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如何使用unity3d制作出真实的物理效果（三）

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电梯直达

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发表于 2011-11-12 11:32:00 |只看该作者 |倒序浏览

         Now the fun part...





         Q: How to simulate stabilizer bars in Unity?


         A: Easily


         Anti-roll bars work by transferring some compression force from one spring to the opposite in the same axle. The amount of the transfered force depends on the difference in the suspension travel among the wheels.


         So first task is to calculate the suspension travel on each wheel, as well as determine whether is grounded or not. We want the travel value to be between 0.0 (fully compressed) and 1.0 (fully extended):


         code:


         groundedL = WheelL.GetGroundHit(hit))


         if (groundedL)


            travelL = (-WheelL.transform.InverseTransformPoint(hit.point).y - WheelL.radius) / WheelL.suspensionDistance;


         else


            travelL = 1.0;


         We multiply the travel diference by the AntiRoll value, which is the maximum


         force that the anti-roll bar can transfer among the springs (we could call this


         the Anti-roll bar's stiffness). This yields the amount of force that will be


         transfered:


         code:


         var antiRollForce = (travelL - travelR) * AntiRoll;


         Finally, we must simply substract the force from one spring and add it to the


         other. We achieve this by adding opposite forces to the rigidbody at the


         positions of the WheelColliders:


         code:


         if (groundedL)


            rigidbody.AddForceAtPosition(WheelL.transform.up * -antiRollForce, WheelL.transform.position);


         if (groundedR)


            rigidbody.AddForceAtPosition(WheelR.transform.up * antiRollForce, WheelR.transform.position);


         Q: What's a good AntiRoll value? Which units is it expressed in?


         A: A good  value is roughly the value of the wheel's spring. Both are expressed in Newtons.


         The spring value means the force the spring can give when fully


         compressed, and the AntiRoll value means the amount of force that can be


         transfered from one spring to another. Having the same values for Springs and


         AntiRoll in the same axle means that the anti-roll bar can transfer up the


         entire force of one spring to the other.


         Q: Can I have the script  for an anti-roll bar?


         A: Sure, here is it.


         As suggestion, I


         believe that an Anti-Roll script should be included in Unity's Standard


         Assets.


         Add two anti-roll scripts to your vehicle, one per axle (front -


         rear). Set the left-right wheels on each one and adjust the AntiRoll value.


         Remember to reset center of mass to the real position, or don't touch it at


         all!


         The full script: AntiRollBar.js


         code:


         var WheelL : WheelCollider;


         var WheelR : WheelCollider;


         var AntiRoll = 5000.0;


         function FixedUpdate ()


            {


            var hit : WheelHit;


            var travelL = 1.0;


            var travelR = 1.0;


            var groundedL = WheelL.GetGroundHit(hit);


            if (groundedL)


               travelL = (-WheelL.transform.InverseTransformPoint(hit.point).y - WheelL.radius) / WheelL.suspensionDistance;


            var groundedR = WheelR.GetGroundHit(hit);


            if (groundedR)


               travelR = (-WheelR.transform.InverseTransformPoint(hit.point).y - WheelR.radius) / WheelR.suspensionDistance;


            var antiRollForce = (travelL - travelR) * AntiRoll;


            if (groundedL)


               rigidbody.AddForceAtPosition(WheelL.transform.up * -antiRollForce,


                        WheelL.transform.position);


            if (groundedR)


               rigidbody.AddForceAtPosition(WheelR.transform.up * antiRollForce,


                        WheelR.transform.position);


            }


         Q: How can I set a real center of mass for my vehicle?


         A: Use  non-overlapping box colliders to roughly resemble your vehicle's shape, then  move the center towards the position of the engine.





         Unity/PhysX calculates the center of mass based on the position and


         volume of the GameObject's colliders. Note that overlapping colliders add more


         mass at the overlap position.


         Typically, you would only need to move the


         center of mass a bit towards the position of the engine (front - rear). For


         instance, if your vehicle has front engine and its front is oriented in the Z+


         direction, you should only need to move the (automatically calculated) center of


         mass 1 meter to the front:


         code:


         function Start ()


            {


            rigidbody.centerOfMass += Vector3(0, 0, 1.0);


            }


         Q: Does it really works so good? Could it really be so simple??


         A: See it  by yourself:

         http://www.edy.es/unity/offroader.html


         You can have


         a good view of how it works by pressing C (change secondary camera), then


         pressing B while driving (alternate Sport / Offroad mode). Press N for


         completely disabling the stabilizer bars (B to reenable, or Enter to restart


         after flipping over).


         Ending words....


         As this is my first post here, please let me introduce myself. I'm a developer now working on dj software. Years ago, while I was a student two friends and me spent about two years trying to develop games in C++ from scratch (no engines at all), because games was what we wanted to do since kids. As you can imagine, very little to no result could be seen in the screen. I discovered Unity about four weeks ago (had heard of it before, but hadn't look at it) and for me it was like seeing The Light. I could just put things on the screen, click play, and worked! I could then fine-tune the scene by looking at code examples and writing scripts in whatever language used by Unity (two weeks ago I realized I was coding in javascript). Magic! Now I'm about to achieve a personal goal of modeling physically real vehicles. This is what I really wanted to do since I was kid and self-learned to code in Basic with by Commodore 64. Overlander, Powerdrift, Hard Driving, and later 4D Sports Driving, Fatal Racing... now I can easily do it!! Even as hobbyist, I'm purchasing the Unity Pro license just because I love it!


         This article is my first contribution to the GREAT Unity community. It's great to have fun by modeling and playing with Unity's vehicle physics.


         Enjoy!


         Edy