Mirroring Weights
Consider Jim’s gloves or his boots. They are exactly the same mesh but they’re different objects on opposite sides and are affected by different bones. There are several faster ways to add weights to these objects, including only having to weight one side of them. If you weight both sides manually, it will be a slow process (especially if it’s a complex model such as the hands) and you may end up with each side having different deformations. However, there are easier ways to do this by either mirroring before or after adding any weights.
Mirroring before Adding Weights
It is recommended that you mirror before you add any weights. If you’re working on a mirrored mesh, you should be sure you’ve enabled the X-Mirror option before you start painting because after weights are painted, fixing them will be usually more tedious and slow.
If possible, join both sides of the mesh; for example, join the two gloves. Activate the X-Mirror option and paint the weights on one side, which should reflect those weights on the other side. After you’re done defining weights, separate the meshes again so they become different objects while keeping their weights.
Mirroring after Adding Weights
If you’ve already painted the weights on one side, you can still mirror those weights. Let’s use the gloves as an example to see how to do this:
1. Delete the glove you haven’t weighted.
2. Select the one you have weighted. Duplicate it with Shift + D. Right click to cancel the transform after duplication. Set the 3D Cursor in the center of the scene with Shift + C. Press Ctrl + M to mirror the glove and X to mirror it on the X-axis. Finally, press Enter to confirm.
3. Go to the Vertex Groups panel. Delete the vertex groups from the new side and replace their names with those from the other side. Let’s look at this tricky process step by step. For this example, let’s use only one bone and let’s pretend that the one you painted was on the left side:
a. In the list, find the names of the vertex groups from the right side (the ones you need now), all of which end with .R (you can find them quickly by entering .R in the list filter). For this example, the one we’ll mirror is the D_hand.L bone weight, so delete D_hand.R.
b. Now find D_hand.L, double click its name, and rename it to D_hand.R. That will cause the current weights in that vertex group to be affected by the right bone instead of the left one.
c. Repeat the process with all the bones you need to mirror.