Today I decided to experiment with the Blender clothes module (see here and here). So I took the hairlock/antistar PC model (the red haired girl) who happen to be wearing a pretty dress, and off we go.
It took me an evening to get something decent up and running. First I’ll describe the minimal setup, then reasons why the initial setup didn’t work out and how I fixed it.

The minimal setup involves three essential steps:

First we enable ‘cloth mode’ on the target (a mesh we’d like to behave as clothe, e.g. a table cloth, a pair of jeans etc…). This means that we tell Blender to compute cloths physics based on gravity and collisions on this object.

Secondly we setup one or several ‘colliders’ – Blender will only evaluate collisions with designated objects.
Finally we ‘bake’ (precalculate) the cloth simulation. While this step is in theory optional (Blender will calculate frames while scrubbing through the timeline), you’ll need a really fast box to avoid it.

I just outline the procedure, if you’re getting lost please refer to the links provided or dig up a YouTube video (no, really!). The caveats section is more interesting.


Minimal setup
  • Setup clothe:
    • select the mesh that you want to use as cloth.
    • select object > physics > cloth > enable clothe
    • enable clothe.
  • Enable clothe collision
    • In cloth tab group, select collision tab
    • enable collisions
    • enable self collisions (recommended)
  • Setup collider
    • select object to collide with
    • select object > physics > enable collisions.
  • Bake and playback
    • Again, select the clothe object, then object > physics > cloth
    • Select the start and end frame. The 250 frames default will have you wait a couple of hours or more. Just pick the frame range you need, or less to experiment with parameters.
    • Press ALT+A to preview the animation for baking is complete.
    • Select Free bake to get rid of the current simulation.

Caveats

  1. The first time I run this simulation I had the character’s skin clipping (showing through) the clothe. Not just a bit, really showing through.
    => If we have a character animated with an armature, and we want to ‘let physics take care of clothing’ we probably don’t want the armature to be enabled for the cloth object. However, with a posed character, that typically means that, in the first frame, the clothe object won’t be fitting, it will overlap the character. The simulation starts with the posed character as is (not the rest position).
    I shifted all keyframes 10 frames forward, then key-framed the character back to rest position (at frame zero) and fitted the clothe correctly. That got rid of clipping (no need to worry about collision quality just yet).
  2. To prepare my models for Open GL rendering I usually split meshes by color (otherwise color bleeding occurs). Unfortunately this also means that each colored piece of clothe becomes a separate object in it’s own right.
    => A cloth object must be ‘in one piece’ for the simulation to work correctly. separating color groups should be done later (e.g. when exporting the animation)
  3. Initially my character didn’t have a trunk (anymore). Why include faces and vertices that won’t ever be displayed? Unfortunately this doesn’t work any good with clothes, because there’s nothing to collide with(!). The solution, obviously, is to have a complete body with a decent shape! In fact we don’t need to use the same object for simulating the clothe and rendering (let alone animation playback in a game).
  4. Physics ‘takes time to boot’. The piece of clothe we’re animating won’t look right at frame 1, it needs to ‘shape up’ following the rules of the simulation.
    If we’re already adding lead frames at the beginning of the simulation to match the shape of the cloth with a ‘rest position’ then this may leave enough lead time for the clothe to start fitting naturally before our animation really begins, otherwise there is a technique (see links above) which allows baking a pre-calculated deformation for the cloth, so we could replace the original mesh by a fitted version.
  5. Why does it look all rounded?
    If we’re using something like 1 blender unit = 1 metre (I do) then the default minimum distance for collision is really set too high. if we’re trying to fit a cloth nicely, it will look good in places, bad in others. In my case I had a sleeveless dress with just a couple of ribbons to fit at the top, so the ribbons looked all round and even bounced unrealistically against… …nothing. Reduce the collision distance as much as possible (there are two settings for this, one in the collision tab for the cloth, one in the collision tab for the character’s body.
  6. Clothe simulation per se isn’t overly time consuming (or the Blender routines are deadly optimized), collisions are. Animating just about 20 frames takes minutes, and my model isn’t really high poly. This isn’t a problem except tuning parameters is time consuming.

Simple experiments

While trying to get around the ’rounded sleeves bouncing in thin air’ issue, I tried a few things…

  • Combining with the sub-surface modifier. It may be fairly obvious that, if we want smooth wrinkles and other clothe-like appearance, we might want to refine the definition of the source mesh. Adding a subsurface modifier does the trick (without the need to generate hard to edit vertex data). The subsurface modifier should be at the top, keep the cloth modifier (gets generated automatically when we check the cloth setting) at the bottom.
    Make sure the resulting subsurface doesn’t overlap the character
  • Pinning. With most garments, not all parts are meant to flow freely. In fact, some parts are stretched. The cloth simulator gives a light ‘free flow effect’ that looks nice in parts and rather terrible in others. So Blender includes a ‘pinning feature’. The idea of pinning is that a selected group of vertices will be very ‘stiff’ compared to other, keeping the default shape of the mesh throughout the simulation (can save a lot of time trying to tune cloth parameters).
    With a posed character, we really can’t use pinning without  adding the armature modifier to the clothe. While this hurts my logic a little (I want the body to the move the clothe, not the armature…) it can be fairly easy to limit the armature’s influence (e.g. use envelopes or vertex groups) to avoid unwanted effects.1

I’m seeing weird glitches when I combine sub-surface, pinning and armature together (sometimes). Aside, my character’s dress seems to bounce up all too willingly. It’s almost amusing but not always desirable; but I’m really just learning how to tune this up and there’s quite a bunch of parameters to go through.

Conclusion

Animating clothes with bones is a total pain (I tried). In contrast, using the Blender cloth functionality definitely has a learning curve, but the results look very promising. I can see many useful applications, even with fairly stiff, low poly clothe models. With a reasonably high poly count it can get really pretty.

I’m expecting the baked cloth animations to go through when I export the animation (because the exporter collapses the modifier stack, and cloth is a modifier) so I may be able to get this working in my game. Frames may need to be offset to allow time for ‘shaping up’ the garment, then I would need to adapt my exporter.

Finally, many animations cycle through. The physics simulation doesn’t take this into account, so beginning and ending frames must be interpolated, either while exporting or later.