OpenGL - FPS style Camera move with object fixed on camera

Question

I am trying to move a camera in a 3d space. So far I have had success in moving and rotating the camera using this code:

void specialKeyHandler(int key, int x, int y) {

    float fraction = 0.05f;

switch (key) {
    case GLUT_KEY_LEFT :
        camAngle -= 0.01f;
        lX = sin(camAngle);
        lZ = -cos(camAngle);
        break;
    case GLUT_KEY_RIGHT :
        camAngle += 0.01f;
        lX = sin(camAngle);
        lZ = -cos(camAngle);
        break;
    case GLUT_KEY_UP :
        iX += lX * fraction;
        iZ += lZ * fraction;
        break;
    case GLUT_KEY_DOWN :
        iX -= lX * fraction;
        iZ -= lZ * fraction;
        break;
    default:
        break;
    }

}

Of course I have these variables defined. lX, lY, lZ are for LookAt variables and iX, iY and iZ are for camera's eye.
The camera moves as required, but now I want to attach an object on the "camera's eye" which shall move with the camera. Just like a Weapon in a FPS game.
This is what I have for that:

void display()

{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    drawEarth();
    drawWalls();

    glColor3f(255,0,0);
    glPushMatrix();
    glTranslatef(iX+0.05,iY, iZ-0.05);
    glRotatef(camAngle, 0,1,0);
    glutWireCone(0.005,0.1,20,20);  // object to go with camera
    glPopMatrix();

    glutSwapBuffers();

}

This code currently does bind the object with the camera while moving up and down. But when I rotate the camera by pressing left and right keys, object stays there and camera moves on its own..
Any solution?

Here's the full opengl code, if anyone wants to run:

#include <GL/glut.h>
#include<iostream>
#include<math.h>
using namespace std;

void display(void);

void reshape(int, int);
void mouseHandler(int button, int state, int x, int y);
void keyBoardHandler(unsigned char c, int x, int y);
void specialKeyHandler(int key, int x, int y);
float angle = 0;
bool camDefault = true, perspectiveOrtho = true;

float iX =0  , iY = -0.8, iZ = 0, lX = 0, lY = -0.8, lZ = -1, uX = 0, uY = 1, uZ = 0; 
float camAngle = 0;
int main(int argc, char** argv)

  {
glutInit(&argc, argv);
glutInitWindowSize(512, 512);
glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE);
glutCreateWindow("FPS Camera");
glutDisplayFunc(display);
glutIdleFunc(display);
//look from negative x axis towards origin. with y on top
glutReshapeFunc(reshape);
glEnable(GL_DEPTH_TEST);
glClearColor(0,0,0,1);
glutMouseFunc(mouseHandler);
glutKeyboardFunc(keyBoardHandler);
glutSpecialFunc(specialKeyHandler);
glutMainLoop();

return 0;
   }


void drawEarth() {

    glColor3f(0,255,0);
    glBegin(GL_QUADS);
    glVertex3f(-1,-1,-1);
    glVertex3f(-1,-1,1);
    glVertex3f(1,-1,1);
    glVertex3f(1,-1,-1);
    glEnd();
}

void drawWalls() {

    glColor3f(0,0,255);
    glBegin(GL_QUADS);
    glVertex3f(-1,-1,-1);
    glVertex3f(-1,-1,1);
    glVertex3f(-1,1,1);
    glVertex3f(-1,1,-1);
    glEnd();


    glColor3f(0,234,255);
    glBegin(GL_QUADS);
    glVertex3f(-1,-1,1);
    glVertex3f(-1,1,1);
    glVertex3f(1,1,1);
    glVertex3f(1,-1,1);
    glEnd();


}

float unitVector(float dir, float x, float y, float z) {
    float sumSqr = pow(x,2) + pow(y,2) + pow(z,2);
    return dir / (sqrt(sumSqr));
}
void display()

{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    drawEarth();
    drawWalls();

    glColor3f(255,0,0);
    glPushMatrix();
    glTranslatef(iX+0.05,iY, iZ-0.05);
    glRotatef(camAngle, 0,1,0);
    glutWireCone(0.005,0.1,20,20);
    glPopMatrix();

    glutSwapBuffers();

}



void reshape(int width, int height)

{

    glViewport(0,0,width,height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(90,1, 0.001, 1000);
    gluLookAt(iX,iY,iZ,iX+lX,lY,iZ+lZ,uX,uY,uZ);
    glMatrixMode(GL_MODELVIEW);

}

void updateLookAt() {
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective (90, 1 ,0.001, 1000);
    gluLookAt(iX,iY,iZ,iX+lX,lY,iZ+lZ,uX,uY,uZ);
    glMatrixMode(GL_MODELVIEW);
}


void keyBoardHandler(unsigned char c, int x, int y) {
    switch(c) {
    case 'f':  // go left
        iX-=0.01;
        lX -= 0.01;
        cout<<endl<<"S pressed";
        break;

    case 's':  // go right
        iX += 0.01;
        lX += 0.01;
        break;

    case 'e': // go up
        iZ += 0.01;
        lZ += 0.01;
        break;

    case 'd': // go down
        iZ -= 0.01;
        lZ -= 0.01;
        break;

    default:
        break;
    }

    updateLookAt();

}

void mouseHandler(int button, int state, int x, int y) {
    if(button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN) {
        if(camDefault) {
            glLoadIdentity();
            gluLookAt(-1,0,0,0.0,0.0,0.0,0.0,1,0);
            camDefault = false;
        }

        else {
            glLoadIdentity();
            gluLookAt(0,0,0,0.0,0.0,0.0,0.0,1,0);
            camDefault = true;
        }
    }

    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) { 
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        if(perspectiveOrtho) {
            gluPerspective (90, 1 ,0.00001, 1000);
            perspectiveOrtho = false;
        }

        else {

            glOrtho(-1,1,-1,1, -1,100);
            perspectiveOrtho = true;

        }
        glMatrixMode(GL_MODELVIEW);

        glLoadIdentity();
        if(camDefault)
                gluLookAt(0,0,1,0.0,0.0,0.0,0.0,1,0);
            else 
                gluLookAt(-1,0,0,0.0,0.0,0.0,0.0,1,0);


    }
}



void specialKeyHandler(int key, int x, int y) {

    float fraction = 0.05f;

switch (key) {
    case GLUT_KEY_LEFT :
        camAngle -= 0.01f;
        lX = sin(camAngle);
        lZ = -cos(camAngle);
        break;
    case GLUT_KEY_RIGHT :
        camAngle += 0.01f;
        lX = sin(camAngle);
        lZ = -cos(camAngle);
        break;
    case GLUT_KEY_UP :
        iX += lX * fraction;
        iZ += lZ * fraction;
        break;
    case GLUT_KEY_DOWN :
        iX -= lX * fraction;
        iZ -= lZ * fraction;
        break;
    default:
        break;
    }


    updateLookAt();
}

Answer 1

You have two problems.

The first problem is on your coordinates system.

When you apply :

glTranslatef(iX+0.05,iY, iZ-0.05);

You place the object at the camera position, and add an offset to see him. But this is in the world coordinates system. So when you rotate the camera, the object doesn't move cause it doesn't follow the camera's coordinates system.

You need to translate to the camera center, rotate the camera, then add the offset.

The second problem is on the rotation. Your "camAngle" variable is in radian, cause you use it with cos() and sin(). But in OpenGL, glRotate take the angle as a degree value. You need to convert this angle in angle : angle *= 180/PI. (180/PI ~= 57.2957795)

Here the corrected display function :

void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glColor3f(255,0,0);

    glPushMatrix();
        glTranslatef(iX,iY,iZ); // Translation to the camera center
        glRotatef(-camAngle * 57.2957795, 0,1,0); // Rotate to correspond to the camera
        glTranslatef(0.05,0,-0.05); // Offset to draw the object

        glutWireCone(0.005,0.1,20,20);
    glPopMatrix();

    drawEarth();
    drawWalls();
    glutSwapBuffers();
}