Hi, all,

I have a simple question, how could I create an attribute that can increment a random number. through time? Simple expression like $X+rand($FF) seem doesn't work because $X keep referring to the default value. I try many way but still can't make it work…

Thanks in advanced.

POPs are a common way to perform feedback based solutions like this because they function iteratively on a single geometry over time. In other words, when deciding on the new value of an attribute, you have still have the old one available to you. For example, putting $TX + 5 into the first component of the position POP's position field would increment the x position by 5 units on every cook.

Hi, thanks for the reply.

I see…but how about in SOP? Basically what I am trying to do is to move some points of a geometry a random amount of distance per frame. Does it mean I need to do some kind of referencing to a pop network in order to achieve this?

Thanks.

The simplest way is to have a reference to a POP network. Basically you would feed the points you want to walk randomly into the POP network and use them as source location at frame 1.Then at the SOP level you can use the popnet output as regular geometry points for further manipulation.

You can take a look at the attached hip file

you can do some feedback in the DOPs sop solver. or ask for the old feeback SOp back… ;-)

oh yeah, forgot to mention… please note, that there may be other and better ways of doing it other than what I posted (like, the way paul mentioned). I am just a developer not an artist; i have no clue what Houdini does, but I have a pretty good idea how it does it

Its OK.

But I still don't get it, the hip you upload is actually emitting particle from the point, but the point of the geometry itself is not moving (deforming). I try read back the position of the particles but no luck…

cheers

you need to turn the points from the POP net back into the geometry. I modified the example to show how to use AttribTransfer to achieve that (ie, transfer point positions). The geometry/topology stays the same but the points move around.

Edit: you may want to use some other, more reliable, method to assign the point positions. I used attrib transfer because it was the first thing that popped into my mind. It assigns the new point position to the closest POPnet point, but there should be a way to assign it based on point number (and you may be able to match up the point numbers between the birthing geometry and POPnet by setting the Emission Type of the Source POP to ‘ordered’ rather than ‘random’ )

Thanks rafal, I get it now. Thanks a lot for taking the time to help me.

Cheers

in terms of feedback etc, you can just use the “channel” sop and reference an attribute in a CHOPs network if you wanted to avoid simulation and be able to scrub backwards etc.

Just import the attribute as animated geometry and use the expression CHOP.

If you wanted to simply add a random value on each frame (or index) you could do something like this …

oc($C,$I-1) + rand($C+$I)

don't have houdini in front of me but I think that will work. Then read it back into the channel SOP as animated geometry.

Hi, Andrew, thanks for the suggestion, I am trying that out too, seems working quite nicely. I need to learn more about chop though.

Thanks again for everyone's help!