A Car with Wheel Joints

The first thing we are going to address in this tutorial is the geometry of our car. It is possible to use any other custom made model for the vehicle appearance. Every wheel and the frame will be individual independent nodes in the world, so make sure that your vehicle model includes separate meshes for the wheels and the frame.

Right now we are going to take the vehicle model from the content addon included in the SDK and then add some physical parameters to the geometry to achieve a viable driving simulation.

1. Let's create an empty C++ project.

2. Download the Eastern European Countryside add-on from Add-On Store at https://store.unigine.com/ and add it to the project by dragging into the project data/ folder in the Asset Browser. In the Import Package window that opens, click the Import Package button and wait until the add-on contents are imported.

3. Launch the Editor via the Open Editor button. For your vehicle you can choose any of the minibus nodes located in data/countryside/minibus or import your own model in any supported file format.

   

   A minibus vehicle model has the body frame as the parent node and its wheels as child nodes.

   

4. In the Asset Browser, right-click on a minibus node you like the most and choose Place as Node Content. Place the vehicle on the ground in the world. As we don't plan to use multiple instances of the car, we simply put all its parts directly to the scene, without the NodeReference container.

   

Let’s create a camera for a third-person view. While the camera follows the car, the player can control the viewing direction (rotate the camera) with the mouse. To achieve this, perform the following:

1. In the World Nodes window, delete the first_person_controller containing the default camera.
2. Create a custom Persecutor Camera to follow the car. Go to Create -> Camera -> Persecutor and then place it near the vehicle.
3. Check the Main Player option in the Parameters tab of the PlayerPersecutor to make it the default camera in the world.

4. Delete the default Persecutor_Target node from the World Nodes window hierarchy and set our minibus as the target node for the PlayerPersecutor camera instead.

   

5. To put the camera closer to the vehicle, set the Max Distance parameter for the PlayerPersecutor to 8.

Now let’s create a C++ component to implement driving and steering functionality for the car. To achieve this, do the following:

1. To start writing code we should open our project in an IDE. Go to SDK Browser and choose the Open Code IDE button.

2. In an IDE create a new C++ class (*.h and *.cpp files) and call it Car. Make sure it inherits from the Unigine::ComponentBase class.

3. Get references to joints from the wheel nodes and define variables for the velocity, acceleration and torque values.

   Car.h

   Source code (C++)

   #pragma once
   #include <UnigineComponentSystem.h>
   
   #include <UniginePhysics.h>
   #include <UniginePlayers.h>
   #include <UnigineControls.h>
   #include <UnigineGame.h>
   #include <UnigineMathLib.h>
   
   class Car : public Unigine::ComponentBase
   {
   public:
   	// declare constructor and destructor for our class and define a property name
   	COMPONENT_DEFINE(Car, ComponentBase)
   	// declare methods to be called at the corresponding stages of the execution sequence
   	COMPONENT_INIT(init);
   	COMPONENT_UPDATE(update);
   	COMPONENT_UPDATE_PHYSICS(updatePhysics);
   
   	// driving parameters
   	PROP_PARAM(Float, acceleration, 50.0f);
   	PROP_PARAM(Float, max_velocity, 90.0f);
   	PROP_PARAM(Float, default_torque, 5.0f);
   
   	// wheel joints
   	PROP_PARAM(Node, wheel_bl);
   	PROP_PARAM(Node, wheel_br);
   	PROP_PARAM(Node, wheel_fl);
   	PROP_PARAM(Node, wheel_fr);
   
   protected:
   	void init();
   	void update();
   	void updatePhysics();
   
   private:
   	// wheel joints
   	Unigine::JointWheelPtr joint_wheel_bl;
   	Unigine::JointWheelPtr joint_wheel_br;
   	Unigine::JointWheelPtr joint_wheel_fl;
   	Unigine::JointWheelPtr joint_wheel_fr;
   
   	// settings of input controls 
   	Unigine::ControlsPtr controls;
   
   	// current driving parameters
   	float current_velocity = 0.0f;
   	float current_torque = 0.0f;
   
   };

   Car.cpp

   Source code (C++)

   #include "Car.h"
   
   using namespace Unigine;
   
   REGISTER_COMPONENT(Car);
   
   void Car::init()
   {
   	// get the wheel joints from the nodes
   	if (wheel_bl)
   		joint_wheel_bl = checked_ptr_cast<JointWheel>(wheel_bl->getObjectBody()->getJoint(0));
   
   	if (wheel_br)
   		joint_wheel_br = checked_ptr_cast<JointWheel>(wheel_br->getObjectBody()->getJoint(0));
   
   	if (wheel_fl)
   		joint_wheel_fl = checked_ptr_cast<JointWheel>(wheel_fl->getObjectBody()->getJoint(0));
   
   	if (wheel_fr)
   		joint_wheel_fr = checked_ptr_cast<JointWheel>(wheel_fr->getObjectBody()->getJoint(0));
   
   	// get the settings of input controls relevant to the player (camera)
   	controls = Game::getPlayer()->getControls();
   }

4. Then add control inputs processing to determine the current torque and velocity to be applied to wheel joints.

   Car.cpp

   Source code (C++)

   void Car::update()
   {
   	// get the time it took to render the previous frame
   	float deltaTime = Game::getIFps();
   
   	// process control inputs
   	if (controls)
   	{
   		// set the torque and increase the current velocity if the forward button is pressed
   		if (controls->getState(Controls::STATE_FORWARD) != 0)
   		{
   			current_torque = default_torque;
   			current_velocity = Math::max(current_velocity, 0.0f);
   			current_velocity += deltaTime * acceleration;
   		}
   		// set the torque and decrease the current velocity if the backward button is pressed
   		else if (controls->getState(Controls::STATE_BACKWARD) != 0)
   		{
   			current_torque = default_torque;
   			current_velocity = Math::min(current_velocity, 0.0f);
   			current_velocity -= deltaTime * acceleration;
   		}
   		// exponentially decrease the current velocity when neither throttle nor brakes are applied
   		else
   		{
   			current_velocity *= Math::exp(-deltaTime);
   		}
   }

5. To move the car forward or backward, we are going to use joint motors of the front wheels:

   • Setting a positive velocity value will move our car forward.
   • Setting a negative velocity value will move our car backward.

   Each physics frame, we apply current angular velocity and torque to the wheel joints based on the user input.

   Car.cpp

   Source code (C++)

   // actually apply the torque and velocity to the front wheel joints
   void Car::updatePhysics()
   {
   	joint_wheel_fl->setAngularVelocity(current_velocity);
   	joint_wheel_fr->setAngularVelocity(current_velocity);
   
   	joint_wheel_fl->setAngularTorque(current_torque);
   	joint_wheel_fr->setAngularTorque(current_torque);
   }

To provide the ability to turn left or right as well as the ability to slow down with brakes, we shall implement some additional logic that turns the front wheels and stops the rear ones. To achieve this, do the following:

1. Add the following code to the Car component to add the front wheels turning and braking support.

   Car.cpp

   Source code (C++)

   PROP_PARAM(Float, turn_speed, 50.0f);
   PROP_PARAM(Float, car_base, 3.0f);
   PROP_PARAM(Float, car_width, 2.0f);
   
   float current_turn_angle = 0.0f;
   
   float max_turn_angle = 30.0f;

   Car.cpp

   Source code (C++)

   void Car::update()
   {
   	// get the time it took to render the previous frame
   	float deltaTime = Game::getIFps();
   
   	current_torque = 0.0f;
   
   	// process control inputs
   	if (controls)
   	{
   		/*...*/
   		// turn the front wheels based on the direction input
   		if (controls->getState(Controls::STATE_MOVE_LEFT) != 0)
   			current_turn_angle += deltaTime * turn_speed;
   		else if (controls->getState(Controls::STATE_MOVE_RIGHT) != 0)
   			current_turn_angle -= deltaTime * turn_speed;
   		else
   		{
   			// get rid of the front wheel wiggle
   			if (Math::abs(current_turn_angle) < 0.25f)
   				current_turn_angle = 0.0f;
   
   			// align the front wheels if there are no more direction input
   			current_turn_angle -= Math::sign(current_turn_angle) * turn_speed * deltaTime;
   		}
   
   		// apply braking by maxing out the angular damping if the brake button is pressed
   		if (controls->getState(Controls::STATE_USE) != 0)
   		{
   			joint_wheel_bl->setAngularDamping(10000.0f);
   			joint_wheel_br->setAngularDamping(10000.0f);
   		}
   		else
   		{
   			joint_wheel_bl->setAngularDamping(0.0f);
   			joint_wheel_br->setAngularDamping(0.0f);
   		}
   	}
   
   	// clamp the velocity and the front wheels turn angle
   	current_velocity = Math::clamp(current_velocity, -max_velocity, max_velocity);
   	current_turn_angle = Math::clamp(current_turn_angle, -max_turn_angle, max_turn_angle);
   
   	// figure out the correct turn angle for the front wheels
   	float angle_0 = current_turn_angle;
   	float angle_1 = current_turn_angle;
   	if (Math::abs(current_turn_angle) > Math::Consts::EPS)
   	{
   		float radius = car_base / Math::tan(current_turn_angle * Math::Consts::DEG2RAD);
   		float radius_0 = radius - car_width * 0.5f;
   		float radius_1 = radius + car_width * 0.5f;
   
   		angle_0 = Math::atan(car_base / radius_0) * Math::Consts::RAD2DEG;
   		angle_1 = Math::atan(car_base / radius_1) * Math::Consts::RAD2DEG;
   	}
   
   	joint_wheel_fr->setAxis10(Math::rotateZ(angle_1).getColumn3(0));
   	joint_wheel_fl->setAxis10(Math::rotateZ(angle_0).getColumn3(0));
   }

2. Before you can use the C++ component you should initialize the Component System. Modify the init() method of the AppSystemLogic class as shown below (AppSystemLogic.cpp file).

   AppSystemLogic.cpp

   Source code (C++)

   #include <UnigineComponentSystem.h>
   
   int AppSystemLogic::init()
   {
   	// initialize ComponentSystem and register all components
   	ComponentSystem::get()->initialize();
   
   	return 1;
   }

3. Build and run the project via your IDE (press Ctrl + F5 in Visual Studio), the Component System will generate a property-file for the Car component.
4. Switch back to UnigineEditor and select the root minibus node in the World Nodes window. Then click Add New Property in the Parameters window and assign the newly generated Car property.

5. Next, assign wheels (ObjectMeshStatic nodes) to the corresponding fields of the Car property as specified below.

   

Congratulations! Now you can finally drive the car around the world. If the current physics of the car is not suitable for your needs, you can always fine tune the parameters of the wheel joints and achieve the results you need. Also, see the Basic Programming How-To Livestream for the hands-on vehicle implementation and discussions. The full source code of the Car component is given below.

#pragma once
#include <UnigineComponentSystem.h>

#include <UniginePhysics.h>
#include <UniginePlayers.h>
#include <UnigineControls.h>
#include <UnigineGame.h>
#include <UnigineMathLib.h>

class Car : public Unigine::ComponentBase
{
public:
	// declare constructor and destructor for our class and define a property name
	COMPONENT_DEFINE(Car, ComponentBase)
	// declare methods to be called at the corresponding stages of the execution sequence
	COMPONENT_INIT(init);
	COMPONENT_UPDATE(update);
	COMPONENT_UPDATE_PHYSICS(updatePhysics);

	// driving parameters
	PROP_PARAM(Float, acceleration, 50.0f);
	PROP_PARAM(Float, max_velocity, 90.0f);
	PROP_PARAM(Float, default_torque, 5.0f);
	PROP_PARAM(Float, turn_speed, 50.0f);
	PROP_PARAM(Float, car_base, 3.0f);
	PROP_PARAM(Float, car_width, 2.0f);

	// wheel joints
	PROP_PARAM(Node, wheel_bl);
	PROP_PARAM(Node, wheel_br);
	PROP_PARAM(Node, wheel_fl);
	PROP_PARAM(Node, wheel_fr);

protected:
	void init();
	void update();
	void updatePhysics();

private:
	// wheel joints
	Unigine::JointWheelPtr joint_wheel_bl;
	Unigine::JointWheelPtr joint_wheel_br;
	Unigine::JointWheelPtr joint_wheel_fl;
	Unigine::JointWheelPtr joint_wheel_fr;

	// settings of input controls 
	Unigine::ControlsPtr controls;

	// current driving parameters
	float current_velocity = 0.0f;
	float current_torque = 0.0f;
	float current_turn_angle = 0.0f;

	float max_turn_angle = 30.0f;

};

#include "Car.h"

using namespace Unigine;

REGISTER_COMPONENT(Car);

void Car::init()
{
	// get the wheel joints from the nodes
	if (wheel_bl)
		joint_wheel_bl = checked_ptr_cast<JointWheel>(wheel_bl->getObjectBody()->getJoint(0));

	if (wheel_br)
		joint_wheel_br = checked_ptr_cast<JointWheel>(wheel_br->getObjectBody()->getJoint(0));

	if (wheel_fl)
		joint_wheel_fl = checked_ptr_cast<JointWheel>(wheel_fl->getObjectBody()->getJoint(0));

	if (wheel_fr)
		joint_wheel_fr = checked_ptr_cast<JointWheel>(wheel_fr->getObjectBody()->getJoint(0));

	// get the settings of input controls relevant to the player (camera)
	controls = Game::getPlayer()->getControls();
}

void Car::update()
{
	// get the time it took to render the previous frame
	float deltaTime = Game::getIFps();

	current_torque = 0.0f;

	// process control inputs
	if (controls)
	{
		// set the torque and increase the current velocity if the forward button is pressed
		if (controls->getState(Controls::STATE_FORWARD) != 0)
		{
			current_torque = default_torque;
			current_velocity = Math::max(current_velocity, 0.0f);
			current_velocity += deltaTime * acceleration;
		}
		// set the torque and decrease the current velocity if the backward button is pressed
		else if (controls->getState(Controls::STATE_BACKWARD) != 0)
		{
			current_torque = default_torque;
			current_velocity = Math::min(current_velocity, 0.0f);
			current_velocity -= deltaTime * acceleration;
		}
		// exponentially decrease the current velocity when neither throttle nor brakes are applied
		else
		{
			current_velocity *= Math::exp(-deltaTime);
		}
		/*...*/
		// turn the front wheels based on the direction input
		if (controls->getState(Controls::STATE_MOVE_LEFT) != 0)
			current_turn_angle += deltaTime * turn_speed;
		else if (controls->getState(Controls::STATE_MOVE_RIGHT) != 0)
			current_turn_angle -= deltaTime * turn_speed;
		else
		{
			// get rid of the front wheel wiggle
			if (Math::abs(current_turn_angle) < 0.25f)
				current_turn_angle = 0.0f;

			// align the front wheels if there are no more direction input
			current_turn_angle -= Math::sign(current_turn_angle) * turn_speed * deltaTime;
		}

		// apply braking by maxing out the angular damping if the brake button is pressed
		if (controls->getState(Controls::STATE_USE) != 0)
		{
			joint_wheel_bl->setAngularDamping(10000.0f);
			joint_wheel_br->setAngularDamping(10000.0f);
		}
		else
		{
			joint_wheel_bl->setAngularDamping(0.0f);
			joint_wheel_br->setAngularDamping(0.0f);
		}
	}

	// clamp the velocity and the front wheels turn angle
	current_velocity = Math::clamp(current_velocity, -max_velocity, max_velocity);
	current_turn_angle = Math::clamp(current_turn_angle, -max_turn_angle, max_turn_angle);

	// figure out the correct turn angle for the front wheels
	float angle_0 = current_turn_angle;
	float angle_1 = current_turn_angle;
	if (Math::abs(current_turn_angle) > Math::Consts::EPS)
	{
		float radius = car_base / Math::tan(current_turn_angle * Math::Consts::DEG2RAD);
		float radius_0 = radius - car_width * 0.5f;
		float radius_1 = radius + car_width * 0.5f;

		angle_0 = Math::atan(car_base / radius_0) * Math::Consts::RAD2DEG;
		angle_1 = Math::atan(car_base / radius_1) * Math::Consts::RAD2DEG;
	}

	joint_wheel_fr->setAxis10(Math::rotateZ(angle_1).getColumn3(0));
	joint_wheel_fl->setAxis10(Math::rotateZ(angle_0).getColumn3(0));
}

// actually apply the torque and velocity to the front wheel joints
void Car::updatePhysics()
{
	joint_wheel_fl->setAngularVelocity(current_velocity);
	joint_wheel_fr->setAngularVelocity(current_velocity);

	joint_wheel_fl->setAngularTorque(current_torque);
	joint_wheel_fr->setAngularTorque(current_torque);
}