Edge Artifacts in the Sim Routine

Simulation routines do hate edges. Of course this is crystal clear to you, but for those who want to tell their friends, this is why. You can see some example in the way I calculated velocities and accelerations in the gravity post above.

The velocity in frame 2 was derived from the positions in frame 1 and 3. But since there is nu frame 0, the velocity at frame 1 either cannot be determined or has to be derived in another way. Frame 1 is the edge and so is frame 60, the edge  width is 1 (frame).
The acceleration, the 2nd pass in the run, at frame 3 was derive from the velocities at frame 2 and 4. So, the acceleration at frame 2 became at question given the debate on the velocity in frame 1, and the acceleration at frame 1 became questionable even more. The same holds for frames 60 and 59, and the edge width has become 2 now.

Because I only needed one average value I decided to ignore the values in the edge, but for cloth sims this is not the best idea. You do want the edges of a dress or towel look good, do you? And the sim runs hundreds of times, as per frame, and steps per frame, and internal iterations per step. You can't afford to loose all that info. 

This is the main puzzle for simulations. Not the algorithm itself, but how to deal with the edges. how to keep the edge width limited, how to reduce the effects in the edge area, and how to prevent the edge effects from rippling into the main area. As I was loosing 1 frame at each pass, on both sides of my calculation. After 20 passes I would have lost 40 frames out of 60, and had to take the average of only 20 values. This would have raised my measurement error, made the result less accurate and less fit for decision making, to say the least.

The Poser Cloth Room sim has to face two kinds of edges. One: the boundaries of the cloth mesh. Two: the limits of the parameter values. They're different kinds of limits, but anyway.

So I ran the sims for gravity and air damping investigation. The cloth did not collide to anything but the ground in the last frame only, and as discussed above air effects were sort of ruled out. Initial density was 1, the lead sheet.
During the entire fall of the cloth, the edges showed serious effects (all renders will follow in the next posts). At the touch on the ground, a few of them faded out but most others remained. Essentially, the effect was on the front edge only. About.

But, when I did the air damping sims at low cloth densities (0.01 and 0.0001 instead of 1.0) those edge effects were completely absent, at any frame. So I started in alter the density value from its upper limit (1.0) down. At 0.9 I found a reduced distortion, on 0.8 I found a further reduced distortion but shifted to the side edge. At 0.7 the further reduced effect had shifted to the back edge, and at 0.5 the effect was almost gone at all.

Hence, my impression is that the Cloth Room sim is not only sensitive at the cloth boundaries, but might be sensitive to the limits of the parameters as well. And why did the effect remain after dropping on the ground, I had expected the cloth to flatten out.

At second thought, the fold, sheer and stretch parameters were still at their very max. The cloth stiffness might have prevented the final settlement. So, I reduced those parameters as well, from 1000 to 10 while keeping density at 1. This made the effects disappear indeed after the ground impact, at max cloth density. They did not disappear that much in the earlier frames, while falling down.

All effects gradually grow from the edges in, from about the start till ground impact, and tend to worsen over time.

So, what can be inferred form this?
To me it means that extremes have to be avoided in parameter values, I'll do with the lower half of the allowed ranges. It also means that some artefacts which are seen in my results and others might originate from these edeg anomalities. In a still, one has the option to make them fade out by just adding a few frames after the final pose is reached. I don't know. It also implies that density might be the final dial to iron things out a bit. Might. Don't know for sure. Further investigation is required. Will do, in due time.

To me this also means that experiments with low resolution meshes, and parameters at their limits, might produce anomaly-driven results only. They may be of limited value for our further understanding and solution of the issues that were flagged. Any extrapolation should be made with great caution. Research at the vertex level might reveal interesting results, but might turn out to be th wrong way home as well. There is a health warning in there. 

Time for pictures, and time to stop. From now, I will focus on the issues as in the title of this thread, and just try to shine some light into the Cloth Room in general.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though