Object Visibility

Approximate your object to a sphere: pos + r.

Transform pos into camera coordinate system: VecMatMul( &pos, viewMatrix ).

Clip if in front of the "front clip" or behind of the "back clip": if( pos.z < frontClip || pos.z > backClip ) -= clipped =-
(That takes care of front and back frustrum planes)

It's not that easy with left/right/top/bottom planes, because they are not parallel to anything...

Easy-Cheesy way (not very precise):
1. Project pos (which is already in camera coordinate system) on screen:

posXScreen = ( (  pos.x / pos.z ) * focus ) + screenWidthHalf;
posYScreen = ( ( -pos.y / pos.z ) * focus ) + screenHeightHalf;
    

Where "focus" is focus distance - distance from camera to a projection plane:

focus = screenHeightHalf / tan( vertFOVHalf );
   

2. Project object radius on-screen:

screenR = ( r / pos.z ) * focus;
   

3. Now you have screenX, screenY - object position ON SCREEN and screenR - object radius ON SCREEN. So...

if( screenX + screenR < 0 || screenX - screenR > screenWidth || screenY + screenR < 0 || screenY - screenR > screenHeight ) -= clipped =-
   

Another way (more precise):
1. Take four normals from every frustrum plane (using cross products).
2.

if( VecDot( center, &frustrumNormals ) < -r ) -= clipped =-  

Yep, that requires some crazy math knowledge... I can give you code if you want.

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Cheers

Meduzza!


Edited by - Meduzza on September 14, 2000 10:33:32 AM