I’m wondering if I can do this with Stoke…
I have a Cloth simulation with lots of little strands blowing in the wind. I’d like to form a shape with the strands blowing in a custom direction, which wouldn’t be uniform. Kind of like they had been brushed into a shape.
Can I create this ‘Shape Force’ with Stoke and add it as a force for a Cloth Simulation?
Yes you could, but the method of defining the vectors in the field is up to you.
One thing you can do (which I have tested already) would be to add a Hair modifier to a custom mesh, comb it the way you want it. Then create a PRT Hair (from the Krakatoa toolset) from the Hair, create a Stoke Field Magma that samples the PRT Hair and uses the Tangent channel to define the Vector field to be used as Velocity. Then create a Stoke Field Force object from the Stoke Field Magma, and pick it as your SpaceWarp in the Cloth sim.
You can even go a step further and animate the Hair using a regular Wind SpaceWarp to produce a “constrained wind” where the effect of the regular Wind on the Hair is converted into the new Wind via the above process.
Alternatively, you could create a long Spline, add a Normalize Spline modifier to add a lot of knots, then collapse to EditableSpline. Clone to the side as often as you want. Attach all clones to a single ESpline. Go to Vertex Mode and use Soft Selection to tweak the “flow” of the splines and produce a custom field the way you want. Optionally, you can even do this in 3D by cloning the plane of splines in the 3rd dimension. Or clone the splines radially in 2D or 3D, and tweak them (just like you would do with the Hair above, minus the combing tools). Then make the PRT Hair from the ESpline -> Stoke Field Magma from Tangents -> Stoke Field Force to turn the Tangents of the samples along the splines to a Force field…
Or you could just model a custom mesh, then clone it and use Push to produce several layers (like a matryoshka doll), attach them together, then use Stoke Field Magma to sample the surface Normals and turn to a vector field.
In all these cases, some multiplier would be needed to tweak the scale of the field, as SpaceWarps are an acceleration field (the Force is applied over time to produce Velocity), while Stoke itself uses the fields directly as Velocity. So a Tangent field might require some scaling to produce the effect you are after.