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Houdini Pyro Collision


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Hi there everyone.

This must be a common question but I just cannot seem to get the Pyro Collisions to work correctly there is defiantly something I am missing and I would greatly appreciate some assistance.

So this is the issue I am having. Below you can see the Multi Field going through the Collision Field where there are no holes.

This is the Multi Field.

Here is the Collision Field on its own so you can see its a solid Piece.

This is the Temperature Field.

This is another angle of the actual Geo at the same frame as the images Above.

So I just wanted to mention what I have observed and what I have tried.
first my observations, the Multi filed is only partly interacting with the collision field, in that some of the smoke travels along the ceiling of the roof before finding an exit and pushing outwards.
Secondly have I have tried, Is using the Houdini shelf Deforming collision object but it only dose 2 frames then hits my 32gb memory cap and my system crashes and restarts. This is if the mode in the Static Object is set to Volume Sample. If I try Ray Intersection it dose not work, there are holes in the collision Field.

so the method I am Trying to get right (Based on the Images above) is the secondary Volume Source set to collision.
Here are my VDBs and I am merging in the Cached RBD Data.

And this is my Volume Source for that VDB Surface

If you need anything Please Let me know !

Have a Great day,


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Wow those where some Quick Reply's.

So @sunchao360 I can share the file with you Shortly, My internet is really slow.
@3dome that was at a max Axis of 150, I am trying a quick one at 600 but a 300 test gave me the same results.

The results for the 600 Res where the same, Here is a screen shot from above and below

As you can see from the Bottom view below (Excuse the Pun) you can see that it is traveling along the  ceiling though I cannot tell whey it is going through at the start of the explosion and the Explosion is within the collisions for sure.


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locking a default alembic node doesn't work ;) would be nice if you could filecache out a single frame of the complete house and lock or stash that

Edit: or just unpack + convert after the alembic and lock that - might be easier with the way u set up the sim. or upload that thing to somewhere

Edited by 3dome
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7 minutes ago, JodyLS27 said:

Ahh that' s good to know, I didn't know that about Alembic files, do you know why that is ?

Below is the version with the house Cached and Locked.


because it comes in as packed alembic (which is represented as points) and just stores a path to the geo on them.

the new cache is also just packed alembic. what we need is alembic1 - transform1 - unpack - convert - filecache (locked)

Edited by 3dome
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just did a simple test in your scene where my basic boxes acting as ceilings were 0.1 (=10cm) thick/thin. Smoke passed through. Had a look at the pyrosolver, 'correct collisions' was turned off. Any reason why you did that? Turing it back on fixed it.

If that does not help with your house geo, you might want to turn on Relationships -> Collisions -> Extrapolate into collisions and see if that does the trick.

Edited by 3dome
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Alright cool so I just had a thought, what if me not unpacking and converting to polys from the alembic first, then fracturing and packing it up was the issue in the first place ? I am going to do a test with it now to see if that Fixes my issue, in the mean time I have another file below with the newly Unpacked --> Converted --> Cached Geo of the house. 

As for the Correct collision and Extrapolate into Geo, I actually had those turned on and was getting the issue as well which is why I turned them off to see if I got a different result. 
Also I tested with them on and it still is not working so I will try my solution above and see if it fixes it :)

Also with my Fractures (If you want to use my Geo) The fractures are split into a two vertical columns (Using the network Box), Use the top ones as it is just basic Voronoi fractures for testing.




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@3dome It worked !!!!!!
Thank you so much for all your Assistance and Patience.

Another thing I found just before trying the Sub Steps was that I had to go to : Pyro Solver --> Advanced --> Collisions: and set Fields to correct to a (*) for all of the Pyro Fields to be affected.

Thanks again for all the assistance !

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