How Backface Culling Works?

[pythonboy]

How backface culling works? It is so cool.It renders in viewport only what camera sees.I need to know how this works so i can make a script that will do some specific operations only on the faces that are seen by camera.

[joojaa]

it multiples the vector along camera direction (not ea digital camera does not have angle to it since the world is actually widened before this phase) and then take a dot product between these vectors.

since dot product returns with normalized vectors

cos(angle Between Vectors) you know if the result is smaller than (and equal but numeric precicion is so keep 0) 0 that its facing away thus can be discarded

[pythonboy]

vector $hey = <>;
vector $wow = <>;
dot $hey $wow;

That is dot vector.I understand that.

But what are the two vectors in my case?The first one from camera to each face???The second,what???

I found on the web that dot product of two vectors is same as the cos(angle between the two vectors),that is kinda cool.Damn i am falling in love with math.hehe

[joojaa]

Well since it works in camera space of the ones are allways <> because thats the eye ray in camera space and the second is cameraspace face normal! (which in turn can be computed by taking cross(point1-point2,point3-point2) or any combination really), maya knows this at all time tough so you can just ask maya for it)

Offcourse if you do some sort of raytracing then the eye ray can vary.

Anyway understand that back face culling does NOT hide all faces, just the ones hat absolutely wont participate in the render, theres a lot of extra faces that don't contribute and tahts the job of the z buffer, or trace (in which case you do not really need back face culling).

[pythonboy]

So how would you do this?

When you rotate in viewport the face that camera sees have one shader and the ones that camera don't see get another shader?

[joojaa]

Duplicate the surface and flip its normals. them mark them both as backface culling and assign different shader to the other surface.

(the wh renderer OGL does not do 2 sided materials, in one pass, as this would mean youd need 2 times as complex shaders and possiby pushing recursion depth of all non 1500$+ cards out)

PS. mel has and should not have any acces to the gpu shards of things... its like you are trying to use a hammer whan your meaning to use the entire factory. But your vantage point don't let you see that.