Stich,

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im edges.

I have two questions, as I am experiencing strange behavior.

In SOPs.

I have a an open cap cylinder NURBS surface, and trimmed NURB plane surface with inner circle forming the trim. I want to create a blend surface between the bottom base of the cylinder and trim edge of the plane. But StitchSOP refuses to pick up the trim edge, and as a result the blend surface is all wrong. Thus far I have been able to do nothing about.

Secong SOP problem. The Blend SOP. I have two NURB surfaces that share copositional edges. I have piped both of those surfaces into MergeSOP, then appended the Blend SOP to round the shared NURB edge. Obviously nothing is happening, after I select the surface anf RMB, or I would not be posting.

My friendly critic of those two problem is that I think the implementation is a little confusing to the user. Its not obvious and not intuitive. The blue directions on top are not helping, and after following them and RMB, nothing happens. So Houdini is not communicating to the user in this case.

The third confusing thing I am encounteting is particles leaking through a Collision POP after bouncing two or three times successfully.
I have the following POP network.

Source (which emmits 25 particles on impulse acivation on 1st frame only). So the total number of particles in this POPnet is 25. The source geometry is a NURB sphere and emmission happens in volume.

Appended to SourcePOP is ForcePOP (which is referencing Force object from Objects)

Appended tp ForcePOP is InstancePOP which is instancing a metaball geometry.

Appended to Instance and is the last POP is the CollisionPOP which is referencing a sculptued NURBS fruitbowl, just a single untrimmed NURB surface. The CollisionPOP has following attributes
Collision tab: Collision tolerance: .25, Ignotr Transform Object, and Geometry Moves with time are OFF.
Behavior tab: Behavior: Bounce On Collision, Hit Index: 0, Bounce Accuracy: 1, Gain Tangent: .25, Gain Normal: .4

OK, what happens, is that particles bounce couple of times correctly, then magically ‘leak’ through the surface, on third or fourth bounce.
Why is the leak happening and how to stop it?
I noticed that setting Collision Tolerance to higher number reduces the chances of partcle leaking, but does not stop it altogether. Some still leak. Also setting that number higher bring the collision point closer to the bounding volume face of the collision geometry.

Thank you.

David Rindner
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DavidRindner
I have a an open cap cylinder NURBS surface, and trimmed NURB plane surface with inner circle forming the trim. I want to create a blend surface between the bottom base of the cylinder and trim edge of the plane. But StitchSOP refuses to pick up the trim edge, and as a result the blend surface is all wrong. Thus far I have been able to do nothing about.

Use Bridge SOP instead.
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DavidRindner
im edges.
Why is the leak happening and how to stop it?

Hi David,

You can reduce the leakage by upping the oversampling of the particle system. This will take smaller time-steps and the velocities will be smaller and more accurate per cook, resulting in more accurate collision detection.

Find the Oversampling parameter in the POP SOP or in the POPsheet parameters.

Good luck,
J
jason iversen, houdini procedural pipeline supervisor @ weta digital
also, http://www.odforce.net [www.odforce.net]
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DavidRindner
Collision tab: Collision tolerance: .25, Ignotr Transform Object, and Geometry Moves with time are OFF.
Behavior tab: Behavior: Bounce On Collision, Hit Index: 0, Bounce Accuracy: 1, Gain Tangent: .25, Gain Normal: .4

Assuming that your bounce accuracy is actually 1, and that you didn't mean to type 10, that could be the source of the problem. It is possible for a particle to hit the collision surface early in the time increment for the frame, bounce, and at the end of the frame end up outside the surface.

Setting this number higher will allow more bounces to occur within a frame, and possibly stop those pesky leaky particles.

Failing that, Jason's suggesting of increasing the oversampling for the entire animation should work.
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Strange! BridgeSOP is not really doing it. Its not producing a surface.

Here the set up

I am starting with two trimmed NURBS surfaces (TrimSOP#1 and TrimSOP#2). With Footprint tag turned ON.
From top icon menu I select BridgeSOP.
I select first surface then with SHIFT key depressed I select second, then RMB to complete.
Houdini adds MergeSOP with both TrimSOPs as Merge members, and appends BridgeSOP to MergeSOP.
However no surface is actually created, and changing options in BridgeSOP parameters is not helping.
I am wondering where I am making a mistake.

Dave Rindner
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