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Clip space headache

Started by nexekho Sep 10, 2010 at 2:32 PM 4 replies 800+ views

Original Post

Sep 10, 2010 at 2:32 PM

I need to know if a point is behind the eye. I have this point in clip space. I know the near and far clip planes. How can I calculate what the Z position of the eye is in clip space if my near plane is 1.0 and my far plane is 1024? It should be just beyond -1.0, I know that much, but not how to calculate the actual value.

Many thanks.

Zipster

Sep 10, 2010 at 3:20 PM

Clip space isn't the best place to be performing this kind of test, since things get wacky after the homogeneous divide -- view space would be better because all you have to check is Z < 0 (or Z > 0, depending on your coordinate system).

nexekho

Sep 10, 2010 at 4:18 PM

As in before the perspective is applied? I didn't think of that, I'll try it, thanks.

nexekho

Sep 11, 2010 at 2:05 PM

Just to wrap up, that works great, thanks. My frustum culling is now flawless. (needed to flip the X/Y coordinates of the bounding box coordinates behind the eye)

Eric Lengyel

Sep 14, 2010 at 2:32 AM

You might find slide 7 of the following presentation interesting:

http://www.terathon.com/gdc07_lengyel.pdf

Points behind the camera have positive values greater than (f+n)/(f-n) in clip space.

Zipster

Sep 14, 2010 at 2:48 AM

Quote:
Original post by Eric Lengyel
You might find slide 7 of the following presentation interesting:

http://www.terathon.com/gdc07_lengyel.pdf

Points behind the camera have positive values greater than (f+n)/(f-n) in clip space.

Ah yes, for some reason I forgot that the asymptotes were at Z=0 and Z'=(f+n)/(f-n) after the divide! Good reminder :) I guess that what happens when you step away from graphics for too long :/

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