Metaballs and Marching Squares | Hacker News Reader

Metaballs and Marching Squares

jamie-wong.com

109 pointszodiac19 comments

For a nice description of doing this in 3D: http://paulbourke.net/geometry/polygonise/

I used that in the 90s to do a Lisp implementation: http://people.cs.uchicago.edu/~wiseman/3D/marching-cubes/

Something really cool about this approach is the marching squares algorithm is highly parallelizable which would make this a good candidate for GPU acceleration provided that there is enough memory.

I implemented marching cubes in the 00's to extract isosurface meshes from PET scans. As far as I can remember I had it operating real-time on regular CPUs. The volumes were in the order of 512^3 if my memory serves me well. I guess GPUs could help for very large volumes though.

You wouldn't need marching cubes to operate real time - it runs once, generates an iso-surface mesh which can be efficiently rendered in hardware.

Edit: my thesis advisor was the first to prove that there were exactly 15 distinct configurations of voxels in MC, which gives you the ability to perform constant time lookup for the (configuration, rotation) for vertices on the edge of any particular voxel. http://graphics.stanford.edu/courses/cs164-10-spring/Handout...

The thing is there was slider to move the isosurface boundary.

What he's referring to is a transfer function.

The marching cubes are not just evaluated to find the surface where Voxel(xyz)=value

But rather F(Voxel(xyz))=value

What gets interesting is that both pre-interpolated, and post-interpolated versions are interesting.

This is the basis of the Voxel Farm engine, which Miguel talks about in this early blog post: http://procworld.blogspot.com/2010/11/opencl-first-iteration...

I have used alpha threshold trick to achieve metaballs for my fluid simulation[1][2]. I wonder how efficient Marching Squares method is.

[1]https://jsexperiments.herokuapp.com/sph/ [2]https://github.com/asadlionpk/SPHjs

It's pretty efficient, O(N), where N is the number of cells (partially) covered by something (which means some pruning of the cells that need to be processed is desirable otherwise you'd have to evaluate all of them for quadratic complexity). The algorithm is also embarassingly parallel, every cell can be processed independently so it scales perfectly with the number of processors available (if you forget about contention effects).

So metaballs aren't distinct mathematical objects that you can model on their own (like a fractal or a cylinder)- rather, they're artefacts that are created when sampling in certain ways circles/spheres intersecting?

They don't inherently have to be sampled.

Just like you can define the perimeter of a circle given constants (x_0, y_0, r):

(x-x_0)^2 + (y-y_0)^2 = r^2

you can define the perimeter a metaball on a list of (x_i, y_i, r_i) as described in the post.

No.

Metaballs are a thing, and marching cubes is one way of implementing them. Another approach is ray-casting.

Metaballs are a family of implicit equations that are visually interesting and easy to set up. They all come in the form of a summation of some function of distance from a control point over a set of control points. The appearance of spheres/circles intersecting is a consequence of the popular falloff-type functions.

You can think of metaballs as a bunch of glowy points. The implicit surface that is shown is all of the points where the glows add up to some target value.

Blumenthal, "Introduction to Implicit Surfaces".

Possibly quite dated, though.

Quick Desmos graph, if you want to play around with some of the math with less of the programming:

https://www.desmos.com/calculator/mvnrzlrhrz

I remember when almost every demo had metaballs in them. Now that's been replaced with distance functions/raymarching, which can produce far more interesting scenes.

I did some 3d metablobs using raymarching in a 4k exe.

https://www.youtube.com/watch?v=sBcwQCBpEiE

Blender has 3d metaballs, if you want to play around with them without too much fuss.

You don't care, but I got to shake hands with Bill Lorensen (marching cubes), Alvy Ray Smith (cofound Pixar, co-invented the alpha channel), Jim Blinn (need I say more?), Tony DeRose (really improved Subdivision Surfaces), Anthony Apodaca (Director of Graphics Research and Development at Pixar Animation Studios, co-created RISpec)...

I'm like a Computer Graphics groupie.

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