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[dspencer]

Hi,
I'm using openGL with C++ for a project. I have a video camera on a movable entity. This camera shows up on your screen as an image on a TV, located slightly to the right of the center of the screen. To draw the scene being viewed by the camera, I'm setting the Perspective matrix to a slightly right aligned glFrustum(). However, this makes figuring out the coords I need for the scene that exists around the TV image is quite difficult. Furthermore, I would liek to be able to change the location of the image without entirely redoing this code.

So, the question is - is it ok to change the Perspective matrix in openGL in the middle of a scene? I'm under the impression that this is a very bad idea. If it is bad, why? Also, how else might I do this?

Thanks for your help

[rogerlevy]

As far as I know it's perfectly legal to do this and this would be one use it was intended for.  You could first draw your TV in the foreground (with the TV's details specified in local coordinates), push the p. matrix, translate it, draw the inner tv image, then pop it back.  Pretty easy.

[Saint]

No this is not a bad idea at all and in my opinion no different from changing between perspective projection when rendering a scene and orthographic projection when rendering GUI. You can of course change the viewport or compensate for the change in the modelview matrix or geometry, but there is nothing principally wrong with changing the perspective matrix mid-scene.  

Don't overdo it, though, I've seen cases where people used the projection and modelview matrices at random, not caring about the matrix mode, and that kind of neglects the point of having different modes.

[Ivan]

The projection matrix uses a stack just like the modelview matrix. So whatever operations you're doing is using the current state of the projection matrix stack, so there's no problem as long as you push the current matrix onto the stack, and then pop it afterwards (just remember to set the matrix mode to projection when you push the stack, a common mistake)

[dspencer]

That is just lovely! Thanks again for the prompt replies!

[Will Vale]

Setting the viewport might be easier though

[Zaphos]

Technically the minimum stack depth for the projection matrices is 2, while for modelview it's 32 ... so might want to be careful pushing with that stack.  Not sure if that minimum stack depth is an actual limit in practice, though?