Cross-sections

Gestalt

Does anyone know how I could go about baking cross-sections (could just be alpha masks)? I was inspired by how SotC did 'volumetric' fur, and I'd like to try making an 8 layer (16 tri) tileable patch based off of cross-sections taken from a high-poly patch of grass.

Neox

A greyscale heightmap should do the job, your shader could clamp the "crosssection" later on based on the shell distance.

Gestalt

I don't mean a height-map. Technically the blades would be able to 'cross over' eachother which a height map wouldn't account for. I might see about using Zbrush to cut slices of the high-poly patch and render an alpha projected from the top of each slice.

Neox

You can just offset the uvs of each shell, maybe even with a direction map, this would i'm pretty sure for SotC they didn't do fur with direction, but tweaked the direction on the characters once they had all their maps

Vailias

Couldn't you use a standard baking process with a very small ray cast distance?

Gestalt

Yeah that would work. The only problem I can think of would be that it would bake everything within the distance and not just a cutout. Moving the plane for each cut and leaving the shortened ray distance should make it work I think.

Vailias

Well why not just use multiple planes to start with? Lay out the uv islands so they don't touch. And bake at a high enough res to get the sizes you need.

Set your ray distance to 0 if you really want 2d cross sections.

Gestalt

I put something together in UDK to test out the idea. I didn't take it very far, do anything fancy with the material or anything, and I'm not sure if it's worth pursuing since glancing angle don't look too great, but this is what I have.

16 tris, 1 8bit Gray-scale 512x1024 texture, no shadowing; uses diffuse, spec, and opacity mask; there are 4 patches of grass

Eric Chadwick

Not bad.

I did a lot of work with volume textures a couple years ago. In Max you can generate these easily from geometry, using an Ortho camera pointing down, animating the near/far clips, and rendeirng a sequence. Or you can use a Boolean, moving two "cut" operands down to reveal the slices you want.

More techniques.
http://wiki.polycount.com/HairTechnique

Gestalt

I used planes with a ray distance to capture the detail, but I think that I actually set up the distance incorrectly, so fixing that might help a decent amount. I use Maya normally although I used Blender for this, so I think I'll have to keep working with the ray distance method.

I'll probably keep working on this afterall, but this time I'll include fins, add some layers, adjust the material to look more like grass, vert paint so that I can add color variation to the tops of the grass, darken the bottom layers, and add wind to move the top layers slightly.

Eric Chadwick

Ahhh, now I remembered, the problem with clipping planes is you need to cap off the ends of the cut geometry, or you get holes. Better to use Boolean or something similar like animated Slice in Editable Poly combined with Cap.