Who doesn't love a game that incorporates cool real-time 3D characters? You do, right? Everyone wants the most amazing characters possible for their game. So why doesn't everyone have them? Because, it's not that easy. There are so many options available for the artist and programmer that it can be truly overwhelming. On that account, here is a guide to help you plow through these choices.
Who doesn't love a game that incorporates cool real-time 3D characters? You do, right? Everyone wants the most amazing characters possible for their game. So why doesn't everyone have them? Because, it's not that easy. There are so many options available for the artist and programmer that it can be truly overwhelming. On that account, here is a guide to help you plow through these choices.
Now as far as which decisions you should make, well that really is up to your production team. I assure you that while the information included is not all-encompassing, you will be able to make well-educated decisions. There is no definitive "right" answer because so much depends upon budget, time, manpower, target platform etc. But, do allow me to step on my soapbox for my two cent(ence)s worth. Early planning must be done! These decisions need to be carried through the entire production because once you head down any of the paths, the time and money spent to change is most likely just not worth it. (Yeah, so I do preach a bit.)
Mesh construction techniques:
The first and largest fork in the road comes early on. How am I going to handle the character mesh? While there are many ways of designing 3D real-time character engines, the character meshes themselves are usually one of two types:
a hierarchy of separate objects
single mesh characters
Characters of this type have a separate object for each body part in the character. These separate objects are connected together in some form of hierarchy. Then, by applying rotations and translations to each body part, you can create fluid motion. You see this type of character in many games, but some classic examples of this technique include Virtua Fighter and Tomb Raider.
Benefits of this method:
Characters that are created from a hierarchy of separate objects are very adaptable.
You only need to store a transformation for each object and the vertex information once, thus the animation file is very small.
By having the character in a hierarchy, it is fairly easy to create new animations. If your engine supports an Inverse Kinematic (IK) or Dynamic system, you can actually generate new animations on the fly. This is the key benefit of creating your characters this way.
Drawbacks of this method:
Since the actual position of each vertex in the model is determined by the hierarchy, you need to do some extra work to get the mesh to the screen. Each object in the mesh must be transformed by the complete hierarchy above it. This makes for quite a bit more math per vertex. For example, if your hierarchy looks like figure 2 (see below), the mesh of the left hand must be transformed by the hips, chest, chest2, left collar, left upper arm, left lower arm, and finally the left hand.
Where the objects in the character connect, you get gaps. Since the objects in the character are separate, you need to consider how they go together. By clever modeling, you can hide these seams, but this is not always possible. You may notice that many characters of this type wear armor or clothing that hides these seams.
A single mesh character is not made of separate parts. As the name implies, the character is one single mesh. The character is animated by changing the position of the vertices in the mesh directly. This can be done by having a table of positions or offsets for each frame or just by switching meshes as the character animates. Quake characters are the classic single mesh characters.
Benefits of this method:
Single mesh characters look better, especially when modeled with low polygons counts. There are no seams in the character and the mesh looks smoother.
The characters are fast. Since there is no need to transform each vertex through a hierarchy, there is a lot less math required to display these characters.
Drawbacks of this method:
The characters are not as adaptable. Since the character must be animated by an external system, it is not as easy to come up with unique motions.
The animation files are larger. Since you need to store a position or at least a delta for every vertex for every frame, much more data must be stored. While you can compress this a great deal, it is a limiting factor in the length of the animation.
If you have journeyed thus far and understand the foundation, you are ready to embark with me to a greater horizon; the future. Could it be possible to incorporate the flexibility of a hierarchy of objects with the good looks of a single mesh? As always, we want more! And much to our delight and fancy, we can have more. With 3D hardware becoming more common and ripping processors' speed, the sky is the limit (well, at least until we think of something else we want). Thus we get:
Deformation and realtime skinning of skeletons
To demonstrate, let us take a look at a character's arm...
The simplest method to make this arm look better would be to cover the gap by connecting vertices at the end with a few extra polygons. |
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While this looks better, it tends to stretch badly and look a bit deformed as the arm moves. |
The real key to making things look better is to add a series of vertices that are influenced by the rotation of both bones. If we add a single set of vertices and weight them so they are influenced 50/50 between the two bones, the look is improved. |
This helps with the stretching problem, but the pinching at the joint is still pretty noticeable. By allowing the bones to influence the end vertices of the objects, the character looks much better. In this image, the vertices are weighted 33%/66%, 50%/50%, and 66%/33%, around the joint. |
It is clear that we can achieve the most realistic look and most flexible animation by combining real-time weighted mesh deformations with a skeletal hierarchy. By combining these techniques with motion captured animation, dynamic simulation, and Inverse Kinematic engines, we can create convincing 3D characters. The cpu power saved by using hardware rendering enables game programmers to push the envelope and add these enhancements to their games.
As gamers, we know what we want. As programmers, artists, producers, we know what we can do. But as innovators, we must incorporate what we know and get what we want. We know that what is cool today will be obsolete tomorrow. Yet, this keeps us moving toward our mystical ideal of the coolest real-time 3D characters.
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