Do you mean on how many tris/verts, or how they are placed? There are certain “rules” pertaining to modeling for game model, e.g. three loops per joint. But also due to computer limits you want to keep the count of verts/tris low.
Yes, keeping the vertices count under control is very important in a game engine, because your animated models will all be moving real time, calculations and video display must then occur very quickly to maintain an acceptable framerate.
For a rendered movie/vfx , due to your animation being rendered and so nothing is going to be real time so you don’t have any need to care about the performance of your model or keeping their polycount under control.
edit 2 : additionally modern graphic cards deal with models in game engines better if they have very few textures (so by example it’s a good idea to have a model having 1 2048x2048 texture instead of 16 512x512 textures)
Poly count is most important, but you still should try to retain the best topology you can at that limit. You can cheat by adding triangles in places that don’t really bend, or at the edge of the bending area.
oh. yeah. Stuffs usually done in quads for texturing, animating etc., and then converted later. You should probably convert some triangles beforehand since game engines can be a little weird with splitting quads. I generally just tri certain quads that effect the sillouette.
the 3 problems you need to look for before exporting to whatever game engine are :
inverted normals, they’re easy to obtain accidentally during a modelling session in Blender (in past version of blender just beveling could give inverted normals by example) , so fixing those before export is a priority because in most game engine it will look very ugly.
Spotting inverted faces can be done with :
In Object mode using Textured display, you’ll see where the problems are immediately
In Edit Mode, turn Backface Culling on (and the Double Sided OFF ), you’ll see the problematic areas easily too.
deformed quads (non planar) are usually not a problem, but if a quad is very deformed , a game engine can simply visually make it disappear, so in a running animation by example you can suddenly notice a hole popping up in a part of your model, the deformed quad being displayed then deforming too much disappear then re-appear.
Converting such a quad into 2 triangle fixes the problem entirely, one of the reasons most people would convert their models fully in triangle too before they’re in a game engine, just check that the triangulation is done in a way it complement your model (as a quad can give 2 triangles in several way depending on where the edge cutting the quad into 2 tris is placed).
assign materials and for the materials assign a texture with the coordinates set to UV (oh and do not use Cycles Render mode, use Blender Render mode, most of the exporters will not export the node system of material from Cycle into something a game engine may read)
when you begin it’s easy to overlook that and just unwrap your model and assign the texture into the UV/Image editor, but if you assign a material and the texture to it, the UV shouldn’t be exported.
While i remember, another must do is to check the scale and rotations.
When you rescaled/rotated an object while being in Edit Mode, you don’t have to be concerned with this, things will be good.
But if you have rescaled/rotated an object while being in Object Mode, your model scale and rotation will be thrown off and will very badly impact your export.
While in Object mode, press N and at the top of it, the Transform tab, check the XYZ number given for rotation and scale
XYZ Rotations must be 0 0 0 and XYZ scales must be 1 1 1 to avoid problems in export.
If they’re not those exact numbers, it is because you scaled/rotated in Object mode, and you need to apply those transformations in order to export correctly.
Just press CTRL+A and “Rotation&Scale” (or click Object -> Apply -> Rotation&Scale), you’ll see the transformations number becoming what they should be to export correctly.