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# Some help with 2D collision detection concepts please? :)

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I was just contemplating how collision detection works with objects moving at different speeds. say a bullet (1pixel) had a velocity of 1 (pixel per frame) in this case you can just check if the bullet is about to collide with something on each frame by checking if the next pixel ahead is solid... no problem right? now what if we gave it a velocity of 5 pixels per frame? in this case every frame, it is picked up and moved five frames (in whatever direction) so therefore it is skipping 4 pixels along its path. we don't want this because what if there were a solid wall in those 4 pixels it would go right through it (instead of bouncing, exploding, etc...) ill draw an example of what i mean to make it more clear: so what can we do about this? i think it would work, to on every frame, check the five pixels ahead of the bullet for solidity before moving it.. but would that be too slow? is there a better way to handle this? anyone have some insight on this? thanks jakerrz [edited by - jakerrz on November 3, 2002 12:29:01 PM]

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use math

if you can define the level as a set of line segments, you can test collisions and that using dot products..or simpler methods

simple example:

you''ve got a line segment, and you have the equation of it,
you substitute the x and y pos of the 2 points into the equation of the line, and if the sign is different, you''ve crossed it, otherwise you havent.

you can also use the normal of the "surface" ie the wall that the line represents dotted with the velocity vector and that helps with finding out where you are...

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Like Qatal said: treat your collidable objects as vectors, and test collisions based on the space swept out during movement.In this case, you could create a line segment that represents the movement of the bullet, and a line segment for the wall. Then you check to see if the two line segments intersect. If so, their intersection point is where the collision occurs.

Note: this technique does not work well when you try to solve more complicated systems, such as polygons that can rotate. However, it will model what you are trying to do just fine.

A simple bresenham-based pixel checking approach to collision detection will likely be faster than solving for the intersection point, however you need to profile to be sure.

One advantage of the vector method is that you can have non-integer velocities/angles/positions.

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Collision detection? Hmm... I have the perfect solution for that. A short time ago when I was creating my first game I created a routine to do collision detections between 2 objects and between a point and an object. Let me make you an acount on my website. I''ll put up this library and you can download it along with some documentation. My web address is www.geocities.com/francoissoft/Developers.htm... well that''s my programming page. You''ll need a password and a username to get in so I''ll generate one for you. Just give till the end of this week, maybe tommorow. I''ll send it to your e-mail address. Cool?

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kool that sounds great
I''d like to see that
Thanks!!

jakerrzero@hotmail.com

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