If you have to rig a model that's very dense with the random awful edge flow, weighting becomes your worst nightmare and you just can't do nothing about having some modeller to fix it or anything.
Some would say, ok, dude, build a low-res proxy mesh, skin it, weight it, copy weights... In some cases - fine. But, not if the mesh has 'thickness' and requires a very wide range of motion like this dragon wing here. (Sorry, I can't show the full view of the model)
With a very short deadline I had, that's when the influence object comes in. I modeled a simple low-res plane that matches the shape of the wing and sits right in the center of the real mesh thickness.
I skin the plane to some joints. Painting weights on this was a piece of cake compared to the one before.
Then I added the plane as the influence object of the real wing. (Select the real mesh, then the plane. Go to skin>edit smooth skin>add influence with 'use geometry' checked)
Everything worked great except for when I scale the whole rig - this is what I got.
When you scale the rig down the influenced area become thicker, and when you scale it up it becomes thinner. Normally, if the influence object is not skinned to any joint this problem shouldn't occur. I used to end up leave it as is and inform animator or whoever using it that scaling's not gonna work and I have no idea how to fix it (SHAME).
Eventually, I found the solution and I'm very proud to share it with you guys.
1. Don't skin the influence object. Duplicate it, skin the duplicate instead.
2. The influence mesh and the base mesh have to scale with the rig. (parent, scale constraint)
3. Pipe the deformation using blend shape from the duplicated mesh to the influence
mesh with origin option set to 'world' not 'local'.
4. F***, the solution sits right under my nose for years.
Now, you should have a scalable rig with influence poly object working correctly!