blag/content/posts/2021-07-27-procedural-meshes.org

---
title: "(post on procedural meshes needs a title)"
author: Chris Hodapp
date: "2021-07-27"
tags:
- procedural graphics
draft: true
---

(TODO: a note to me, reading later: you don't need to give your entire
life story here.)

(TODO: pictures will make this post make a *lot* more sense, and it
may need a lot of them)

Context Free is one of my favorite projects since I discovered it
about 2010.  It's one I've written about before (TODO: link to my
posts), played around in (TODO: link to images), presented on, as well
as re-implemented myself in different ways (see: [[https://github.com/hodapp87/contextual][Contextual]]).  That is
sometimes because I wanted to do something Context Free couldn't, such
as make it realtime and interactive, and sometimes because
implementing its system of recursive grammars and replacement rules
can be an excellent way to learn things in a new language.  (I think
it's similar to [[https://en.wikipedia.org/wiki/L-system][L-systems]], but I haven't yet learned those very well.)

I've also played around in 3D graphics, particularly raytracing, since
about 1999 in PolyRay and POV-Ray.  POV-Ray is probably what led me to
learn about things like implicit surfaces, parametric surfaces, and
procedural geometry - its scene language is full of constructs for
that.  Naturally, this led me to wonder how I might extend Context
Free's model to work more generally with 3D geometry, and let me use
it to produce procedural geometry.

[[http://structuresynth.sourceforge.net/index.php][Structure Synth]] of course already exists, and is a straightforward
generalization of Context Free's model to 3D (thank you to Mikael
Hvidtfeldt Christensen's blog [[http://blog.hvidtfeldts.net/][Syntopia]], another of my favorite things
ever, for introducing me to it awhile ago).  See also [[https://kronpano.github.io/BrowserSynth/][BrowserSynth]].
However, at some point I realized they weren't exactly what I wanted.
Structure Synth lets you combine together 3D primitives to build up a
more complex scene - but doesn't try to properly handle any sort of
*joining* of these primitives in a way that respects many of the
'rules' of geometry that are necessary for a lot of tools, like having
a well-defined inside/outside, not being self-intersecting, being
manifold, and so forth.

Tools like [[https://openscad.org/][OpenSCAD]], based on [[https://www.cgal.org/][CGAL]], handle the details of this, and I
suspect that [[https://www.opencascade.com/][Open CASCADE]] (thus [[https://www.freecadweb.org/][FreeCAD]]) also does.  In CAD work, it's
crucial.  I experimented with similar recursive systems with some of
these, but I quickly ran into a problem: they were made for actual
practical applications in CAD, not so much for my generative art, and
they scaled quite poorly with the sort of recursion I was asking for.

Implicit surfaces (or one of the many
equivalent-except-for-when-it's-not names for this, e.g. F-Reps or
distance bounds or SDFs or isosurfaces) handle almost all of this
well! They express CSG (TODO: link to CSG) operations, they can be
rendered directly on the GPU via shaders, operations like blending
shapes or twisting them are easy... for more on this, see [[http://blog.hvidtfeldts.net/][Syntopia]]
again, or nearly anything by [[https://iquilezles.org/][Inigo Quilez]], or look up raymarching and
sphere tracing, or see [[https://ntopology.com/][nTopology]], or Matt Keeter's work with [[https://www.libfive.com/][libfive]]
and [[https://www.mattkeeter.com/research/mpr/][MPR]].  They're pure magic, they're wonderfully elegant, and I'll
probably have many other posts on them.

However, there is one big issue: turning implicit surfaces to good
meshes for rendering /is a huge pain/, and while many renderers can
handle implicit surfaces directly, Blender's renderers cannot. I will
have other posts going into more detail on this subject, but for now,
take it on faith.  This is why I did not try to use implicit surfaces
for this project.  (TODO: Make those posts.)

With these limitations in mind, around 2018 June I had started jotting
some ideas down.  The gist is that I wanted to create
"correct-by-construction" meshes from these recursive grammars.  By
that, I meant: incrementally producing the desired geometry as a mesh,
triangle-by-triangle, in such a way that guaranteed that the resultant
mesh had the desired detail level, was a manifold surface, and that it
was otherwise a well-behaved mesh (e.g. no degenerate triangles, no
self-intersection, no high-degree vertices, no triangles of extreme
angles) - rather than attempting to patch up the mesh after its
creation, or subdividing it to the necessary detail level.  For
something similar to what I mean (though I didn't have this in mind at
the start), consider the [[https://en.wikipedia.org/wiki/Marching_squares][marching squares]] algorithm, which is
guaranteed to produce closed, manifold meshes.

(TODO: Illustrate this somehow)

The form it took in my notes was in sort of "growing" or "extruding" a
mesh per these recursive rules, building in these guarantees (some of
them at least) by way of inductive steps.

My meandering path to implementing it went something like this:

- Write some very ad-hoc Python to generate a mesh of a parametric
  conversion of my annoying spiral isosurface from 2005 by breaking it
  into planar "slices" or "frames", which move along the geometry and
  then are connected together at corresponding vertices.  (TODO: Add
  link to the automata_scratch repo, whatever it's renamed to)
- Explore [[https://github.com/thi-ng/geom][thi.ng/geom]] and pretty quickly give up - but in the process,
  discover [[https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.42.8103][Parallel Transport Approach to Curve Framing]].
- Implement that paper in Python, reusing the basic model from my
  prior code. (See [[https://github.com/Hodapp87/parallel_transport][parallel_transport]])
- Again continue with this model, allowing more arbitrary operations
  than parallel frame transport, eventually integrating most of what I
  wanted with the recursive grammars.  (See
  [[https://github.com/Hodapp87/automata_scratch/tree/master/python_extrude_meshgen][automata_scratch/python_extrude_meshgen]])
- Keep running into limitations in python_extrude_meshgen, and start
  [[https://github.com/Hodapp87/prosha][Prosha]] in Rust - partly as a redesign/rewrite to avoid these
  limitations, and partly because I just wanted to learn Rust.
- Realize that Rust is the wrong tool for the job, and rewrite *again*
  in Python but with a rather different design and mindset.

(this is, of course, ignoring many other tangents with things like
shaders)

(TODO: Maybe split these off into sections for each one?)

I put some serious effort into [[https://github.com/Hodapp87/prosha][Prosha]] and was conflicted on shelving
the project, but the issues didn't look easily solvable.  Part of
those issues were implementation issues with Rust - not that Rust
could have really done anything "better" here, but that it just wasn't
the right tool for what I was doing.  In short, I had spent a lot of
time and effort trying to badly and unintentionally implement a Lisp
inside of Rust instead of just picking a Lispier language, or perhaps
using an embeddable Rust-based scripting language like [[https://github.com/koto-lang/koto][Koto]] or [[https://github.com/rhaiscript/rhai][Rhai]]. I
had ignored that many things that functional programming left me very
accustomed to - like first-class functions and closures - were
dependent on garbage collection.  When I realized this and did a big
refactor to remove this entire layer of complexity, I was left with
very little "core" code - just a handful of library functions, and the
actual recursive rules for the geometry I was trying to generate.
That's good and bad: things were much simpler and vastly faster, but
also, it felt like I had wasted quite a lot of time and effort.  I
have some more detailed notes on this in the Prosha repository.

Part of the issues also weren't Rust implementation issues - they were
deeper issues with my original "correct-by-construction" mesh idea
being half-broken.  It half-worked: I was able to produce closed,
manifold meshes this way, and it could be tedious, but not *that*
difficult.  However, all of my attempts to also produce "good" meshes
this way failed miserably.

(TODO: Can I find examples of this?)

The crux is that the recursive rules I used for generating geometry
(inspired heavily by those in Context Free) were inherently based
around discrete steps, generating discrete entities, like vertices,
edges, and face, and it made no sense to "partially" apply a rule,
especially if that rule involved some kind of branching - but I kept
trying to treat it as something continuous for the sake of being able
to "refine" the mesh to as fine of detail as I wanted.  Further, I was
almost never consistent with the nature of this continuity: sometimes
I wanted to treat it like a parametric curve (one continuous
parameter), sometimes I wanted to treat it like a parametric surface
(two continuous parameters), sometimes I wanted to treat it like an
implicit surface (with... theoretically two continuous parameters,
just not explicit ones?).  It was a mess, and it's part of why my
Prosha repository is a graveyard of branches.

The recursive rules were still excellent at expressing arbitrarily
complex, branching geometry - and I really wanted to keep this basic
model around somehow.  After some reflection, I believed that the only
way to do this was to completely separate the process of meshing
(refinement, subdivision, facetization...) from the recursive rules.

This would have been obvious if I read the guides from [[https://graphics.pixar.com/opensubdiv/overview.html][OpenSubdiv]]
instead of reimplementing it badly.  Their [[https://graphics.pixar.com/opensubdiv/docs/subdivision_surfaces.html][subdivision surface]]
documentation covers a lot, but I found it incredibly clear and
readable.  Once I understood how OpenSubdiv was meant to be used, it
made a lot of sense: I shouldn't be trying to generate the "final"
mesh, I should be generating a mesh as the /control cage/, which
guides the final mesh.  Further, I didn't even need to bother with
OpenSubdiv's C++ API, I just needed to get the geometry into Blender,
and Blender would handle the subdivision on-demand via OpenSubdiv.

One minor issue is that this control cage isn't just a triangle mesh,
but a triangle mesh plus edge creases.  I needed a way to get this
data into Blender.  However, the only format Blender can read edge
creases from is [[http://www.alembic.io/][Alembic]].  Annoyingly, its [[http://docs.alembic.io/reference/index.html#alembic-intro][documentation]] is almost
completely nonexistent, the [[https://alembic.github.io/cask/][Cask]] bindings still have spotty Python 3.x
support, and when I tried to run their example code to produce some
files, and Blender was crashing when importing them.... and this is
all a yak to shave another day.  I instead generated the mesh data
directly in Blender (via its Python interpreter), added it to the
scene, and then set its creases via its Python API.

After the aforementioned refactor in Prosha, I was able to quickly
translate the Rust code for most of my examples into Python code with
the help of some library code I'd accumulated from the past projects.
Debugging this mostly inside Blender also made the process vastly
faster.  Further, because I was letting Blender handle all of the
heavy lifting with mesh processing (and it in turn was using things
like OpenSubdiv), the extra overhead of Python compared to Rust didn't
matter - I was handling so much less data when only producing the
control cage, not the full mesh.

I'm still a little stuck at how to build higher 'geometric'
abstractions here and compose them.  I have felt like most of the
model couples me tightly to low-level mesh constructs - while Context
Free and Structure Synth definitely don't have this problem.  This is
particularly annoying because a lot of the power of these recursive
grammars comes from their ability to be abstracted away and composed.

(TODO: Show some examples)