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Grain Stepping / Obstacle Subframe Interpolation Issues


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I posted this a few days ago on the primary Houdini forum, but haven't gotten much traction with it, as it's a pretty strange bug. 


In short: I've animated the movement of a box, with grain pouring out of the box. Unfortunately, the grain doesn't leave the box cleanly: as it pours out, it creates a stair-step pattern on the bottom edge - one stairstep per frame - as if the box collision geometry isn't interpolating correctly during the subframe timeslot.


Some notes:


- Collision geometry is a single piece of geo that is translated inside Houdini via keyframes rather than an Alembic Cache or geometry sequence. 

- I added a time-blend node as recommended by jeff in the Houdini forum.  

- The "Re-evaluate SOPs to Interpolate Geometry" box in my static object node in my DOP network is checked.

- Increased simulation substeps to 30 (The grain-stepping effect seems to generate one step per frame, so increasing the simulation substeps doesn't appear to have any effect. ) 

Despite all of that, the stair-stepping still occurs. Any thoughts would be appreciated! 






Edited by LukeLetellier
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Time Blend SOP won't do anything for you since your geometry is static. You are translating at OBJ level and you have subframe data. Since it's only tranforming as well, you can un-tick "Use Deforming Geometry" on your Static Object DOP - then it only have to calculate the collision geo once.


Substeps for the grains only affect the grains themselves. Which is not the issue here, so you could bump that down to 1 again. The issue is your collision object that you need substeps for in the simulation. So instead we increase the DOP Network substeps, at 3 I get this: 



Seems to work a lot better, a lot faster than what you had. I only did what I already described, but heres the hip anyway. Hope it helps!


Edited by Skybar
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Thank you so much for explaining the reasons behind your decisions - as that helps me understand a lot of what's going on behind the scenes - and it all makes sense, and obvious enough that I should have figured it out myself! After making that adjustment on this side of things, it works beautifully. 


Thanks so much for your help!


-- Luke

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