Which view vector to use for back-face culling?

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LindsayWard 101 Oct 13, 2004 at 05:41

Hi there,
I have a theoretical question about back-face culling.
A back-facing polygon is one where the angle between the normal for the polygon’s front face and the viewing vector (towards the eye) is less than 90 degrees. This can be tested using the dot product of these two vectors and the sign (positive or negative) tells us if the face is facing the front or the back.
The question is whether to use:
- the vector from the lookat point (centre of view) to the eye/camera, or
- the vector from a point on the polygon to the eye/camera
In some situations, the results are the same, but not all, it seems.

I’ve read a few textbooks, and have mixed results. Most talk about the second option, but Hearn & Baker (3rd ed) uses the first option, and I’m not sure if it’s always right.
It might have something to do with whether the polygon is actually visible or not - such as if the polygon is in front of the viewer it’s front-facing, but as it moves further to the side and the angle using the second vector changes, when the dot product changes sign, the polygon is actually outside of the field of vision… maybe.

Does anyone have any suggestions or knowledge they can add?
Thank you,
Lindsay

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Mihail121 102 Oct 13, 2004 at 06:58

Ok, here is how to do it:

you have a view vector formed by the this difference:

LookAtPoint - CameraPosition

You also have your polygon (positioned in world, scaled and rotated). You also have the transformed normal of this polygon.

To find if this polygon is back facing, find the dot product of the view vector and the transformed polygon normal and check if it’s negative. If it is - polygon is not visible!

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anubis 101 Oct 13, 2004 at 09:44

you have to take the vector from the polygon to the view point since you try to find the angle between the surface normal and that vector. if you take the vector from the eye to the surface you get another angle. try drawing the two vectors originating from the same point. you can easily see which vector gives you the angle you want and that inverting one of them gives you a different angle

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Nick 102 Oct 13, 2004 at 13:04

Back face culling can be done in different stages. You can do it in object space, by transforming the camera position to that space. For every polygon, take the vector from one of the polgyon’s vertices, and compute the dot product with the polygon’s normal. The sign determines what way the polygon is facing. This approach has great performance advantages if you can precompute the normals of the model (i.e. when they are static). The only cost is the dot product.

You can also do it in camera space. The method is the same, but now the normal has to be recomputed with a cross product. If the model is static we could also transform the normal from model space to camera space, but this actually takes longer.

The last approach is in screen space. Here you just need to know whether the polygon’s normal is facing to the screen or not. So only the z-component of the cross product is required. That’s fast, but it requires all vertices to be transformed to screen space. In many implementations this is done anyway, to avoid complexity in the vertex pipeline and the vertex cache. I currently use this method myself, and have seen no significant performance advantage from doing culling in earlier stages.

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LindsayWard 101 Oct 15, 2004 at 01:39

Thanks for the answers so far guys.
I understand that the sign of the dot product of the front-face polygon normal and the viewing vector gives me whether or not it is front-facing. Whether I use the vector to the camera or from the camera only dictates whether positive is front or back.
The question is whether to use the vector between the camera (eye) and the lookat point or between the camera and a point on the polygon.
It is clear to me that we get different answers for these options because if the polygon were behind the camera and facing it, it would be front-facing with the latter of the two options above, and back-facing for using the former. Is it true that if we do it after clipping then we don’t need to worry about this difference? What if we have a really wide field of view like 180 degrees and the polygon is right at the edge of the view? It seems we could get different answers again.
Nick, you talked about what space you do it in, does that affect which view vector we use?

Thanks again,
Lindsay.

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Reedbeta 167 Oct 20, 2004 at 18:15

As far as I know, back-face culling should always be computed using a vector from a point on the polygon to the eye. (Be careful that you do not use a vector from the eye to the polygon, as this will give the wrong result.)

Using the vector from the lookat point to the camera would be correct only if the lookat point happens to lie in the same plane as the polygon.

You can do this in any space you want, so long as you make sure all the relevant vectors (point on polygon, eye, normal) are in the same space. There is no mathematical reason why one space would be preferred over others, but as Nick described, there is usually one space in which the culling is most efficient.