Trail sop from particles group

[titor]

Hi everyone.

I'm here beacause i'm having some troubles with particles group and trail sop.

In my scene i have particles and if their age reaches 5 then they are put in a group called RUBANS, and then what i'd like to do is use this group to do a trail with the trail sop.

But I can't figure out how to get my group of particles outside the popnetwork in order to trail only the particles that are grouped.

Here's my file if you can give it a try. Thanks

ruban1.hip

[titor]

Nobody has a small lead about that ?

[Matt_K]

Hey Titor,

I haven't managed to look at your scene, but one thing you could try is:

Add a specific colour to the group in your POP Network with a color POP, then in SOPs delete points by colour to leave only the one's that need the trail added.

Hope that helps,

Matt.

[Nerox]

Hi Titor,

That's actually really easy: inside the pop network you already have the particles isolated in a group. All you have to do is delete the points that aren't in the group and then apply the trail SOP. And mix in the particles that you've deleted before and your done.

So:

Below the POP network, create an delete node, set the group to RUBANS and select points and delete NON selected, feed this in to the trail sop. Next, in case you want to preserver the deleted points, duplicate the delete node and switch it to Delete Selected, create an merge node to bring the deleted particles back in to your tree.

Hope this helps.

Cheers!

[Macha]

Something is weird there. Immediately I thought of the same method that Nick described but for some reason it doesn't work.

An alternative would be similar to Matt's idea but with an integer attribute to tag. You might have to rename that attribute in the sop network.

Strange...

[Macha]

Well, now it does work. How strange. Very strange.

Particles don't update very well, especially if you scrub. It can lead to weird probs. I wonder if that was related to it.

ruban1.hipnc

Edited March 6, 2011 by Macha

[titor]

Hi everybody and thanks for your answers.

@Matt : I'll try to do that and use the Cd attribute to get more used to expressions.

@Nerox : Thanks, it seemed to be the right way to do it, but i didn't manage to achieve it, the way to use delete sop is still a bit confusing to me.

@Macha : Yeah i find that it's kinda strange the way it interact in the viewport, it seems to work when i render out file on disk.

I think now that if i want my trails to appear later i just need to tweak in my popnetwork the way the particles are transferred to my group RUBANS, right ?

I'll try this tomorrow and keep you up with the results

[titor]

Hi everyone.

I did a quick render last night to see how trails behaved.

And i noticed this strange artifact (see in the attached file)

Where does it come from ? Is it possible that beacause of all my particles are in my group RUBANS, then the trail sop acts weirdly ?

[Nerox]

Hi everyone.

I did a quick render last night to see how trails behaved.

And i noticed this strange artifact (see in the attached file)

Where does it come from ? Is it possible that beacause of all my particles are in my group RUBANS, then the trail sop acts weirdly ?

The problem is that you have changing point numbers, this is because particles die after 10 secs, apparently the trail SOP doesn't like this (it makes sense though). You can verify this by changing the life expectancy of the particles to a greater number.

The following is not a very efficient solution but it's the only one that I can think of. Don't let particles die off, rather trail them all and delete the trails based on their age. This is possible because the trail gets the same attributes as the original particle. See the attached file.

To get back on my previous post, the solution I described works fine for me. I've incorporated this in to the attached file as well. Also, no offense on your behalf, but you said: 'I don't really get the delete SOP'. This rises the question, did you build this scene your self? Because it has a digital asset and a VOPPOP with a nice little network in it. I would expect that a delete node would have passed your path before you would get in to more complicated stuff like VOP's. If you get VOP's, the delete node is a piece of cake, just look at the help card and you'll get there.

ruban-nick_V01.hipnc

[mawi]

Or add the trail in POPs with the split node and build the curve in SOPs with a add node.

ruban1.hip

Edited March 6, 2011 by mawi

[Nerox]

Or add the trail in POPs with the split node and build the curve in SOPs with a add node.

Nice one :-)

[titor]

Hey everyone.

Nick you were right about the age of the particles source of the artifacts.

And yes I build this scene myself, I admit I got some help from another student to deal with VOP but I'm still learning Houdini.

When i said that the use of the delete sop is a bit tricky, it's just because I often mess up with the use of groups and the rest of the options, and never get the result I want. I might haven't take the right path to start my learning ^^, 'cause you're right VOPPop are also some complicated stuff.

Thank you mawi for your help.

Your setup seems lighter than the one I intended to use.

[pclaes]

As an intermediate step and a tutorial on trails, this might interest you too:

http://www.3dbuzz.com/xcart/product.php?productid=58&cat=11&page=1

It has a bit of vops in it, but mostly expressions. This uses the add sop to build the trails based on a specific attribute. There is a trail asset in there that allows you to do much more. At the time that was inspired by some of the trails in "Harry Potter and the Chamber of Secrets" by some of the work done by Double Negative, and also by the end fighting sequence in The Chronicles of Riddick. They are worth looking at for inspiration.

[Nerox]

Hey everyone.

Nick you were right about the age of the particles source of the artifacts.

And yes I build this scene myself, I admit I got some help from another student to deal with VOP but I'm still learning Houdini.

When i said that the use of the delete sop is a bit tricky, it's just because I often mess up with the use of groups and the rest of the options, and never get the result I want. I might haven't take the right path to start my learning ^^, 'cause you're right VOPPop are also some complicated stuff.

Thank you mawi for your help.

Your setup seems lighter than the one I intended to use.

I didn't mean to discourage in any way, I was just curious. I can imagine that it depends on the person, but for me it's very important to organize my network, name nodes properly and even color them. Some people work very well in chaos, I don't. I guess some organization could help you as well, and have a look at the help card of the delete node (you can find it by clicking the question mark on the parameter window.

VOP's is something you have to get your head around, jet it's a very powerful environment and the great thing is that VOP's work very similar in the different editors. In SOP's it works on a per point basis, in SHOP's it's per pixel and POP's obviously works per particle. I believe 70% (ish) of Houdini nodes can be rebuild with VOP's, well that say's something. I always use VOP's, especially when you hit the point where the script editor isn't wide enough for your written expression, go to VOP's and your done with 5 - 6 nodes. (VOP stands for Vex OPerators, or Vertex EXpression, it's basically visual programming, just Awesome!)

[titor]

I didn't mean to discourage in any way, I was just curious.

I can imagine that it depends on the person, but for me it's very important to organize my network, name nodes properly and even color them. Some people work very well in chaos, I don't. I guess some organization could help you as well, and have a look at the help card of the delete node (you can find it by clicking the question mark on the parameter window.

VOP's is something you have to get your head around, jet it's a very powerful environment and the great thing is that VOP's work very similar in the different editors. In SOP's it works on a per point basis, in SHOP's it's per pixel and POP's obviously works per particle. I believe 70% (ish) of Houdini nodes can be rebuild with VOP's, well that say's something. I always use VOP's, especially when you hit the point where the script editor isn't wide enough for your written expression, go to VOP's and your done with 5 - 6 nodes. (VOP stands for Vex OPerators, or Vertex EXpression, it's basically visual programming, just Awesome!)

I know what you meant, in fact I started using houdini only 5 or 6 months ago, and I admit that my goal in using Houdini is to use a new software and discover something different from Maya. I won't discourage myself, I really find this software fun

Also I'm using it to try to achieve specific tasks on a short movie we're working on with a friend, as a consequence I only look roughly into all directions at the same time.

I know that it might not be an efficient way to do things, but I think that it still is interesting to see what it is possible to do with a software.

And yeah VOPs are great, I have some knowledge in programming and it's a really interesting network, but I still need to get deeper in it.

In the future, I'll try to look more often at the help

Thank you all.

Edited March 7, 2011 by titor