camera class for opengl

7/27/2019 Camera Class for OpenGL

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A Camera Class for

OpenGLJohn McGuiness

October 2006



Necessity for a Camera Class

Existing available camera tool - gluLookAt()

Basic utility which encapsulates a series of rotate and translatecommands

Allows viewing along an arbitrary line of sight with an “Up” vector defined

Need extra transformations to provide greater flexibility

Need to modify “Forward” and “Along” vectors as well as

“Up” to improve on gluLookAt() Camera class may be built to encapsulate commands for

greater ease of use



“Up”, “Forward” and “Along”

The three camera view vectors are defined as shown:



Proposed Camera Features

The camera class should:

Provide motion along the view vectors as well as arbitrary axes(in some cases)

Provide rotation about the view vectors as well as arbitrary axes

(in some cases)

Maintain the camera’s own orientation by keeping the viewingvectors orthogonal to each other

Need to define motion for two possible types of camera:

Land camera – e.g. for road vehicles simulation Air camera – e.g. for flight simulation



Camera Motion Walking

This is motion along the Forward vector (or Z-axis):



Camera Motion Strafing

This is side to side motion on the Along vector (or X-axis):



Camera Motion Flying

This is vertical motion on the Up vector (or Y-axis):



Camera Rotation Pitching

This is rotation about the Along vector – looking up and down



Camera Rotation Yawing

This is rotation about the Up vector – looking left and right



Camera Rotation Rolling

This is rotation about the Forward vector – twisting left and right



Camera Class Declaration

The camera class uses a type called Vector3D whichprovides storage and common operations (e.g. dotproduct, cross product etc.) for vectors

#include "Vector3D.h"

The enumerated type defined below is used todistinguish between the two types of camera

enum CAM_TYPE { LAND_CAM, AIR_CAM };




First, we need private variables for each view vector aswell as the camera type and current position:

class Camera {

private:CAM_TYPE CameraType;

Vector3D Position;

Vector3D Along;

Vector3D Up;

Vector3D Forward;...




Various construction/destruction, update and controlfunctions are then declared publically:

class Camera {

...public:

Camera(CAM_TYPE ct = LAND_CAM); // Default: land

virtual ~Camera();

void SetCameraType(CAM_TYPE ct);

Vector3D GetPosition();void Reset();

void Update();

...




Finally, the motion and rotation functions are declared. The boolean array, Wall[4], is an extra feature which

modifies the motion of land cameras if they have to slideagainst walls (as opposed to going through them)class Camera {

...

public:

...

void Pitch(GLfloat theta);

void Yaw(GLfloat theta);

void Roll(GLfloat theta);

void Walk(GLfloat delta, bool Wall[4]);

void Strafe(GLfloat delta, bool Wall[4]);

void Fly(GLfloat delta);

};



Setup and Control Functions

The code listing on the following two slides is fairly self-explanatory

It comprises the basic constructor and destructor as wellas a function to alter the camera type

The Reset() function sets the camera position to (0,0,0)and aligns the viewing axes with the local drawingcoordinate system

Note that the default Forward vector points along thenegative Z-axis




Camera::Camera(CAM_TYPE ct) {

SetCameraType(ct);

Reset();

}

Camera::~Camera() {

}

void Camera::SetCameraType(CAM_TYPEct) {

CameraType = ct;

}




Vector3D Camera::GetPosition() {

return Position;

}

void Camera::Reset() {

Position = Vector3D(0.0, 0.0, 0.0);

Along = Vector3D(1.0, 0.0, 0.0);

Up = Vector3D(0.0, 1.0, 0.0);

Forward = Vector3D(0.0, 0.0, -1.0);

Update();

}



Building the View Matrix

The last function called by Reset() is probably the mostimportant

The Update() function applies all changes made to the

viewing axes and camera position, and updates the viewin MODELVIEW mode

In actual fact, as with gluLookAt(), the perception of camera motion is achieved by moving the objects aroundthe scene while keeping the camera at a fixed position

Instead of using translations and rotations, a view matrixmay be built, meaning that just one OpenGL function callis needed – glLoadMatrix()




First we obtain the camera virtual position coordinatesusing the dot product of pairs of the view vectors:

void Camera::Update() {

GLfloat x = DotProduct(Along, Position);

GLfloat y = DotProduct(Up, Position);

GLfloat z = DotProduct(Forward, Position);

...

These will be used to translate the camera (or rather,the scene) to its correct position




The translation part of the view matrix is shown below:

1 0 0 0

T = 0 1 0 00 0 1 0

–x –y z 1

Note that we must remember to make z positive, sincefor convenience we have taken “Forward” as meaningthe direction into the screen which is opposite toOpenGL convention (Z-axis is positive outwards)




The rotation part of the view matrix is built from theview vectors as shown:

A.x U.x –F.xR = A.y U.y –F.y

A.z U.z –F.z

Again, the Forward vector is reversed




Combining these two matrices, we get:

1 0 0 0 A.x U.x –F.x 0 A.x U.x –F.x 0

V = 0 1 0 0 . A.y U.y –F.y 0 = A.y U.y –F.y 00 0 1 0 A.z U.z –F.z 0 A.z U.z –F.z 0

–x –y z 1 0 0 0 1 –x –y z 1

The code on the following slides shows the rest of theimplemented function




void Camera::Update() {

...

Glfloat ViewMatrix[4][4];

ViewMatrix[0][0] = Along.x;

ViewMatrix[0][1] = Up.x;ViewMatrix[0][2] = -Forward.x;

ViewMatrix[0][3] = 0.0;

ViewMatrix[1][0] = Along.y;

ViewMatrix[1][1] = Up.y;ViewMatrix[1][2] = -Forward.y;


...




...

ViewMatrix[2][0] = Along.z;

ViewMatrix[2][1] = Up.z;

ViewMatrix[2][2] = -Forward.z;


ViewMatrix[3][0] = -x;

ViewMatrix[3][1] = -y;

ViewMatrix[3][2] = z;


glMatrixMode(GL_MODELVIEW);

glLoadMatrixf((GLfloat *)&ViewMatrix);

}



Camera Rotation Functions

The Pitch(), Yaw() and Roll() functions change thedirection of the Forward, Along and Up vectorsrespectively

In each case, the rotation will result in the alteration of asecond view vector, leaving one unchanged

The second modified vector is found by calculating thecross product of the other two vectors

This means that mutual orthognality is maintained forthe three vectors




Looking at a yaw from above, we can see how tocalculate the new direction of the Along vector:




Thus, for the Yaw function definition, we have:

void Camera::Yaw(GLfloat theta) {

Along = Along * cos(theta * DEG2RAD)+ Forward * sin(theta * DEG2RAD);

Along.Normalize();

Forward = CrossProduct(Along, Up) * -1.0;

Update();

}

Pitch() and Roll() on the following slide look very similar



Camera Rotation Functionsvoid Camera::Pitch(GLfloat theta) {

// Invert UP/DOWN for air camerasif(CameraType == AIR_CAM) theta = -theta;

Forward = Forward * cos(theta * DEG2RAD)

+ Up * sin(theta * DEG2RAD);

Forward.Normalize();

Up = CrossProduct(Forward, Along) * -1.0;

Update();

}

void Camera::Roll(GLfloat theta) {

if(CameraType == LAND_CAM) return; // Not for land cams

Up = Up * cos(theta * DEG2RAD)- Along * sin(theta * DEG2RAD);

Up.Normalize();

Along = CrossProduct(Forward, Up);

Update();

}



Camera Motion Functions Walk(), Strafe() and Fly() are a little easier to implement

In each case, all we have to do is add the correct scaledvector to the camera’s Position vector and update

As with rotation functions, motion functions work slightlydifferently depending on the type of camera being used

For example, when walking forward with a land camera,if the view has been pitched upwards, we do not want tomove up the camera’s forward vector, but rather along a

modified vector with the Y componet set to 0 – this willachieve the effect of staying on the ground rather thantaking off into the air.




The Walk function with wall handling also implemented:

void Camera::Walk(GLfloat delta, bool Wall[4]) {

if(CameraType == LAND_CAM)

Position -= Vector3D(Forward.x *!(Wall[0] && Forward.x * delta > 0.0 ||

Wall[1] && Forward.x * delta < 0.0),

0.0, Forward.z *

!(Wall[2] && Forward.z * delta > 0.0 ||

Wall[3] && Forward.z * delta < 0.0))* delta;

else Position -= Forward * delta; // Air camera

Update();

}




Similarly, the Strafe function is defined as follows:

void Camera::Strafe(GLfloat delta, bool Wall[4]) {

if(CameraType == LAND_CAM)

Position -= Vector3D(Along.x *!(Wall[0] && Along.x * delta > 0.0 ||

Wall[1] && Along.x * delta < 0.0),

0.0, Along.z *

!(Wall[2] && Along.z * delta > 0.0 ||

Wall[3] && Along.z * delta < 0.0))* delta;

else Position += Along * delta; // Air camera

Update();

}




Finally, flying is, of course, only allowed for air cameras:

void Camera::Fly(GLfloat delta, bool Wall[4]) {

// Don't allow for land cameras

if(CameraType == LAND_CAM) return;

Position += Up * delta;

Update();

}

Although flying through walls has been allowed here,this would be implemented in the same manner as theprevious two functions



References

Frank D. Luna, 2003, Introduction to 3D Game Programming with DirectX 9.0 , Wordware Publishing,Inc.

Silicon Graphics Inc., 1997, OpenGL Programming Guide,Chapter 3 – Viewing , Addison-Wesley PublishingCompany

Philipp Crocoll, The Advanced CodeColony Camera,

http://www.codecolony.de/



camera class for opengl

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