Playing Card "3D" rotation in OpenGL - perspective?

Question

Hi all,

(Mac OS X, Cocoa, C/C++, OpenGL, somewhat mathematically challenged.)

Project is a real-time (30fps) poker game, with a 2D view. The OpenGL view is set up for Orthographic projection using glOrtho; the camera is always looking directly down at the cards/table. To date, it's essentially been a 2D image composition engine.

My playing cards are pre-built textures (with alpha), rendered thusly:

glBegin(GL_TRIANGLE_STRIP);
    glTexCoord2f(0.0, 0.0);
    glVertex3d(startDraw_X, startDraw_Y, 0.0);

    glTexCoord2f(1.0, 0.0);
    glVertex3d(endDraw_X, startDraw_Y, 0.0);

    glTexCoord2f(0.0, 1.0);
    glVertex3d(startDraw_X, endDraw_Y, 0.0);

    glTexCoord2f(1.0, 1.0);
    glVertex3d (endDraw_X, endDraw_Y, 0.0);
glEnd();

This works well, and provides 1:1 pixel mapping, for high image quality.

QUESTION: how can I (cheaply, quickly) "flip" or distort my card using OpenGL? The rotation part isn't hard, but I'm having trouble with the perspective aspect.

Specifically, I have in mind the horizontal flipping action of UIViews in iPhone applications (e.g. Music). The UIView will appear to flip by shrinking horizontally, with the appearance of perspective (the "rear" coordinates of the view will shrink vertically.)

Thanks for any help or tips.

Answer 1

Instead of using glOrtho to set up an orthographic projection, use glFrustum to set up a perspective projection.

Answer 2

Since you game is 2D, I assume it is a top-view camera, right ?

In this case, you can setup your projection matrix as a projection one, put your camera at the center of the table, at a decent height (> 1.2 x width of table depending on the FoV) and not change anything else.

You may have to set your card's (and other game items) height a lot closer from the table than what they may currently be (since in ortho, a delta of 10 meters along Z won't be seen).

Mixing ortho and perspective is technically possible, but I don't quite see the advantage. However, you may have special assets in your game that require special treatments...