I bought an animation pack, but each animation is its own FBX file. When I import one into Blender, the armature is already in the starting pose of the animation. I have a mesh that’s in a tpose that I want to use the animations on.
Since I’ve already rigged the mesh, I was thinking I’d pose the mesh to match the animation’s starting pose, then I’m guessing there’s a way to make that pose the resting state of the mesh, and then rerig the mesh with the animation’s armature. Would that be the best way to do it? If so, how would I change the resting state of the mesh to be the new pose?
You don’t want the armature’s rest pose to be the first frame of animation, you want it to be the rest pose of the .FBX armature.
What is that rest pose? There are a number of ways to see it. Quickest is to enter edit mode on the .FBX armature. That’s the pose you want. (Note-- you also want the same parenting that the .FBX has.)
Another way is to switch to “rest pose” in properties/armature/skeleton. With the armature selected, of course.
How to copy that rest pose to a different armature’s rest pose? You can use “copy transforms” constraints on each bone to copy the transforms of the original armature to your new armature, then use “apply as rest pose”, then optionally clear the constraints-- but really, you’re better off just using the old armature rather than your new one.
What if you have a mesh that needs weighting to the new armature rest pose? The best way to do this is to model the mesh to fit the pose you actually need. If you didn’t do that, it can be complicated, because it’s not going to be very clear how your bones are going to need to roll to have your model look right in its new rest pose.
How do you copy animations to a new armature? You can leave those copy transform constraints in place and bake the animation, which has some advantages. You can also just assign a particular action to the armature-- any action that’s in the file, even one on a different armature-- by choosing a different action for the armature object in the dope sheet/action editor view.
TL;DR: Your best bet is to know what pose you want to target, and to build your mesh and armature in that pose from the start. Techniques to adapt are possible but you’re probably best off redoing it from scratch. If you’re beginning, you need the practice anyways.