Uniform edge lengths are a reasonable proxy for mesh quality, since equilateral triangles have good angles. But if you have more specific criteria, you can usually do better. "Hodge-Optimized Triangulations" provides a nice discussion of this perspective: geometry.caltech.edu/pubs/MMdGD11.p…
A popular approach in graphics is something like Botsch & Kobbelt, "Remeshing for Multiresolution Modeling" (Sec. 4): graphics.rwth-aachen.de/media/papers/r…
Basic idea: iteratively move vertices to equalize areas, split long edges, collapse short edges, & flip edges to improve vertex valence.
In this class of algorithms, I really like Chen & Holst, "Efficient mesh optimization schemes based on Optimal Delaunay Triangulations": math.uci.edu/~chenlong/Pape…
Here there's a clear notion of optimality, & an efficient preconditioner. For surfaces: restrict to tangential motions
Here the idea is to parameterize the mesh & nicely sample according to varying density in the plane. I don't sense this idea is used too much these days, due to the need to parameterize.
Though on second thought, there are methods that use sophisticated field-aligned parameterization to get very high-quality almost-regular meshes with few irregular vertices, such as Nieser et al, "Hexagonal Global Parameterization of Arbitrary Surfaces": mi.fu-berlin.de/en/math/groups…
Combines the strengths of field-aligned & local iterative methods to get extreme performance/scalability, albeit with more irregular vertices than, say, Nieser et al
Finally, a fun example where meshes with unit edge lengths show up is in Isenberg et al, "Connectivity Shapes": cs.unc.edu/~isenburg/rese…
The idea: if you have perfect unit edge lengths, you can throw away vertex positions and recover the geometry *purely* from the connectivity!
In simulation, the award-winning "El Topo" package of Brochu & Bridson uses near-unit edge lengths to provide high-quality free surface tracking: github.com/tysonbrochu/el…
(By the way, "award winning" in these posts refers to the SGP software award @GeometryProcess!)
Bruno will be able to give better details about the algorithms used, and interesting applications like optimal transport that I didn't mention yet.
It's also important to mention that uniform edge lengths provide more detail than necessary in large regions with low curvature.
Other methods adapt to local curvature/feature size, like Dunyach et al, "Adaptive Remeshing for Real-Time Mesh Deformation": hal.archives-ouvertes.fr/hal-01295339/d…
Likewise, the Holst & Chen paper mentioned earlier allows spatial adaptivity, by providing a density function: math.uci.edu/~chenlong/Pape…
There are sure to be countless references I'm missing both to algorithms & applications—but that should provide a few useful pointers!
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I realized this can be coded up in < 5 minutes in the browser via @UsePenrose, and gave it a try. Pretty darn close! 🧵
[2/n] To be clear, this 🧵 isn't about finding a better packing—or even finding it faster. Wizards like @KangarooPhysics surely have better tricks up their sleeves 🪄
Instead, it's about an awesome *tool* for quickly whipping up constraint-based graphics: penrose.cs.cmu.edu
[3/n] The "17 squares" problem provides a great demonstration of how Penrose works.
In fact, if you want to use this thread as a mini-tutorial, you can try it out at penrose.cs.cmu.edu/try/
Let's find out by trying to re-create a bunch of classic graphics images using #dalle2! A thread. 🧵 [1/n]
Left: original image
Right: DALL-E 2 image
In each case I tried many times & show the best result. Full query string given.
Let's start with a real classic: a chrome and glass ball over a checkerboard, from Turner Whitted's 1980 paper, "An Improved Illumination Model for Shaded Display": cs.drexel.edu/~david/Classes…
Pretty good job on reflection/refraction! Was hard to get the colors I wanted. [2/n]
Two more classics for the price of one: the Stanford Bunny in a Cornell Box.
Models in engineering & science have *way* more complexity in geometry/materials than what conventional solvers can handle.
But imagine if simulation was like Monte Carlo rendering: just load up a complex model and hit "go"; don't worry about meshing, basis functions, etc. [1/n]
Here's one fun example: heat radiating off of infinitely many aperiodically-arranged black body emitters, each with super-detailed geometry, and super-detailed material coefficients.
From this view alone, the *boundary* meshes have ~600M vertices. Try doing that with FEM! [3/n]
Here's another fun question: given two loops around an (infinite) pole, can you remove one loop without breaking it?
Amazingly enough... yes!
This is a surprising example of what's called an "ambient isotopy": a continuous deformation of space taking one shape to another. 1/n
People have made some great drawings of this transformation over the years (sometimes using a loop rather than an infinite pole—which is equivalent), but it can still be hard to interpolate between individual drawings in your head.
(...does a movie make it any clearer?!) 2/n
What's also fun about the motion in the movie above is that it was created without* human input: instead, the computer tries to nudge the shape around so that every point is as far as possible from itself. You can read all about it in this thread:
Disclaimer: this is a simplified exercise for a final exam and should not be used for serious scientific work! It omits important forces and uses nonphysical constants.
What's the nicest way to draw a shape with many "holes"?
We can use the principle of repulsion to explore this question: each point of the shape behaves like a charged particle, trying to repel all others. Surface tension prevents everything from shooting off to infinity. 1/n
For millennia people have been drawn to the question: what are the "nicest" possible shapes that exist?
This is really a basic question about nature: these shapes exist outside space and time; the same shapes can be discovered by civilizations anywhere in the universe. 2/n
"Nicest" could mean the most symmetric—for instance, the ancient Greeks discovered there were five so-called Platonic solids where every face and every vertex looks the same: the tetrahedron, cube, octahedron, dodecahedron, and icosahedron. 3/n