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Thread: FPS rotation problem

  1. #1

    FPS rotation problem

    I'm working on a first-person perspective game engine,and I've run into a stupid problem.I use glTranslate and glRotate to move my scene around.And it works great.But,when the rotation begins,it is always in relation to the camera's starting position.The farther away I get from that initial position,the wider my turning circle.I'm pretty sure that I need to track the position of the view and rotate from that new position,but I have no idea how to do it.Any suggestions would be greatly appreciated...


  2. #2
    Member Contributor
    Join Date
    May 2000
    Ottawa, Ontario, Canada

    Re: FPS rotation problem

    Here is the code I use; may not do everything you want for example its not flight sim type of movement but it does allow you to move around on a horizontal plane, you change elevation with pageup/pagedown keys - sort works like an elevator.
    The first part is the function itself, the second part is the code that is in my rendering thread. This stuff comes from OpenGL Superbible. Have fun!

    void UpdatePosition(struct CAMERA *pCamera)
    float fLinearVelocity = 0.0f;
    float fAngularVelocityY = 0.0f;
    float fAngularVelocityX = 0.0f;
    float fAngularVelocityZ = 0.0f;
    float fTime=0;
    float fXDelta=0;
    float fAngle=0;
    float fZDelta= 0;
    double fSeconds=0;
    float elevation = 0.0f;

    LARGE_INTEGER currentTime;

    // Keyboard Input
    // Check for forward or backwards Motion
    fLinearVelocity = 6.0f;

    fLinearVelocity = -6.0f;
    // check for elevation
    elevation = 0.25f;

    elevation = -0.25f;

    // Check for spin left/right (Y Axis rotation)
    fAngularVelocityY = -45.0f;

    fAngularVelocityY = 45.0f;

    // Check for looking up and down
    if(GetAsyncKeyState(0x41)) // A key
    fAngularVelocityX = -9.50f;

    if(GetAsyncKeyState(0x5a)) // Z key
    fAngularVelocityX = 9.50f;

    // Adjust position and orientation. Get the time since the last
    // check. If the velocity = 0 (no keypress or mouse movement)
    // then the motion will be nil...
    // D = vt
    fTime = (float)(currentTime.QuadPart - lastTime.QuadPart)/
    lastTime = currentTime;

    fSeconds = (double)(currentTime.QuadPart - globeTime.QuadPart)/

    // Update Rotation angles (clamp the X rotation)
    fAngle = fTime * fAngularVelocityX;
    fAngle += pCamera->orientation[0];
    if((fAngle < 90.0f) && (fAngle > -90.0f))
    pCamera->orientation[0] = fAngle;
    pCamera->orientation[1] += fTime * fAngularVelocityY;
    // Update linear position
    fTime = fTime * fLinearVelocity;
    fXDelta = fTime * (float)(sin(DEGTORAD(pCamera->orientation[1])));
    fXDelta += pCamera->position[0];
    fZDelta = fTime * (float)(cos(DEGTORAD(pCamera->orientation[1])));
    fZDelta += pCamera->position[2];

    pCamera->position[1] += elevation;

    pCamera->position[0] = fXDelta;
    pCamera->position[2] = fZDelta;

    // Halt the camera at the boundaries of the Virtual World.

    if(pCamera->position[1] > 40.0f)
    pCamera->position[1] = 40.0f;

    if(pCamera->position[0] > 100.0f)
    pCamera->position[0] = 100.0f;

    if(pCamera->position[0] < -100.0f)
    pCamera->position[0] = -100.0f;

    if(pCamera->position[2] > 100.0f)
    pCamera->position[2] = 100.0f;

    if(pCamera->position[2] < -100.0f)
    pCamera->position[2] = -100.0f;
    // end of updatecamerpostition

    ** rendering thread code


    glRotatef(cameraData.orientation[0], 1.0f, 0.0f, 0.0f); // look up, down
    glRotatef(cameraData.orientation[1], 0.0f, 1.0f, 0.0f); // turn
    glRotatef(cameraData.orientation[2], 0.0f, 0.0f, 1.0f); // advance/retreat

    glTranslatef(-cameraData.position[0],cameraData.position[1], cameraData.position[2]);

  3. #3

    Re: FPS rotation problem

    Cool! Thanks a lot.I'll give this a try tonight.


  4. #4

    Re: FPS rotation problem

    Make sure you rotate the camera BEFORE you translate the camera.

    The rotation is always done with respect to the origin (0, 0, 0), so if you rotate first, then translate, you'll get the correct result.

    Hope that helps (matrix calculations can be such a bitch when you're first learning them)

  5. #5

    Re: FPS rotation problem

    I looked back at my original code,and it turns out that I forgot a glLoadIdentity() before I did my translation and rotation.STOOOPPPIDDD!
    That fixed my problem.Thanks for the other suggestions,though...

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