The Viewer is a program which allows you to access the meshes and animation files from the game's database and preview them.
How Mesh Animation Works Edit
Meshes are specified as .msh files while animations are specified as .anm files. Clicking on a .msh file will show you the raw mesh in the preview area of the interface. If you then click on a .anm file, the mesh you selected will adopt the animation of that particular file.
Animations are possible because the mesh is attached to an invisible skeleton which specifies which parts of the mesh will move. If two characters are very different in stuctures it is very likely that their skeletons will be very different as a result. For instance, A Centaur and a Satyr will have very different skeletal structures; this is easy to spot out because they have different number of limbs overall. If you try to transfer any animations from the Satyr to the Centaur .msh mesh file, the satyr's animation will appear distorted.
This happens because a skeleton with a structure which is not compatible with the mesh is forced in a mesh which doesn't fit its purpose. On the other hand, there are several meshes which do share a similar skeleton structure. This is done by the game designers to save themselves time and avoid creating the same numbers of animations for so many monsters every time. Imagine how tedious it would be to create an animation for when the character is idle, an animation for when the character is attacking, one for when it is walking or running and another for when it dies, then repeat this procedure for the 100 and over monsters present in the game. It would be a very long task!
For this reason some animations can be used on those monster meshes which look overall the same. For instance, a tigerman may make use of its own set of animations, or since it is has two pawed legs, two arms, a tail, a head and a jaw it could borrow the animation set of either the ratmen or the shadowstalkers without any serious aberration. The same is true for the shadowstalkers and the ratmen making use of each other's animations or those of the tigermen.
Other Features Edit
1. File Name You can see the types of bones present in any animation file if you goto View > Properties > Animation Tab. This window also gives you the address line which lets you reference to that particulat .anm file in the Art Manager when you are editing a character. Similarly, the View > Properties > Mesh Tab lets you reference to the .msh file you have displayed. The Viewer is therefore a useful tool for previewing the compatibility between meshes and animation sets.
2. Editing the Skeleton In the Viewer you can edit the skeleton of the mesh, even if in a limited way. For instance you can edit the size, rotation and location of the attachment points (the position where equipment will be attached) or add new ones or remove them altogether. Most of these features are however meant for experienced programmers and designers.