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DOPS/POPS without Shelf Tools (transitioning to Houdini)

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I'm new to Houdini, transitioning from Max(TP, pflow, Magma...etc.). I started by messing around probably how everyone else did by using the shelf tools. However, I'm now trying to really dive into it using the application as it is intended. I figured that I would start off with a very straightforward and simple example from scratch - NO SHELF TOOLS.


What I did first was actually create what I intended to make using the shelf tools first and then see what was going on and then start from scratch. In my example I just want to Have a Torus fall and have particles emit off the surface as it falls. The Rigid Body part is working perfectly, but the POP part of it is where I'm having problems.


As soon as I add POPS into the mix (even if I make a different DOP network) I keep running into the problem that the particles seem to emit from a "ghost" of the torus geometry. One thing I noticed and can't seem to find an answer for are these 3 things.

  1. When I use the shelf tool to emit particles from the surface, it fills in information into the "DOP Import" Node that it creates. The information is created in the Object Mask of the node. I can't seem to find what an Object Mask actually does (I'll revisit this later).
  2. The shelf tool creates a SOP node with a DOP I/O and I don't know why.
  3. When using the shelf tools to make this setup, I noticed that If I deleted the information that was in the Object Mask of the DOP Import node at the end of the tree, the particles in the DOP Network wouldn't work at all.

To revisit, what I was saying previously - I don't know what an Object Mask does/is for, and the information that was added when using the shelf tool route looked incredibly arbitrary to me. I included some images of my basic setup. Here is the top level Object view: 38j5L5n.jpg


You can see what I'm talking about where the torus falls, but the particles are emitting from the "Ghost" geo.


This is where I suspect something is wrong, If I had to guess, the Object Mask has something to do with saying to use the geometry but not interfere because it's a mask? (I have the DOP Import node set to Bypass so that everything would show up.)



And lastly, here is my DOP tree. When I select the Torus SOP this is what happens. But with the shelf tools, it used info in the Object Mask as the SOP Path.





I know this is super detailed for such a simple setup, but I'm trying to understand what I'm doing and also what/why Houdini is what it is. Any and all help is much appreciated as I've been looking for an explanation to this for well over a week. I look forward to having that moment when it all clicks. Thanks! :)



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1 - object mask is which DOP objects you import in form the network. So in your example above putting - rdbobject1 - in that field would only import that geometry. Putting * in there is a wildcard, which means import everything. It has nothing to do with what is used in the simulation, just what is pulled out of it back to SOPs.

You can pull in multiple things by writing the names of the objects in sequence, seperated by a space. i.e - rdbobject1 groundplane1 popobject1.


2 - The shelf tool is just trying to help you by giving you a place you can get your particles back into SOPs.


3 - that's because by deleting the info in that field you stopped importing the particles. If you dived into the dopnet, you'd see they were still there.


I often find the shelf tools to be overcomplicated. They often try to do a little too much for me. Theres plenty of nodes created you usualy dont need. I also dislike working at the main obj level. I almost always nest my objects all together. The flow is much simpler to me. This is a personal preference though. After you use Houdini more you'll see the pros and cons of each method and choose your own style.


Ive attached a very simple example of the way I set these up by hand. Everything is in the torus_object1 SOP node. The dopnet node has some of the functionality of the dopimport node as it has an object parameter to bring objects back in to SOPs. This acts like object mask on dopimport nodes.


Hope this helps. It took me a long time to wrap my head around this stuff. The learning curve is alittle steep.


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Thanks Richard!


I will give this a look. The one main thing about Houdini isn't even the concepts of how everything works as far as "math" is concerned. It's just trying to get my head around the workflow. You also mentioned something that threw me off a bit when I first started with the shelf tools - adding extra nodes AND putting them in an order I thought was mandatory.


For awhile I couldn't understand why if you did a simple rigid body setup from the shelf tools, it would add gravity after the solver. I know you can, but coming from every other package, it makes sense for me to put it before. In short, I thought the way the shelf tool set things up was the proper way. So it's refreshing to hear that I'm not just being told "Forget doing it from scratch, the shelves are the way to go!"


Again, thank you so much for the help. I've spent a week trying to dissect and understand.

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DOP's doesn't process top down like SOP's. in SOP's each node is kinda a self contained function. Process the incoming data, spit it out.

Whereas in DOP's it's from the bottom up, and is the ingredients to feed into the simulation engine. 

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Ah, that makes more sense. I'm looking at it how Thinking Particles processes data in Max - Down the chain and then back up. I finally got a look at your file. I prefer it this way where it's essentially self-contained/precomped. The only thing that confuses me is in the example above, when I go to the top Object level, the torus disappears.

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The torus disappears because its only used as a source for the particles. Its not brought back out from the sim. You can use a merge node to add it back in if you like


I guess this is the con about having everything in one node. If I knew I wanted to display the torus and the particles, I probably wouldn't use my approach. For me though, that's a rare case.

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That's actually a good point. In production the only thing I'm usually using geo for is matte's or whatever info they contain for comp that would be useful to them concerning FX. I said it before but I enjoy this approach because I am a stickler for organization and I'd rather everything pertaining to an object be just that - within itself. I'm glad that this was done on purpose (hiding the geo) because I thought I was doing some magical combination of clicks that was screwing it up.


Like I said before, I'm new to Houdini, but I was recommended this site because I heard it was the best forum for Houdini. You guys are super helpful! Especially with something as mundane as emitting particles from a torus. I really appreciate the help and advice. :D

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