WIP#30: Pushing and Sliding (Patreon)

Now, this one's a lot of fun! I've been working on some more advanced and versatile methods of getting realistic skin and soft body deformation quickly and easily, and I'm very happy with these results! There are also a couple of exciting geometry nodes graphs involved as well, which I'll do my best to explain!

So first up, we're getting some nice squishing and bulging, using this rather noodly node graph: 

What's going on here is more of a principle than a concrete technique like what we'll cover in a moment, essentially for each point on dash's skin, it's measuring how close it is to soarin's body, and if it's very close we're pushing in sharply, and then at medium distances it's bulging out with a nice smooth falloff. That's a nice way to sort of fake the conservation of mass and change in shape of a soft body as pressure is applied. The key here is that "geometry proximity" node at the beginning and the "set position" nodes at the end - everything else is just math to tweak the shape. What's nice about this principle is that is sort of gives a kind of collision "fudge factor" where her skin automatically presses in where he touches her.

Additionally, we also have some more conventional techniques applied that I've talked about in some detail before that is, armature animation and shape key tweaking. In this situation though, I'm not using lattices since the geometry nodes essentially achieve the same effect in a much more general way. The armature and shape key animation is also very simple - there is only one bone being animated for each cheek, and there's just one shape key to cover the special shape when he really grips her! that's it!

So now, what's going on here?

This... is a lot more advanced. What I've set up is a system that attempts to model the various forces of friction, elasticity, pressure and suction at play with her vaginal walls - without actually doing any kind of actual simulation. The first relationship here is shrinkwrap: it's quite simple to create a dummy mesh that shrinkwraps to his shaft and so deforms around it goes in and out, including releasing it entirely and retaining its normal shape when not penetrated - however, this is quite a crude model of how the inner walls of a vagina can actually move. Especially, the particularly hot phenomena that we want to exaggerate: the "grip" often associated with really good pussy.

So, what's going on here, under the hood? If you look closely, you can actually see that her inner walls are "rolling" out a little bit as he slowly extracts himself. This technique makes the most sense to me if I describe it as "fold and slide", and now we have to use two intermediate meshes. First, the folding part is relatively simple. we can use a "pull" shape key on the shrinkwrap mesh so that part of its inner surface can be dragged along with the glans.

The real key is that instead of just directly follwing this shrinkwrap mesh with a surface deform, the skin needs to be able to "slide" along the complex, folded and changing surface created by the shrinkwrap object. This is a deformation mode not provided by any of blender's inbuilt modifiers, but it should be possible, in theory, using UV coordinates! This is actually a common technique in retopology, where you can define the vertices of an object you're creating not in the regular world space, but in the UV space of another object. This way, all the points you create are guaranteed to be on the surface of the target object, and you get your UV map kinda for free, since that's how you made the mesh! Of course, if we can create one object on the surface of another by matching up their UV coordinates, we can also slide around on that suface just the same way! Here's what I came up with:

The key here is we can read the UV coordinates of our target object at the bottom there, and then use a "transfer attribute" node to get those coordinates to make sense to our geometry, but only if we first transform it to its own UV coordinates as well, so they overlap. The stuff on the left side is the control math required to actually slide the thing around on the surface - and I'm using a cheeky little sigmoid function to avoid overlapping vertices and generally get a more well behaved effect. Here's a demonstration of the effect of this node graph in isolation, watch the black line:

And that's it! the last step is that surface deform modifier to the actual body geometry that we render, so we can control the impact of this deformer, limiting it to just the the vaginal walls and not the rest of her body.

The really exciting part about all of this is that having worked this technique out, it becomes essentially just a ready made and versatile deformer that can be applied to all sorts of situations very easily - there's also a lot of mileage in a UV surface deformer like this in transferring topology, textures and other information between potentially very different meshes, and I'm sure there are many more uses besides! Do let me know if any of y'all would be interested in downloading these node graphs, I'd be happy to make them available!

Also, as a much less technical addendum, I've developed a look for scenes with RD set on a cloud! What I've done is actually sculpt a kinda flowy looking cloud mesh, take a look!

And here's how it looks with volume shading and sky illumination:

It's an interesting artistic challenge, since in the show for pegasi clouds are these tangible, physical things, so in a more naturalistic rendering style one has to find the way to re-introduce that cartoonish look to make it work. Let me know what y'all think!