Funny Blog Post Title

Hey everyone

This blog post is going to be kind of all over the place. This week has been really weird, but weird good.

To show off the XML I developed and posted about last time I've upload a video of folding all four corners to the center (Verlet integration scheme). I was going to upload a fold called a pleat, where you fold it back and forth along parallel creases, but it does not do anything. For some reason the folds cancels each other out and nothing happens. These are some of the bugs I mentioned previously and still working on (see below).

Another development was the discovery of this paper: http://www.csc.calpoly.edu/~zwood/research/pubs/origamiCATA06.pdf .

It is very similar to my proposal. They talk a lot about how their cloth sim will work with the membrane (structural, shear and bend) forces and flexure forces(rotation). They also reference actual materials, but they don't go in depth (kind of like what I will do later). It is interesting to note that they most complicated thing that they were able to fold was the traditional dog model. The things that I am doing differently are the GPU implementation and the memory changing aspect of the paper.

One thing to note is that they reference implicit integration, but they say they don't use it. They use a scheme called Newmark integration. I had never heard of this, but the implementation was easy so I tried it. The integration depends on two values, beta and gamma, to balance out the damping of the integration. I spent a few hours playing around with values and regardless of what values I used for beta and gamma, my paper would always explode. It is possible that I never found that sweet spot, but it seems like Newmark integration is not for me. To me it seems that Newmark is a dampened Euler integration system, so to me it makes sense that it would explode.

So bugs have been cropping up everywhere, including problems with acos. Arccos only returns values from 0 to pi (which I tested). So I think I still need to find negative angles to correctly calculate values of the forces, although the direction of the fold maybe enough. But I am still playing with the implementation of the rotation spring. In the new paper mentioned above they seem to find the angle differently, so I may try their implementation. Another bug seems to be gravity. While gravity does what I want it to, as does the collision with the ground, it seems to have ramifications for the fold. If there is no gravity, the fold does not complete but actually swoops around in a weird manner. So I want to play with forces and try to find a good way to get things folding.

The last thing I started this week was implementing a memory model for the paper. This was my idea of simulating how paper is changed physically when you roll or crease it. I created a new type of bend spring class that holds data for a rotation spring. The rotation spring is placed perpendicular to the bend spring when the bend spring exceeds a certain stretch or compression. The rest angle of the rotation spring is also dependent on the stretch / compression. I tried doing it on a large system, but it is a delicate system and the system seemed to explode when the rotation springs are added. The problem might be stemming from the high spring constant on rotation springs. This means it is more of a balancing issue rather than an implementation problem. I'll post a video when I get it to a better state.

Another approach I took to the memory was simply relaxing the rest length of a bend spring. That certainly had an interesting effect, as if the paper was wet and was suddenly drying. It is a little hard to reproduce sometimes, but basically the paper stays curled and does not unfold because the bend springs are not as strong. It is an interesting approach and something I may consider going forward.