Hey everyone ,
I’m pretty new to Houdini , so the thing is that i don’t quite understand the way that data flows in H. For instance , it might seem that it goes from the to the bottom : you can create a point and then, let’s say copy it. Things start to get worse with stamping - now points ( higher level ) know what’s done in copy SOP ( lower level ). It’s kinda confusing , or am i missing anything ?
Top to bottom.
If you watch some of the tutorials, you’ll see the instructor putting down nodes top to bottom.
As in: start with a circle, feed it into an extrude, then add a transform, etc.
You might want to watch one or two of the tutorials starting from the sidefx learning page.
Sidefx Getting Started
Thanx for the reply! 
Well yeah , that’s pretty clear , i guess my question is more about how stamping works. I mean it seems that top to bottom rule works a bit different in this case.
The left input is cooked multiple times, once for each copy. Each time, the stamp variable values are first set so that they can be used when cooking the copy.
Houdini evaluates SOPs in two passes:
[ol]
[li]Start at Display/Render Flag, and follow nodes/expressions to evaluate dependencies
[/li][li]Then starting at the end of that dependency list, evaluate the nodes to generate geometry
[/li][/ol]
If you don’t have any expressions or channel references, then the node connections are all you need to follow. If you hit d in the Network View, and go to the Dependency tab, there are some useful options to visualize dependencies.
Also you can see the second pass using the Performance Monitor (in a not-so-straight-forward-way) if you:
[ol]
[li]Create a fresh new scene, set the Cook Mode to Manual
[/li][li]Setup a scene (i.e. copying spheres onto a point, or even something complicated like a Pyro Shelf setup)
[/li][li]In the Performance Monitor, click Start, then click View > Event Log; make sure you toggle on Enable Output in the lower right of the Performance Monitor window.
[/li][li]Now, click the Refresh Button (next to the Cooking menu that was set to Manual mode)
[/li][li]That output from top to bottom is the evaluation pass (second pass)
[/li][/ol]
That’s complicated, but can be helpful. There are some threads on odForce and the SideFX forums where SESI folks chime in on evaluation, this one comes to mind: http://forums.odforce.net/topic/17105-short-and-sweet-op-centric-lessons/?p=104263
Hope that helps!