Path: chuka.playstation.co.uk!news
From: Phil Gooch <philg@easynet.co.uk>
Newsgroups: scee.yaroze.programming.2d_graphics
Subject: What to do after a collision has been detected (related to previous post)
Date: Mon, 07 Sep 1998 14:37:40 +0100
Organization: PlayStation Net Yaroze (SCEE)
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OK, so I've detected a collision of a sprite with the terrain of my
'road'. What I have done is put all the allowable 'squares' you can be
on into one texture page (identified as ROAD) and everything else
(trees, grass, roadside edge etc) into another texture page (identified
as ROADSIDE). My array data contains info for which texture page the
square is read from so I can do (pseudo-code)

if ( (mapData[car.mapY][car.mapX] & TPAGE_ID) == ROADSIDE)
    // the car is on a square from the ROADSIDE texture page, so you
have collided with something
    // bounce car off it
    car.veloc.x = -car.veloc.x;
    car.veloc.y = -car.veloc.y;

But this 'bouncing off' is causing me problems, because it is possible
that the car either gets bounced back onto another square that will
register a collision, or be still partly on the collision square after
being bounced (as the car's velocity is being damped by friction on
every frame as well), causing the car to bounce backwards and forwards
ad infinitum.

So, to get round this, I have raised a flag to say: 'if you have
detected a collision, don't detect any more collisions until you are on
a non-colliding square', e.g.

if ( (mapData[car.mapY][car.mapX] & TPAGE_ID) == ROADSIDE)
    // the car is on a square from the ROADSIDE texture page, so you
have collided with something
    // bounce car off it if not already 'colliding'
    if (car.status.collide != terrainCollide)
    {
        car.veloc.x = -car.veloc.x;
        car.veloc.y = -car.veloc.y;
        car.status.collide = terrainCollide;     // now register the
collision so you don't bounce straight back onto
                                                                //
collision square again
    }
else
    car.status.collide == idle;

This prevents the car juddering backwards and forwards that occurs in
certain circumstances, but causes another problem: each collision is
only registered once, until the car is back on a safe square, so it is
possible to drive very slowly towards a terrain obstacle, bounce back
slowly but still be on the same collision square, and then move forward
and drive over it (because the bounce only happens the first time the
collision is detected)

Am I a bit thick or what? I can't seem to find a way out of this one!

Any insight much appreciated.

Cheers

Phil