I remember crossing paths with “normals” in Animation Master. Why do we need a vector to specify which side of the face we are on with a polgon? Are not both sides identical? My follow up question is why are back faces culled to begin with? In what situations would you need a back face rendered?

STATUS:
Reviewing: http://en.wikipedia.org/wiki/Surface_normal

Not 100% sure I understand this post. Are you asking why we need surface normals? Also surface normals are not just used for back face culling but also for the shading models. As for your follow up there are tons of situations where you would need both polygon sides. One quick example I can think of is hair in gaming using polygons and alpha masks. You wouldn’t want pieces of hair to disappear based on the side you are looking at them from. I am sorry if this post is not answering your question(s) but I am not really sure what it is you are asking.

I guess raytraced transparancy would look odd without both sides.

Yes, what are surface normals? Examples do help for what situations you need them and don’t need them. Why do we have to know about them?

You probably don’t need to know about them until they become a problem – and then you do.

Unless you’re writing a shader or something along those lines…

Why do we need a vector to specify which side of the face we are on with a polgon?

Because in 3D computing, programs doesn’t know what side of a face is “inside” or “outside”. Without normals there’s no way to know. And just the visible face has normals, the face that doesn’t visible doesn’t, so you have a plane and one of the 2 faces of a plane is visible, talking in terms of 3D software.

This was devised long time ago , when the computers barely could render 4 colors in 2D, and is a rule that have resisted until nowadays because it works, and many render algorithms are based on this basic property.

Are not both sides identical?
Talking from the wiewpoint of 3D math and hardware: NO.
When you define a plane in any 3D software, you are really defining a face of the plane, not the two. and this is done this way because the time taken to render a scene will depends on how many “faces” you have in the scene, not “planes”. if you have a full scene where all planes have 2 faces, then you have wasted at least 2x the time to render it (not counting all hidden faces that need to be discarded). Any 3D artist must know at least that one plane has only one face and when problems arise for whatever reason, the fist thing to verify is if the faces are “looking” to the right side. Usually all 3D software nowadays manages to do the right thing and the user normally doesn’t worry about these things, except when dealing with complicated geometry or shapes. Of course also depends on the render engine you use…

My follow up question is why are back faces culled to begin with?
You don’t need to render what you don’t see, and this makes posible that the scene you are working render fast without the software need to make useless calculations that end discarded (read: working for nothing). As i write before, has to do with make things go faster. Do this excersise: take a scene, duplicate the scene and invert the normals of all faces in the duplicate, then press “render” and you’ll note that the scene will render slower than the original scene. Now take a scene of… well… 500000-700000 polys and you’ll see what i’m talking about (and i’m being conservative).

This is also true to graphics accelerators: to have one plane with 2 faces, any graphic accelerator need to draw 2 different faces: one with normals inverted respect the other. so the time taken to generate and display them is increased 2x. Not a problem with low numbers, but when using large numbers of polygons, then things will get slower. That’s why if you render a face with the normal pointing to the same wiewpoint that the camera with any game engine, the face will become invisible. The GPU is just ignoring the face. (And this is the right behavior).

In what situations would you need a back face rendered?
If you need to render a still: Never.
If you need to render an animation: Never.
If you need to program a graphics engine that deals with all this: probably many times.

In the bottom, all of this depends on the software/hardware you use. Nowadays you don’t need to worry much about these kind of things, except when problems arise. In general, the only thing you need to know, as an artist, about surface normals is that they need to point to the camera or be visible by the camera, nothing more, nothing less. If you are going to be a software enginner, then you need way more than wikipedia has.

@ stargeizer: Why would a computer bother calculating a hidden face in the first place if it is already hidden? How can front and back faces flip/invert accidentally without normals?

I feel substantially repulsive in saying this, since you put a substantial amount of material in explaining it to me. I am still lost in the forest - half way. Thank you for your time in posting that.

Why would a computer bother calculating a hidden face in the first place if it is already hidden?

A computer doesn’t know the concept of “hidden” or “visible”. So is up to the software used to determine what is “hidden” or “visible”. The normal only helps to determine what side if the face should be directly visible to an eye in front of it or not.

How can front and back faces flip/invert accidentally without normals?

Again, a machine doesn’t know what is a face. A machine only know numbers and thus, is responsability of the programmer to teach the machine what to do with numbers. Remember, a normal only defines what side of the face is visible, and to where the light should be reflected, nothing more, nothing less…

Think of a common mirror. If you see to the mirror, a reflect of your face is seen, but if you look from behind the mirror, there’s no reflect anywhere. Imagine that the normal is in the side of the mirror that reflects, and is pointing to your face, so the light that reaches it is reflected. Well… that’s whats hapening with computing 3D.

I feel substantially repulsive in saying this, since you put a substantial amount of material in explaining it to me. I am still lost in the forest - half way. Thank you for your time in posting that.

No problem at all