Setting up a camera in OpenGL

Question

I've been working on a game engine for awhile. I've started out with 2D Graphics with just SDL but I've slowly been moving towards 3D capabilities by using OpenGL. Most of the documentation I've seen about "how to get things done," use GLUT, which I am not using.

The question is how do I create a "camera" in OpenGL that I could move around a 3D environment and properly display 3D models as well as sprites (for example, a sprite that has a fixed position and rotation). What functions should I be concerned with in order to setup a camera in OpenGL camera and in what order should they be called in?

Here is some background information leading up to why I want an actual camera.

To draw a simple sprite, I create a GL texture from an SDL surface and I draw it onto the screen at the coordinates of (SpriteX-CameraX) and (SpriteY-CameraY). This works fine but when moving towards actual 3D models it doesn't work quite right. The cameras location is a custom vector class (i.e. not using the standard libraries for it) with X, Y, Z integer components.

I have a 3D cube made up of triangles and by itself, I can draw it and rotate it and I can actually move the cube around (although in an awkward way) by passing in the camera location when I draw the model and using that components of the location vector to calculate the models position. Problems become evident with this approach when I go to rotate the model though. The origin of the model isn't the model itself but seems to be the origin of the screen. Some googling tells me I need to save the location of the model, rotate it about the origin, then restore the model to its origal location.

Instead of passing in the location of my camera and calculating where things should be being drawn in the Viewport by calculating new vertices, I figured I would create an OpenGL "camera" to do this for me so all I would need to do is pass in the coordinates of my Camera object into the OpenGL camera and it would translate the view automatically. This tasks seems to be extremely easy if you use GLUT but I'm not sure how to set up a camera using just OpenGL.

EDIT #1 (after some comments): Following some suggestion, here is the update method that gets called throughout my program. Its been updated to create perspective and view matrices. All drawing happens before this is called. And a similar set of methods is executed when OpenGL executes (minus the buffer swap). The x,y,z coordinates are straight an instance of Camera and its location vector. If the camera was at (256, 32, 0) then 256, 32 and 0 would be passed into the Update method. Currently, z is set to 0 as there is no way to change that value at the moment. The 3D model being drawn is a set of vertices/triangles + normals at location X=320, Y=240, Z=-128. When the program is run, this is what is drawn in FILL mode and then in LINE mode and another one in FILL after movement, when I move the camera a little bit to the right. It likes like may Normals may be the cause, but I think it has moreso to do with me missing something extremely important or not completely understanding what the NEAR and FAR parameters for glFrustum actually do.

Before I implemented these changes, I was using glOrtho and the cube rendered correctly. Now if I switch back to glOrtho, one face renders (Green) and the rotation is quite weird - probably due to the translation. The cube has 6 different colors, one for each side. Red, Blue, Green, Cyan, White and Purple.

int VideoWindow::Update(double x, double y, double z)
{
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();

    glFrustum(0.0f, GetWidth(), GetHeight(), 0.0f, 32.0f, 192.0f);


    glMatrixMode( GL_MODELVIEW );

    SDL_GL_SwapBuffers();  
    glLoadIdentity();
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glRotatef(0, 1.0f, 0.0f, 0.0f);
    glRotatef(0, 0.0f, 1.0f, 0.0f);
    glRotatef(0, 0.0f, 0.0f, 1.0f);
    glTranslated(-x, -y, 0);

    return 0;
}

EDIT FINAL: The problem turned out to be an issue with the Near and Far arguments of glFrustum and the Z value of glTranslated. While change the values has fixed it, I'll probably have to learn more about the relationship between the two functions.

Answer 1

You need a view matrix, and a projection matrix. You can do it one of two ways:

1) Load the matrix yourself, using glMatrixMode() and glLoadMatrixf(), after you use your own library to calculate the matrices.

2) Use combinations of glMatrixMode(GL_MODELVIEW) and glTranslate() / glRotate() to create your view matrix, and glMatrixMode(GL_PROJECTION) with glFrustum() to create your projection matrix. Remember - your view matrix is the negative translation of your camera's position (As it's where you should move the world to relative to the camera origin), as well as any rotations applied (pitch/yaw).

Hope this helps, if I had more time I'd write you a proper example!

-Tom

Answer 2

You have to do it using the matrix stack as for object hierarchy,

but the camera is inside the hierarchy so you have to put the inverse transform on the stack before drawing the objects as openGL only uses the matrix from 3D to camera.

Answer 3

My post in this thread may help you:

http://stackoverflow.com/questions/1259461/how-to-fix-weird-camera-rotation-while-moving-camera-with-sdl-opengl-in-c

Answer 4

If you have not checked then may be looking at following project would explain in detail what "tsalter" wrote in his post.

Camera from OGL SDK (CodeColony)

Also look at Red book for explanation on viewing and how does model-view and projection matrix will help you create camera. It starts with good comparison between actual camera and what corresponds to OpenGL API. Chapter 3 - Viewing

Answer 5

Just remember that OpenGl post multiplies.