1,y1) as the coords of its top left 

corner, and the rightmost one (x2,y2). Take the mask entry for row 

|y2-y1| of the topmost sprite and shift it left by the difference in the 

x-coordinates (x2-x1). AND this value with the 1st mask entry of the 

lower sprite. If the result is non-zero then a collision occurred on this 

row. If not, then shift row |y2-y1+1| and compare it to row 2 of the 

lower sprite, and so on, until you reach the bottom of one of the 

sprites. If no collision has been detected by this time, then the sprites 

didn't collide. Simple, eh? 


This routine is quite fast, requiring only a MOV, a SHL and an AND for

each row checked, and only checking those rows that overlap.


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The Legal Bit

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This software is (C) Copyright 1995 Mark Mackey. Permission is given to 

distribute this code freely, or to distribute modified forms of this 

software provided that the author is acknowledged and this copyright 

notice retained. Permission is also given to utilise this code in 

original or modified form in any software provided that the author is 

acknowledged. 


I can be contacted by


Email: mdm1004@cus.cam.ac.uk 

WWW: http://www.ch.cam.ac.uk/MMRG/mdm.html

Snail: c/o Trinity Hall,

           Cambridge CB2 1TJ

           UK


These addresses will be in use until at least October 1996. Please let me 

know if you found this code helpful/useful/rubbish/whatever or if you 

improve it :) 


