# Jumpy raycast-vehicle

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I'm currently trying to get the ships for our game Pulsedrive to hover. For this I'm using a raycast-vehicle-style function which shoots a ray for each of the 4 corners to the bottom to calculate the forces.
That works quite well so far, the problem is just that my ships won't stop to jump up and down.

I read something about the Damping, and even implemented it into the code, but it doesn't seem to help. I used this page as a reference:
http://www.digitalrune.com/Support/Blog/tabid/719/EntryId/91/Car-Physics-for-3D-Games.aspx

The function for calculating the rays goes like this:
 float DoRaycastPhysics(float PrevCompression, vector3 Point, float DeltaTime) { const float SHIP_SUSPENSION_STIFFNESS = 20.0f; const float SHIP_DAMPING = 0.15f; vector3 Pos = Ship.GetLocation(); vector3 Susp = Ship.TransformDirection(Point); // Rotate wheel location float Susp_Length = 2.0; // Rest length float TireRad = 2.0f; // Tire radius vector3 RayOrigin = Pos + Susp; float RayLength = Susp_Length + TireRad; // Shoot the ray vector3 Nrm; float Dist = World.TraceEx(RayOrigin, vector3(0,-1,0), 1000, Nrm); vector3 HitPos = (vector3(0,-1,0)*Dist); // Local float Compression; if(Dist < RayLength) { Compression = Dist - TireRad; // Ray hit something, calculate the Compression }else { Compression = Susp_Length; // Didn't hit, set the compression to "rested"-state } // Calculate the SuspensionForce --- (Problem here?) float SuspensionForce = SHIP_SUSPENSION_STIFFNESS * (Susp_Length - Compression) + SHIP_DAMPING * (PrevCompression - Compression) * DeltaTime; // Apply the force from the hit location in local space Ship.ApplyForce(vector3(0,SuspensionForce,0), HitPos); return Compression; } 

Does anyone see what my problem could be?

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Also, does someone know a good resource for raycast vehicles? There's almost nothing on the web. Probably I can solve the problem myself then.

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This one here seemed very good:
http://www.gamedev.net/topic/394292-demosource-of-marco-monsters-car-physics-tutorial/

You can also look at Box2D and Bullet. They both have some examples of raycast vehicles if I remember correctly

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If the ship keeps jumping up and down, then there might be a problem with your damping. The line

float SuspensionForce = SHIP_SUSPENSION_STIFFNESS * (Susp_Length - Compression) + SHIP_DAMPING * (PrevCompression - Compression) * DeltaTime;

looks a lot like a Hookes law damped spring. The problem might be with the second term:
 + SHIP_DAMPING * (PrevCompression - Compression) * DeltaTime;

where the term

(PrevCompression - Compression) * DeltaTime;

should represent the ships vertical velocity, but probably doesnt since the equation looks wrong. Try to replace it with this algorithm:

(Compression - PrevCompression) / DeltaTime;

Which calculates the velocity based on the mean value theorem. If it doesn't work, try changing the sign or change the SHIP_DAMPING value.

Cheers,
Mike

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Thanks for the reply! Unfortunately your solution doesn't seem to help. Or I am simply using the wrong values.

I have changed the function to this:
 float DoRaycastPhysics(float PrevCompression, vector3 Point, float DeltaTime) { vector3 Pos = Ship.GetLocation(); vector3 Susp = Ship.TransformDirection(Point); // Rotate wheel location float Susp_Length = 4.0; // Rest length float TireRad = 0.5f; // Tire radius float RestLength = Susp_Length; // Resting length vector3 RayOrigin = Pos + Susp; float RayLength = Susp_Length + TireRad; // Shoot the ray vector3 Nrm; float Dist = abs(World.TraceEx(RayOrigin, vector3(0,-1,0), 1000, Nrm)); vector3 HitPos = Susp+(vector3(0,-1,0)*Dist); float Compression=Susp_Length; if(Dist < RayLength) { Compression = Dist - TireRad; // Ray hit something, calculate the Compression } float SuspensionForce; if( Ship.GetLinearVelocity().y > 0) { // Extending SuspensionForce = SuspensionStiffness * (RestLength - Compression) + ExtendDamping * (Compression - PrevCompression) / DeltaTime; }else { // Compressing SuspensionForce = SuspensionStiffness * (RestLength - Compression) + CompressDamping * (Compression - PrevCompression) / DeltaTime; } // Apply the force from the hit location in local space Ship.ApplyForce(vector3(0,SuspensionForce,0), HitPos); return Compression; }

My vehicle is still jumping around.

Maybe I am using the wrong values?
SuspensionStiffness = 30
ExtendDamping = 2.3
CompressDamping = 4.4

I tried others, sure, but none seem to work out.

What works, at least a bit, is to multiply the SuspensionStiffness of the "Extending"-branch with 0.01f, but that doesn't seem a very "right" solution I guess.

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First, this code doesn't seem to make any sense. The force should always be applied, not only when vertical velocity is positive. Also, the two equations are identical.

if( Ship.GetLinearVelocity().y > 0) { // Extending SuspensionForce = SuspensionStiffness * (RestLength - Compression) + ExtendDamping * (Compression - PrevCompression) / DeltaTime; }else { // Compressing SuspensionForce = SuspensionStiffness * (RestLength - Compression) + CompressDamping * (Compression - PrevCompression) / DeltaTime; } 

Instead of all the above, simply write:

 SuspensionForce = -(SuspensionStiffness * (RestLength - Compression) + ExtendDamping * Ship.GetLinearVelocity().y); 

If it still doesn't work, try changing signs and coefficient values.
Cheers,
Mike

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Wow, it works! Thank you, Mike!

Still, I had to change the sign:
 SuspensionForce = abs(SuspensionStiffness * (RestLength - Compression) + ExtendDamping * (Ship.GetLinearVelocity().y)); 

Again, thank you!

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You're welcome, always glad to help. :-)

The abs() does seem a little strange to me though, and if it only works with it, I think there may be a bug hiding somewhere else in your code. Have you tried this:

SuspensionForce = -SuspensionStiffness * (RestLength - Compression) - ExtendDamping * Ship.GetLinearVelocity().y;

Notice there are two signs you can change independently of each other, suspension force and damping force. Good bug huntin'

Cheers,
Mike

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Ah, I forgot to take the "abs()" out when editing my post.

This is what's in my code:
SuspensionForce = (SuspensionStiffness * (RestLength - Compression) + ExtendDamping * (Ship.GetLinearVelocity().y)); 

Works like a charm.

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