View Full Version : Achieving off-axis projection using glFrustum

05-08-2013, 09:38 AM
I just started reading about OpenGL topics specifically about Viewing(Viewing, Modelling, Projection, Viewport transformations) for my current requirements. Specifically, I am trying to achieve a perspective projection for my scene using glFrustum(...) and as I understand glFrustum draws objects that are nearer as larger than the objects that are far.
I have drawn some objects on the screen such as an image, a box etc. just for a test. Here's a very short example of what my draw method looks like:

void draw()


For this initial test with my knowledge of glFrustum, I am just trying to clip a portion of the image on the screen. When the documentation mentioned that glFrustum's arguments are left, right, bottom, top, near, far, I thought of glFrustum to accept screen coordinates for this viewing volume which is why I initially gave it the following arguments: glFrustum(100.0, 974.0, 100.0, 668.0, 3.0, 500.0).
1) With the above, I didn't get any output and I can't seem to get head around the stuff about where the glFrustum's arugments (-1.0, 1.0,-1.0,1.0,3.0,500.0) are coming from?

2) Also, as I specify the viewing volume with glFrustum, I don't see a command that I seem to have it to draw the clipped scene around on the whole screen coordinates? The part of things that come in the viewing volume of frustum are to be drawn but how are these being drawn on the whole screen here?