It annoys me when people, especially the theater companies, use the term "3D" when the actual term for "3D", as most of you know, is stereoscopic. When I was in school or at work, when we use the term "3D", we're referring to 3D animation films. A few examples of 3D animation films are "Shrek", "Kung Fu Panda", "Finding Nemo" and "Toy Story".
So what is 3D?
"3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering into images." (Quote from Wikipedia).
As a 3D artist, I start the process by "Modeling" an object. I have to input every single point of an object into data that the computer can read, until it looks like the object. Usually the first step is to create a cube for example. A cube has 8 vertex (Points), and I continue adding vertex around the cube and move the vertex around until it shapes into the object or character (Credits to Poser). As you can see from the human head, there's an insane amount of points in there. Yup, that's 3D modeling for you. It's very similar to real life sculpting, just that everything is digital.
There's a whole of things I need to do until I move on to the rendering of image, which I'll show later. After modeling an object, we move on to give the object or character a color. This process is called texturing. For example, to texture a piece of wood, I'll first have to model it out, find a good picture of wood, then connect it to the model.
As for animation film, before the model can be animated, they have to be rigged in order for it to move. It's the process of putting in the bones into the character, and the computer will calculate which vertex moves when you move the bones. This process alone is very tedious and it is really hard. Most of the time it includes complex programming, which is intensively used for movies. After rigging, the character is ready for animation!
Animating a character is a very fun process. Once we're done with the rigging, we simply animate the character by moving the bones around. You can find a lot of examples on YouTube but here's a good example.
Animation is not simply just moving the bones around. It involves a lot of acting into it. Normally when animators work, they have a mirror somewhere around them so they can act out themselves and refer. So basically, when you walk into a room full of animators working, you'll see lots of people talking to themselves. It gets funny sometimes.
3D Lighting and Rendering
Lighting? Woah. That's right. In 3D animation, we have to light our scenes as well. As without light, you can't see anything. Most of the job is done by the computer, so all we have to do is to put the lights into the scene, input the data which includes the brightness, color, and how light bounces, and the computer will calculate how the image will look like when the light shines on the object.
It actually sounds very easy when I put it that way, but it actually is very hard work. An example would be an imagethat I've done for one of my assignments. I've spent about 6 hours modeling most of the objects and another 3 to 4 hours to texture them. (This assignment was for a competition too, some of the objects like were pre-modeled). As for the lighting, I have probably spent about at least 40 hours just to test. The rendering of that image took me about 30 minutes.
The reason the computer spent so much time to render is because it has to calculate how the light moves around the scene. As the light touches the object, it will bounce off to surrounding objects. All in all, the computer makes millions or even billions of calculation just to render a single image.
An interesting fact - Some frames in Transformers 2 took as long as 72 hours to render. That means, since there's 25 frames a second for film, it took around 1800 hours to render for a second worth of film time.
So what is Stereoscopic?
Stereoscopic is the technique of creating the illusion of depth in an image by showing 2 different images separately from your left and right eye. Remember the old 3D glasses? The blue and red ones. Basically how it works is by wearing the glasses, the left eye will see a different image from the right eye.
The reason why we can judge the distance of an object when we look at it is because we have 2 eyes. Try closing one of your eyes and look at an object from a distance and it'll be much harder to judge the distance. When you use both your eyes, the 2 images from both eyes will show you the depth of the object, thus making it easier for you to tell the distance.
So how do they film or create 3D stereoscopic film?
They do it by using a stereoscopic camera. A stereoscopic camera has 2 camera lenses that is about the distance of a person's left to right eye, which is about 6.35cm apart. This allows the camera to take 2 separate images/videos at the same time. Same applies to 3D, just rendering 2 images at eyes distance apart. However, it is expensive to do this for 3D animation film because the rendering time would be doubled because of 2 images. Imagine Transformers 2; they would have needed to render 144 hours for a single frame instead of 72 hours if they used stereoscopic!
I hope I've clarified your doubts on stereoscopic and 3D animation. Here are some links on 3D computer graphics from YouTube. It is really amazing how computer graphics are so realistic today.