How to update the forward movement in OpenGL

OpenGL OpenGL: Basic Coding
1

Basically I would like to move in the direction pointed by the cursor, but this doesn’t happen and instead the “forward” direction remains the same it was at the beginning.

For example if I turn 180° and press “w” to walk forward, I walk backwards instead of going where I’m looking at. I’m talking about walking because I’m trying to replicate a first-person camera.

Down here you can see the code I’ve written so far:


// angle of rotation for the camera direction
float angle = 0.0f;

void mouseMove(int x, int y) {
    // this will only be true when the left button is down
    if (xOrigin >= 0) {

        // update deltaAngle
        deltaAngle = (x - xOrigin) * 0.001f;

        // update camera's direction
        lx = sin(angle + deltaAngle)*2;
        lz = -cos(angle + deltaAngle)*2;
    }
}

void mouseButton(int button, int state, int x, int y) {
    // only start motion if the left button is pressed
    if (button == GLUT_LEFT_BUTTON) {
        // when the button is released
        if (state == GLUT_UP) {
            angle += deltaAngle;
            xOrigin = -1;
        }
        else  {// state = GLUT_DOWN
            xOrigin = x;
        }
    }
}

void handleKeys(unsigned char key, int x, int y){
    switch (key)
    {
        case 'w':
            posZ+=4;
            cout << "FORWARD
";
            break;
        case 's':
            posZ-=4;
            cout << "BACK
";
            break;
        case 'd':
            posX-=4;
            cout << "RIGHT
";
            break;
        case 'a':
            posX+=4;
            cout << "LEFT
";
            break;
        case 27:
            exit(0);
            break;
    }
}

void computePos(float deltaMove) {
    x += deltaMove * lx * 0.1f;
    z += deltaMove * lz * 0.1f;
}

//draws the 3D sccene
void drawScene(){
    cout << "ANGLE: "<<angle<<"
";
    if (deltaMove){
        computePos(deltaMove);
    }

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    // Set the camera
    gluLookAt(x, 1.0f, z,
        x + lx, 1.0f, z + lz,
        0.0f, 1.0f, 0.0f);

    glTranslatef(posX, 0.0f, posZ);

    glColor3f(1.0f, 1.0f, 0.0f); //we set the cube color

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, _textureId);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    
    glBegin(GL_QUADS);

    //Front
    glNormal3f(0.0f, 0.0f, 1.0f);
    glTexCoord2f(0.0f, 0.0f);
    glVertex3f(-40.5f, -40.0f, 40.5f);
    glTexCoord2f(1.0f, 0.0f);
    glVertex3f(40.5f, -40.0f, 40.5f);
    glTexCoord2f(1.0f, 1.0f);
    glVertex3f(40.5f, 40.0f, 40.5f);
    glTexCoord2f(0.0f, 1.0f);
    glVertex3f(-40.5f, 40.0f, 40.5f);

    //Right
    glNormal3f(1.0f, 0.0f, 0.0f);
    glTexCoord2f(0.0f, 0.0f);
    glVertex3f(40.5f, -40.0f, -80.5f);
    glTexCoord2f(0.0f, 1.0f);
    glVertex3f(40.5f, 40.0f, -80.5f);
    glTexCoord2f(1.0f, 1.0f);
    glVertex3f(40.5f, 40.0f, 40.5f);
    glTexCoord2f(1.0f, 0.0f);
    glVertex3f(40.5f, -40.0f, 40.5f);

    //Back
    glNormal3f(0.0f, 0.0f, -1.0f);
    glTexCoord2f(0.0f, 1.0f);
    glVertex3f(-40.5f, -40.0f, -80.5f);
    glTexCoord2f(0.0f, 0.0f);
    glVertex3f(-40.5f, 40.0f, -80.5f);
    glTexCoord2f(1.0f, 0.0f);
    glVertex3f(40.5f, 40.0f, -80.5f);
    glTexCoord2f(1.0f, 1.0f);
    glVertex3f(40.5f, -40.0f, -80.5f);

    //Left
    glNormal3f(-1.0f, 0.0f, 0.0f);
    glTexCoord2f(0.0f, 0.0f);
    glVertex3f(-40.5f, -40.0f, -80.5f);
    glTexCoord2f(1.0f, 0.0f);
    glVertex3f(-40.5f, -40.0f, 40.5f);
    glTexCoord2f(1.0f, 1.0f);
    glVertex3f(-40.5f, 40.0f, 40.5f);
    glTexCoord2f(0.0f, 1.0f);
    glVertex3f(-40.5f, 40.0f, -80.5f);

    glEnd();

    glutSwapBuffers();
}
2

Your “go forward” code

case 'w':
            posZ+=4;
            cout << "FORWARD
";
            break;

This works fine assuming positive Z is the forward direction. When you turn, the forward direction is pointing somewhere else, but the code is still incrementing just the Z component of the position. In reality, the position is a three dimensional vector and you want something like

case 'w':
            pos  += directionVector*timeStep;
            cout << "FORWARD
";
            break;

Where directionVector is the forward direction that you update every time you turn. (And timeStep is related to the time between frames, so that you go forward at a consistent speed even if your framerate drops.)

Take a look at this tutorial, it’s what helped me get on the right track with making a camera.