Path: chuka.playstation.co.uk!news
From: "Scott Ward" <scott.ward6@virgin.net>
Newsgroups: scee.yaroze.beginners
Subject: Collision Detection!.....(Joy)
Date: Fri, 18 Jun 1999 16:15:41 +0100
Organization: PlayStation Net Yaroze (SCEE)
Lines: 65
Message-ID: <7kdntq$4fj13@chuka.playstation.co.uk>
NNTP-Posting-Host: p-222-virgin1.tch.virgin.net
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 5.00.2314.1300
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300

I've been looking for different ways of achieving quality collision
detection and I saw a method on the Sony FAQ page which copies images into
main memory and then uses bitwise 'and' to test for collision. Although this
method is supposedly very slow it is pixel perfect when it comes to accuracy
so I really wouldn't mind giving it a whirl.

I got in touch with James Russell at Sony who provided me with some more
information but I'm still unsure of how to do it. Here's a snippet of what
he sent me:

>Firstly you do trivial rejection - if the horizontal and vertical
boundaries of
>the two sprites don't cross in any way, then there is no detection. If they
do
>cross, then you clear a buffer that is the same size as the screen
(although it
>only has to have a 2 bit resolution). Then you copy the first mask into
that
>buffer, shifting the bits as necessary so that the mask is in the same
position
>it's in on the screen. Then you do the same for the second mask, except
instead
>of copying the mask into the buffer, you and it with the buffer and test
the
>result. If any bit is one, then there was a collision.

Right, I'm still pretty new to this games coding business so whilst this
message may seem reasonably straight forward to you lot it has me a little
baffled.

I'd get back in touch with James but I feel this may take some time to sort
out so instead of me hounding the good people at Sony I'd rather someone
offered their time to help me out. To save space on the newsgroup I'll try
to ask as many questions on this one note. Here goes:

1) Although I understand the concept of creating a mask of a sprite how do
you make one? At first I thought that you probably just create an identical
sprite with lower resolution but then, as far I know, 4-bit sprites are the
lowest resolution you can load into the frame buffer. How do you make a
2-bit mask?

2)When James refers to a buffer, does this mean in the frame buffer or a
buffer in memory? If it means create a buffer in memory then I definately
need help. :o\

3)Copying the first mask into the buffer. What form does the mask take? Is
it an image in the frame buffer or is it stored in memory as a series of
bytes?

4)If all this done in the frame buffer then I have no bother moving stuff
about but if it's done in memory how would you go about 'shifting the bits'
and how would you know where to shift them to?


I know this question(s) must seem a bit daunting but even if you can only
answer one of the questions or you think you have a rough idea then I'd be
grateful if you'd get in touch.

Cheers,

Scott  "stuck again.....what a surprise"  Ward.