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Collision problem

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Hi I have two circles when they collide I trade their velocities just for right now, but it seems sometimes they stick together, thats probably because I trade the velocities and i keeps trading it because its colliding, the way I want to deal with this is pull them apart so there not colliding anymore and then trade their velocities. Would I need to find out the angle they collide at? I dont know how to do any of this, If there is a better way please let me know, or if you know a way to do it the way im thinking, thanks.

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it's using te reflection angle. I prefer the vector maths version. Much easier. Incidently, this is also tied with Newton's principle of the conservation of momentum for elastic collisions.

If you don't understand, you should really read about

1) Newton's laws of physics
2) vector maths, trigonometry, linear algebra. And possibly matrices if you wanna go one step further.

That's all there is. You can't do game phyiscs without those well covered. Most of it is A-level phyiscs and maths.

If you do understand those principles, maybe this will help.


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If it helps, here is the (simplified) function that handles the sphere collisions in my game E-Motion Deluxe. I left out the big-small-sphere collision and the new-sphere-spawn part, because that is irrelevant here. It is written in BlitzBasic, a BASIC language, and i'll explain a few things that are particular to this specific basic "dialect" so you'll understand.

For example the code Spheres(4)\ySpeed# means, look in the array called Spheres, take the 4th record and look at the ySpeed# variable (or "property") of that record. In BlitzBasic a # (pound sign) means it is a floating point variable or constant. All variables without a # are integers. And the ; (semi-colon) is the comment indicator.

So here goes. [grin]


; Nr1, Nr2 are Spheres array indexes
; X1#, Y1# and X2#, Y2# are the centre position of the 2 spheres
Function HandleSphereCollision(Nr1, Nr2, X1#, Y1#, X2#, Y2#)

; collision angle, angle of line through centre of the spheres
CollisionAngle = ATan2((Y2 - Y1), (X2 - X1))

; align sphere speed vectors to the collision angle
AngleBall1 = ATan2(Spheres(Nr1)\ySpeed#, Spheres(Nr1)\xSpeed#) - CollisionAngle
AngleBall2 = ATan2(Spheres(Nr2)\ySpeed#, Spheres(Nr2)\xSpeed#) - CollisionAngle

;preserve kinetic energy, E(k), the speed vector
Ek1# = Sqr((Spheres(Nr1)\ySpeed#^2) + (Spheres(Nr1)\xSpeed#^2)) ; ^2 means to the 2nd power
Ek2# = Sqr((Spheres(Nr2)\ySpeed#^2) + (Spheres(Nr2)\xSpeed#^2))

;speed vectors aligned, so now the x-axis IS the collision angle, swap the x-vectors
CollisionXspeed1# = Ek1# * Cos(AngleBall1)
CollisionYspeed1# = Ek1# * Sin(AngleBall1)
CollisionXspeed2# = Ek2# * Cos(AngleBall2)
CollisionYspeed2# = Ek2# * Sin(AngleBall2)

; if spheres are already moving away from each other, then they cannot collide
; this situation can occur when spheres is spawned, and overlaps another sphere
If ((CollisionXspeed2# - CollisionXspeed1#) < 0) Then
;spheres not already moving away, so swap the x-vectors
tmpX# = CollisionXspeed1#
CollisionXspeed1# = CollisionXspeed2#
CollisionXspeed2# = tmpX#
End If

; when colliding, spheres MUST move away from each other, must have at least some speed
If (CollisionXspeed1# > -1*THRESHOLD_FORCE#) Then CollisionXspeed1# = -1 * THRESHOLD_FORCE#
If (CollisionXspeed2# < THRESHOLD_FORCE#) Then CollisionXspeed2# = THRESHOLD_FORCE#

;preserve new calculated kinetic energy
Ek1# = Sqr(CollisionYspeed1#^2 + CollisionXspeed1#^2) ; ^2 means to the 2nd power
Ek2# = Sqr(CollisionYspeed2#^2 + CollisionXspeed2#^2)

;reverse aligning speed vectors
AngleBall1 = ATan2(CollisionYspeed1, CollisionXspeed1) + CollisionAngle
AngleBall2 = ATan2(CollisionYspeed2, CollisionXspeed2) + CollisionAngle

;store new xSpeed and ySpeed values
Spheres(Nr1)\xSpeed# = Ek1# * Cos(AngleBall1)
Spheres(Nr1)\ySpeed# = Ek1# * Sin(AngleBall1)
Spheres(Nr2)\xSpeed# = Ek2# * Cos(AngleBall2)
Spheres(Nr2)\ySpeed# = Ek2# * Sin(AngleBall2)

End Function

[Edited by - BdR on June 1, 2005 1:14:59 PM]

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thanks for all the help guys.
BdR I tried to implement your code but when the two circles touch they disappear, I put a break point to see what where there new velocities and they seem fine.

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I put about 50 circles moving around and somtimes it works, but mostly when the two circles touch the disappear or go up in the top left corner.

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he is using divisions. Check for divide by zeroes, and generally values of vectors. If it works most of the time, likely to be a math bug. can't help you with that.

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No, i'm not using division in my example, there is no dividing at all. Remember that in code Spheres(Nr1)\xSpeed#, the \ (backslash) is a property indicator, not a division operator. Division is done with a / (slash) it's a bit confusing, i know..

In Visual Basic code it would look something like this Spheres(Nr1).xSpeed so which programming language do you use anyway?

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I am using C# .net
here is my code

public void TradeVelocity(xVolume volume)
double THRESHOLD_FORCE = 0.1f;

double x1, y1, x2, y2;

x1 = XPos + Radius;
y1 = YPos + Radius;

x2 = volume.XPos + volume.Radius;
y2 = volume.YPos + volume.Radius;

double CollisionAngle = Math.Atan2((y2 - y1), (x2 - x1));

double AngleBall1 = Math.Atan2(velocity.Y, velocity.X) - CollisionAngle;
double AngleBall2 = Math.Atan2(volume.velocity.Y, volume.velocity.X) - CollisionAngle;

double ek1 = (velocity.Y * velocity.Y) + (velocity.X * velocity.X);
ek1 = ek1 * ek1;

double ek2 = (volume.velocity.Y * volume.velocity.Y) + (volume.velocity.X * volume.velocity.X);
ek2 = ek2 * ek2;

Vector3 CollisionSpeed1;
CollisionSpeed1.X = (float)(ek1 * Math.Cos(AngleBall1));
CollisionSpeed1.Y = (float)(ek1 * Math.Sin(AngleBall1));

Vector3 CollisionSpeed2;
CollisionSpeed2.X = (float)(ek2 * Math.Cos(AngleBall2));
CollisionSpeed2.Y = (float)(ek2 * Math.Sin(AngleBall2));

if((CollisionSpeed2.X - CollisionSpeed1.X) < 0)
float tempX = CollisionSpeed1.X;
CollisionSpeed1.X = CollisionSpeed2.X;
CollisionSpeed2.X = tempX;

if(CollisionSpeed1.X > -1 * THRESHOLD_FORCE)
CollisionSpeed1.X = -1 * (float)(THRESHOLD_FORCE);

if(CollisionSpeed2.X < THRESHOLD_FORCE)
CollisionSpeed2.X = (float)(THRESHOLD_FORCE);

ek1 = (CollisionSpeed1.Y * CollisionSpeed1.Y) + (CollisionSpeed1.X * CollisionSpeed1.X);
ek1 = ek1 * ek1;

ek2 = (CollisionSpeed2.Y * CollisionSpeed2.Y) + (CollisionSpeed2.X * CollisionSpeed2.X);
ek2 = ek2 * ek2;

AngleBall1 = Math.Atan2(CollisionSpeed1.Y, CollisionSpeed1.X) + CollisionAngle;
AngleBall2 = Math.Atan2(CollisionSpeed2.Y, CollisionSpeed2.X) + CollisionAngle;

velocity.X = (float)(ek1 * Math.Cos(AngleBall1));
velocity.Y = (float)(ek1 * Math.Sin(AngleBall1));
volume.velocity.X = (float)(ek2 * Math.Cos(AngleBall2));
volume.velocity.Y = (float)(ek2 * Math.Sin(AngleBall2));


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