Basics of PhysX Vehicle SDK

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Donald Ramsey
6 years ago
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1 PhysX Vehicles 1

2 PhysX Collision and Dynamics SDK (now) owned by Nvidia Most common physics library used in games for this course Includes lots of vehicle specific features Experimenting with a change to allow deeper usage this year Previous years we allowed rigid body physics libraries but required building driving model from scratch Getting a good driving model is by far the biggest challenge for teams, and was often holding back game quality significantly

3 Basics of PhysX Vehicle SDK Essential tasks Set up library Create some meshes Allocate simulation data Allocate actor and add to world Per frame : Setup inputs to drive and steering Per frame : Wheel raycasts Per frame : Tick simulation Caveat : I have not personally built anything with this Sample code : PhysX-SDK/Sample/SampleVehicle

4 Initialization PxPhysics Base context for all operations PxAllocatorCallback* allocator = &gdefaultallocatorcallback; mfoundation = PxCreateFoundation(PX_PHYSICS_VERSION, *allocator, getsampleerrorcallback()); mphysics = PxCreatePhysics(PX_PHYSICS_VERSION, *mfoundation, scale, recordmemoryallocations, mprofilezonemanager);

5 Initialization PxCooking Utility class for creating meshes in physics PhysX vehicles uses meshes for all objects in vehicle system PxCookingParams params(scale); params.meshweldtolerance = 0.001f; params.meshpreprocessparams = PxMeshPreprocessingFlags(PxMeshPreprocessingFlag::eWELD_VERTICES PxMeshPreprocessingFlag::eREMOVE_UNREFERENCED_VERTICES PxMeshPreprocessingFlag::eREMOVE_DUPLICATED_TRIANGLES); mcooking = PxCreateCooking(PX_PHYSICS_VERSION, *mfoundation, params);

6 Initialization PxScene Container for all object in simulation Global world properties (i.e. gravity) Any objects not in vehicle system can be added straight to this with minimal extra work Probably going to want to add at least one static object mscene = mphysics->createscene(scenedesc); mscene.setgravity(pxvec3(0.0f, -9.81f, 0.0f)); mmaterials = getphysics().creatematerial(staticfriction, dynamicfriction, restitution); PxRigidStatic* plane = PxCreatePlane(*mPhysics, PxPlane(PxVec3(0,1,0), 0), *mmaterial); mscene->addactor(*plane);

7 Initialization Setup vehicle support Few essential parameters need to be set No context object, just free functions PxInitVehicleSDK(physics); PxVec3 up(0,1,0); PxVec3 forward(0,0,1); PxVehicleSetBasisVectors(up,forward); //Set the vehicle update mode to be immediate velocity changes. PxVehicleSetUpdateMode(PxVehicleUpdateMode::eVELOCITY_CHANGE);

8 Mesh creation PxConvexMeshDesc convexdesc; convexdesc.points.count = numverts; convexdesc.points.stride = sizeof(pxvec3); convexdesc.points.data = verts; convexdesc.flags = PxConvexFlag::eCOMPUTE_CONVEX PxConvexFlag::eINFLATE_CONVEX; PxConvexMesh* convexmesh = NULL; PxDefaultMemoryOutputStream buf; if(cooking.cookconvexmesh(convexdesc, buf)) PxDefaultMemoryInputData id(buf.getdata(), buf.getsize()); convexmesh = physics.createconvexmesh(id);

9 Set up simulation data void createvehicle4wsimulationdata (const PxF32 chassismass, PxConvexMesh* chassisconvexmesh, const PxF32 wheelmass, PxConvexMesh** wheelconvexmeshes, const PxVec3* wheelcentreoffsets, PxVehicleWheelsSimData& wheelsdata, PxVehicleDriveSimData4W& drivedata, PxVehicleChassisData& chassisdata) //Extract the chassis AABB dimensions from the chassis convex mesh. const PxVec3 chassisdims=computechassisaabbdimensions(chassisconvexmesh); //The origin is at the center of the chassis mesh. //Set the center of mass to be below this point and a little towards the front. const PxVec3 chassiscmoffset=pxvec3(0.0f,-chassisdims.y*0.5f+0.65f,0.25f); //Now compute the chassis mass and moment of inertia. //Use the moment of inertia of a cuboid as an approximate value for the chassis moi. PxVec3 chassismoi ((chassisdims.y*chassisdims.y + chassisdims.z*chassisdims.z)*chassismass/12.0f, (chassisdims.x*chassisdims.x + chassisdims.z*chassisdims.z)*chassismass/12.0f, (chassisdims.x*chassisdims.x + chassisdims.y*chassisdims.y)*chassismass/12.0f); //A bit of tweaking here. The car will have more responsive turning if we reduce the //y-component of the chassis moment of inertia. chassismoi.y*=0.8f; //Let's set up the chassis data structure now. chassisdata.mmass=chassismass; chassisdata.mmoi=chassismoi; chassisdata.mcmoffset=chassiscmoffset; //Compute the sprung masses of each suspension spring using a helper function. PxF32 suspsprungmasses[4]; PxVehicleComputeSprungMasses(4,wheelCentreOffsets,chassisCMOffset,chassisMass,1,suspSprungMasses); //Extract the wheel radius and width from the wheel convex meshes. PxF32 wheelwidths[4]; PxF32 wheelradii[4]; computewheelwidthsandradii(wheelconvexmeshes,wheelwidths,wheelradii); //Now compute the wheel masses and inertias components around the axle's axis. // PxF32 wheelmois[4]; for(pxu32 i=0;i<4;i++) wheelmois[i]=0.5f*wheelmass*wheelradii[i]*wheelradii[i]; //Let's set up the wheel data structures now with radius, mass, and moi. PxVehicleWheelData wheels[4]; for(pxu32 i=0;i<4;i++) wheels[i].mradius=wheelradii[i]; wheels[i].mmass=wheelmass; wheels[i].mmoi=wheelmois[i]; wheels[i].mwidth=wheelwidths[i]; //Disable the handbrake from the front wheels and enable for the rear wheels wheels[pxvehicledrive4wwheelorder::efront_left].mmaxhandbraketorque=0.0f; wheels[pxvehicledrive4wwheelorder::efront_right].mmaxhandbraketorque=0.0f; wheels[pxvehicledrive4wwheelorder::erear_left].mmaxhandbraketorque=4000.0f; wheels[pxvehicledrive4wwheelorder::erear_right].mmaxhandbraketorque=4000.0f; //Enable steering for the front wheels and disable for the front wheels. wheels[pxvehicledrive4wwheelorder::efront_left].mmaxsteer=pxpi*0.3333f; wheels[pxvehicledrive4wwheelorder::efront_right].mmaxsteer=pxpi*0.3333f; wheels[pxvehicledrive4wwheelorder::erear_left].mmaxsteer=0.0f; wheels[pxvehicledrive4wwheelorder::erear_right].mmaxsteer=0.0f; //Let's set up the tire data structures now. //Put slicks on the front tires and wets on the rear tires. PxVehicleTireData tires[4]; tires[pxvehicledrive4wwheelorder::efront_left].mtype=tire_type_slicks; tires[pxvehicledrive4wwheelorder::efront_right].mtype=tire_type_slicks; tires[pxvehicledrive4wwheelorder::erear_left].mtype=tire_type_wets; tires[pxvehicledrive4wwheelorder::erear_right].mtype=tire_type_wets; //Let's set up the suspension data structures now. PxVehicleSuspensionData susps[4]; for(pxu32 i=0;i<4;i++) susps[i].mmaxcompression=0.3f; susps[i].mmaxdroop=0.1f; susps[i].mspringstrength= f; susps[i].mspringdamperrate=4500.0f; susps[pxvehicledrive4wwheelorder::efront_left].msprungmass=suspsprungmasses[pxvehicledrive4wwheelorder::efront_left]; susps[pxvehicledrive4wwheelorder::efront_right].msprungmass=suspsprungmasses[pxvehicledrive4wwheelorder::efront_right]; susps[pxvehicledrive4wwheelorder::erear_left].msprungmass=suspsprungmasses[pxvehicledrive4wwheelorder::erear_left]; susps[pxvehicledrive4wwheelorder::erear_right].msprungmass=suspsprungmasses[pxvehicledrive4wwheelorder::erear_right]; //Set up the camber. //Remember that the left and right wheels need opposite camber so that the car preserves symmetry about the forward direction. //Set the camber to 0.0f when the spring is neither compressed or elongated. const PxF32 camberangleatrest=0.0; susps[pxvehicledrive4wwheelorder::efront_left].mcamberatrest=camberangleatrest; susps[pxvehicledrive4wwheelorder::efront_right].mcamberatrest=-camberangleatrest; susps[pxvehicledrive4wwheelorder::erear_left].mcamberatrest=camberangleatrest; susps[pxvehicledrive4wwheelorder::erear_right].mcamberatrest=-camberangleatrest; //Set the wheels to camber inwards at maximum droop (the left and right wheels almost form a V shape) const PxF32 camberangleatmaxdroop=0.001f; susps[pxvehicledrive4wwheelorder::efront_left].mcamberatmaxdroop=camberangleatmaxdroop;

10 Simulation data setup Three key classes Wheel - Suspension, radius, etc.. Chassis - Body of the vehicle Drive - How are power and steering applied PxVehicleWheelsSimData* wheelssimdata=pxvehiclewheelssimdata::allocate(4); PxVehicleDriveSimData4W drivesimdata; PxVehicleChassisData chassisdata; createvehicle4wsimulationdata (chassismass,chassisconvexmesh, 20.0f,wheelConvexMeshes4,wheelCentreOffsets4, *wheelssimdata,drivesimdata,chassisdata);

11 Setup examples Chassis centre of mass const PxVec3 chassiscmoffset=pxvec3(0.0f,-chassisdims.y*0.5f+0.65f,0.25f); chassisdata.mcmoffset=chassiscmoffset; Suspension spring parameters PxVehicleSuspensionData susps[4]; for(pxu32 i=0;i<4;i++) susps[i].mmaxcompression=0.3f; susps[i].mmaxdroop=0.1f; susps[i].mspringstrength= f; susps[i].mspringdamperrate=4500.0f; for(pxu32 i=0;i<4;i++) wheelsdata.setsuspensiondata(i,susps[i]); //... Engine properties PxVehicleEngineData engine; engine.mpeaktorque=500.0f; engine.mmaxomega=600.0f;//approx 6000 rpm drivedata.setenginedata(engine);

12 Setup actor and drivetrain instance Previous object were all static data, need an instance as well Actor is set of rigid bodies for wheel Drive is instance of drivetrain Actor and drivetrain get linked Actor added to scene PxRigidDynamic* vehactor = physics.createrigiddynamic(pxtransform(pxidentity)); for(pxu32 i=0;i<numwheelgeometries;i++) PxShape* wheelshape=vehactor->createshape(*wheelgeometries[i],*wheelmaterial); wheelshape->setqueryfilterdata(vehqryfilterdata); wheelshape->setsimulationfilterdata(wheelcollfilterdata); wheelshape->setlocalpose(wheellocalposes[i]); // Need to add chassis as well //... PxVehicleDrive4W* car = PxVehicleDrive4W::allocate(4); car->setup(&physics,vehactor,*wheelssimdata,drivesimdata,0); mscene.addactor(*vehactor);

13 Per-frame actions Input setup PxVehicleDrive4WRawInputData carrawinputs; carrawinputs.setanalogaccel(mgamepadaccel); carrawinputs.setanalogbrake(mgamepadcarbrake); carrawinputs.setanalogsteer(mgamepadcarsteer); PxVehicleDrive4WSmoothAnalogRawInputsAndSetAnalogInputs (gcarpadsmoothingdata,gsteervsforwardspeedtable, carrawinputs,timestep,isinair,(pxvehicledrive4w&)focusvehicle); Wheel raycasts if(null==msqwheelraycastbatchquery) msqwheelraycastbatchquery=msqdata->setupbatchedscenequery(scene); PxVehicleSuspensionRaycasts(mSqWheelRaycastBatchQuery,mNumVehicles,mVehicles, msqdata->getraycastqueryresultbuffersize(),msqdata->getraycastqueryresultbuffer());

14 Per-frame actions (cont d) Update simulation PxVehicleUpdates(timestep,gravity,*mSurfaceTirePairs, numvehicles,vehicles,vehiclewheelqueryresults); scene->simulate(mstepsize, NULL, scratchblock, scratchblocksize);

15 Conclusions Use the PhysX vehicle library unless you have a really good reason not to Spoiler : you don t have a good reason Mechanics of setup are complex, don t be afraid to steal the PhysX sample code The driving model in the PhysX sample app is now the baseline, need to make it better if you want good marks on the driving portion of things

Extracting Tire Model Parameters From Test Data

WP# 2001-4 Extracting Tire Model Parameters From Test Data Wesley D. Grimes, P.E. Eric Hunter Collision Engineering Associates, Inc ABSTRACT Computer models used to study crashes require data describing