a real-time 3D rendering engine in JavaScript

[Dave Long]

Sweet! just when I wanted a torus illustration for a Betti number 
calculator.
http://en.literateprograms.org/Betti_numbers_%28Python%29

> It's not even doing backface removal because I'm too dumb to figure 
> out how to do it safely with perspective;

Looking at the sign of the Z-component of the normal after perspective 
(effectively 2D, ready for orthographic projection onto screen pixels) 
ought to do it -- are there just some numerical issues with what you 
tried?  (the "topological" way to determine orientations seems to be to 
take determinants, but they really eat up the flops, so all this 
messing about with normals can be viewed as a design-time 
partial-evaluation to reduce calculation)

A backwards-facing triangle in 3D is also a backwards-wound triangle in 
2D.  If it's possible to change the winding rule for canvas.fill, that 
might be a cheaper method than doing the calculation in JavaScript.

> Maybe there's some way to do the Z-sort without invoking a JavaScript 
> function for every pair of elements to be compared;

It should be possible to left-pad the elements with suitable elements 
so that the string-based .sort() puts everything in Z-order -- but I 
don't think the sort's expensive enough to bother.

> There's, like, no UI. There are some nifty solid-modeling things in 
> the code that aren't obvious from the UI.
> Also, maybe it would be nice to be able to pull out that logic for use 
> in some other kind of mesh-generation code, like something that graphs 
> a height field or a parametric function.

If you pull that logic out, I'll try to do a 3D version of the 
combinator turtle language:
http://en.literateprograms.org/Turtle_graphics_%28JavaScript%29

-Dave