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Backface Culling

Started by MButchers Nov 16, 2001 at 9:02 AM 1 replies 400+ views

Original Post

MButchers

Nov 16, 2001 at 9:02 AM

hello there, im using Mark Morleys code for bounding sphere occlusion and it works great

however i am now implimenting backface removal and for some reason its not working. Im doing the following Get the clip planes for the frustum ( these all face inwards ). ViewNormal = 1st 3 elements of near clip plane (x,y,z). Multiply the quads normal by the objects current transformation matrix. If DotProduct(ViewNormal,QuadNormal) > 0 then RenderQuad. Can anyone see anything obviously wrong, Thanks Mark.

Chock

Nov 19, 2001 at 2:48 AM

Let me try to draw some art:
^
| /
| |
| /
|
*
Well... * is the eye position, the arrow your ViewNormal and the other thing a quad.
Clearly, the quad´s normal is facing in the direction of the ViewNormal, even though the quad is possibly visible (Draw this clearly on a piece of paper, and you will see what I mean).
But you can adjust this simply. Just take any point on your quad. Let´s call it a. Then calculate
ViewNormal = a - eyePosition
and then your test on DotProduct(ViewNormal, Quadnormal) > 0 will work.

I now see something else that could be wrong. You must take care that you multiply the quad´s normal with the linear part of your transformation matrix (no translation). As you only want to rotate the normal, not translate it. So make the fourth row (or column, depends on your math) 0.

MButchers

Author

MButchers

Nov 19, 2001 at 5:22 AM

Cheers,

shall give that a go,
I had already noticed the bug with translating the
normal ( specular highlights wizzing around with
object as it rotates ) All fixed now

Many thanks

Mark.

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