Help on configuring Obi Rope to simulate sewing.

[ploom]

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.

I appreciate the references, I will take a close look at those.

As for the project, I would like to clarify that the device in my suture simulator is not the classic hook-shaped needle and suture you see in simulators (ie. Lapsim from Surgical Science). The needle will always be in a fixed position on the device, which has a pinned particle of the suture attached to it. This particle does not need to move or be removed from the needle at any point.

That being said, unless I misunderstood or am overlooking something down the line, the generated tether constraints are working nicely in terms of minimizing stretching to a negligible amount - in addition to distance constraints being set at 30. Not sure if having both compound the reduced stretching or if I'm wasting resources here.

Another issue I have found is fast-moving colliders seem to phase through the suture with no collisions being detected. If I move slowly, however, I get the results that are expected. Resolution is set at max and collision constraint iterations is at 30. It seems making the colliders larger sort of works, I am just curious if there is another alternative that can allow me to use smaller colliders on an object that will be manipulating the suture.

Thanks for the all the help so far.