# A Camera Class for

Λογισμικό & κατασκευή λογ/κού

13 Δεκ 2013 (πριν από 4 χρόνια και 6 μήνες)

81 εμφανίσεις

A Camera Class for
OpenGL

John McGuiness

October 2006

Necessity for a Camera Class

Existing available camera tool
-

gluLookAt()

Basic utility which encapsulates a series of rotate and translate
commands

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 viewing
vectors orthogonal to each other

Need to define motion for two possible types of camera:

Land camera

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

Along

vector

looking up and down

Camera Rotation

Yawing

Up

vector

looking left and right

Camera Rotation

Rolling

Forward

vector

twisting left and right

Camera Class Declaration

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

#include "Vector3D.h"

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

enum CAM_TYPE { LAND_CAM, AIR_CAM };

Camera Class Declaration

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

class Camera {

private:

CAM_TYPE CameraType;

Vector3D Position;

Vector3D Along;

Vector3D Up;

Vector3D Forward;

...

Camera Class Declaration

Various construction/destruction, update and control
functions 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();

...

Camera Class Declaration

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 slide
against 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 well
as 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 drawing
coordinate system

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

Setup and Control Functions

Camera::Camera(CAM_TYPE ct) {

SetCameraType(ct);

Reset();

}

Camera::~Camera() {

}

void Camera::SetCameraType(CAM_TYPEct) {

CameraType = ct;

}

Setup and Control Functions

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 most
important

The Update() function applies all changes made to the
viewing axes and camera position, and updates the view
in MODELVIEW mode

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

Instead of using translations and rotations, a view matrix
may be built, meaning that just one OpenGL function call
is needed

Building the View Matrix

First we obtain the camera virtual position coordinates
using 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

Building the View Matrix

The translation part of the view matrix is shown below:

1 0 0 0

T =

0 1 0 0

0 0 1 0

x

y z 1

Note that we must remember to make z positive, since
for convenience we have taken “Forward” as meaning
the direction
into

the screen which is opposite to
OpenGL convention (Z
-
axis is positive outwards)

Building the View Matrix

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

A.x U.x

F.x

R =

A.y U.y

F.y

A.z U.z

F.z

Again, the Forward vector is reversed

Building the View Matrix

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

0

0 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 the
implemented function

Building the View Matrix

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[1][3] = 0.0;

...

Building the View Matrix

...

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

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

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

ViewMatrix[2][3] = 0.0;

ViewMatrix[3][0] =
-
x;

ViewMatrix[3][1] =
-
y;

ViewMatrix[3][2] = z;

ViewMatrix[3][3] = 1.0;

glMatrixMode(GL_MODELVIEW);

}

Camera Rotation Functions

The Pitch(), Yaw() and Roll() functions change the
direction of the Forward, Along and Up vectors
respectively

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

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

This means that mutual orthognality is maintained for
the three vectors

Camera Rotation Functions

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

Camera Rotation Functions

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 Functions

void Camera::Pitch(GLfloat theta) {

// Invert UP/DOWN for air cameras

if(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)

-

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 scaled
vector to the camera’s Position vector and update

As with rotation functions, motion functions work slightly
differently 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 to
move up the camera’s forward vector, but rather along a
modified vector with the Y componet set to 0

this will
achieve the effect of staying on the ground rather than
taking off into the air.

Camera Rotation Functions

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();

}

Camera Rotation Functions

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();

}

Camera Rotation Functions

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 the
previous 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