Rendering translucent triangles in models animated by a vertex shader?

Question

Until now, I've been handling translucency by sorting my translucent triangles back to front. This works very well for quads, but I'd like to incorporate translucency in my models now.

I've thought of separating the translucent tris out and sorting them in the same way as my quads. Sorting by their centroids, then streaming the results into an IBO for just them each frame. But the number of triangles in a model, and the need to transform them on the CPU according to a table of bones, and blend shapes, and some other things in my vertex shader... This doesn't seem like a good solution in performance or sanity.

My models are about 4K tris each, with maybe 20 in a scene at worst, and I'd really like to lean into a simple cute style that relies on translucency, which doesn't have to be physically accurate, or draw objects behind 4 or more layers of translucency.

What technique might work well for my situation, in 2021? I'm using OpenGL 3.3 but I'll use another version if new features exist for this.

Answer 1

Afaik, there's no easy solution to what you want to do. However, there are some things that you can do:

• Don't sort triangles at all, and only sort individual draw calls. This is the easiest solution and is utilised by most games, if they use translucent/transparent objects in the first place. Of course, this might not have the best looks (though it works well with convex shapes), but it will have very good performance.
• Order-independet transparency: There are some pixel shader based techniques for rendering transparent/translucent objects without having to sort anything at all. These techniques usually are approximations (there are also some non-approximate algorithms) and tend work the best for things like smoke (where small errors aren't as noticable), or when not many transparent/translucent triangles overlap each other anyway.
• Compute Shaders: You can use a compute shader to transform your individual vertices according to your animation, and then use another compute shader to sort the triangles on the GPU. That is probably the most straight-forward improvement of what you'd otherwise do on the CPU, and there are many examples for sorting stuff on the GPU out there. But, if you've never worked with compute shaders before, it might be a bit hard to wrap your head around their strengths and limitations at first.
• Ray Tracing: This would probably be the most complex way to solve that problem, since you'll need specific hardware, generate corresponding data structures and a few new shaders, and on top of that, even with modern hardware, ray tracing is quite costly (though probably still faster than sorting triangles on the CPU each frame). But it also doesn't need your triangles to be sorted and will actually work perfectly well even if different translucent objects are intersecting/interleaving each other.