How do I change the Projection so it shows my object?

Question

Hello,

I have an 2D object (GL_QUAD) with size (W,H) and sitting at (-W*0.5, -H*0.5).

I'd rather not resize the object, because I need to be able to select points on that object relative to the texture. (i.e. (x,y) in openGL translates to (x,y) on the texture - no scaling needed).

What I want to do is change the camera properties (projection, angle, etc) so that the object, regardless of size, shows up filling the largest axis of the screen. (Aspect Fit?)

I can compare the screen size vs. object size no problem. I can change my camera matrix no problem (modelview and projection). But I don't know how to calculate the transformation I need in order to view the whole object.

I was thinking of:

1. translate camera (0,0,-1)
2. project object's 4 coords with gluProject()
3. test if points exist within screen rect.
4. if not, goto 1.

But that seems... computationally so intensive. I have the screen coords, and the object coords - shouldn't I be able to calculate the solid angle that lines between the 4 corners of each object describe and set it to that?

Thanks,

---

[EDIT]

Here's another non-elegant solution:

void View::getCamera(eq::Matrix4f& camera)
{
    camera = eq::Matrix4f::IDENTITY;
    camera = camera * _rotation;
    camera.set_translation( _translation  );
    float longAxis = 1.0f;

    // Normalize Scale
    @try {
        float tH = (float)getHeight(); // texture coords
        float tW = (float)getWidth();
        float sH = getBaseFrustum().getWall().getHeight(); // screen coords
        float sW = getBaseFrustum().getWall().getWidth();

        float rW = tW / sW;
        float rH = tH / sH;

        if (rH > rW) {

            longAxis *= tH*2.0f;

        } else {

            longAxis *= tW*2.0f;
        }

    }
    @catch (...) {
        //Nothing?
        NSLog(@"Couldn't get Height or Width");
    }

    // Normalize Scale to Screen/Image Size
    camera.scale( eq::Vector3f(_scale/longAxis,-_scale/longAxis,1.0f) );
}

I don't feel like it's a great solution, however...

Answer 1

It sounds like your stuff is all 2D and really doesn't need the benefits of a perspective projection, in such a case an orthogonal projection would be much easier to use. An orthogonal projection makes it trivially easy to fit a quad to fill the screen.

http://www.opengl.org/sdk/docs/man/xhtml/glOrtho.xml