28-10-2017, 10:31 AM
(This post was last modified: 28-10-2017, 10:35 AM by josemendez.)
(27-10-2017, 08:09 PM)ploom Wrote: Thanks for the insight! Coincidentally, I am working on a surgical suture simulator, and for my purposes the generated tether constraints are sufficient. Stiffness seems to be the the next hurdle at the moment.
Would you be able to recommend any additional iterative options I can work with through Obi, or perhaps an external resource that would be able to function in a scientific scenario such as surgical suturing? I appreciate any guidance you can send my way.
Tether constraints work based on the assumption that there's a fixed "reference point" in the rope, such that you can measure stretch relative to it. In Obi, these reference points are fixed particles, that are not simulated.
So thethers stablish a equation that relate free (simulated) particles to their closest fixed particles: "the free particle must not get further away from the fixed one than the lenght of rope between them", if that makes sense.
Unfortunately this requires that at least 1 particle in your rope is going to be fixed at all times, and that stretching in the rope always occurs in the direction opposite to that particle. In a surgical suture simulator this is not always true, so tethers may be of little help to you. The only thing left in your case is to increase the amount of distance constraint iterations, maybe crank up SOR factor to 1.5 (this is only used in emergency situations, to boost up the power of each iteration) and see if it's enough. If your thread bends/coils too easily, also increase bend constraint iterations.
Also, be aware that Obi does not consider twist/shear forces in the rope. Only stretch and bend are considered. This might be a hindrance for your purposes.
I can reccomend some research papers that have dwelved into this particular problem. However there's no commercial physics engine capable of simulating surgical thread at the required level of detail in realtime that I'm aware of.
This is probably the most appealing research work on the subject:
http://robotics.stanford.edu/~latombe/pa.../paper.pdf
They use a method known as alternating follow-the-leader (FTL) which guarantees zero stretch for simple cases. They also use capsule based collision detection for more robust knot making. They do not, however, calculate twist/shear either. Their work can be coupled with http://www.cg.informatik.uni-mainz.de/fi...t-Rods.pdf in case you absolutely need these.