doing things to for each sop loops

 

How to get information about the current loop

Sometimes you need to use the current loop or the current piece number in an expression on one of the nodes in the same loop. You can get this information with a  Block Begin SOP node that uses the Fetch Metadata method.

1. Set up a looping block using the instructions above.

2. Select the  Block Begin SOP node, and then in the Parameter Editor click Create Meta Import Node.

   This button adds a second Block Begin SOP node to the side of the existing one. This node is set up to generate an empty geometry with some detail attributes.

   The detail attributes are as follows:

   numiterations

   The expected total number of iterations, taking into account the Max Iterations and Single Pass parameters on the  Block End SOP node of the loop.

   iteration

   The current iteration number, always starting at 0 and increasing by 1 each loop.

   value

   In piecewise loops, this is the current value of the attribute. For example, the piece integer or name string, or if there is no attribute, the current point or primitive number.

   In simple repetition loops, this is a floating point value starting at the Block End SOP’s Start Value and increasing by the specified Increment each loop.

   ivalue

   In simple repetition, this is an integer version of value. This can be useful if the value is naturally an integer (for example, starts at 1 and increments by 2) and/or if values are over 24 million (where floating point numbers lose precision).

3. To grab the value of these attributes in a node inside the loop, do the following:

   • In a Houdini expression, use the detail expression function.

     For example:

     detail("../block_begin2", "iteration", 0)
     

   • In an Attribute Wrangle SOP node, use the detail VEX function.

   • In Python, use hou.Geometry.attribValue():

     node("../block_begin2").geometry().attribValue("iteration")

another way to cut or slice objects

Use 2 clip SOPs in a For Each loop. The "detail" function access the iteration number. These little expressions are pasted in the X, Y or Z origin boxes. The first cuts to the top of the division, The second cuts to the bottom of it (technically to the top of the previous iteration - which is what the "-1" is for.

 -8.26+detail("../foreach_count1", "iteration",0)*4

-8.26+(detail("../foreach_count1", "iteration",0)-1)*4

This does not create interior detail - but is good for troublesome geo.

Copying different objects to points

Sometimes you need to copy a bunch of different objects to points. Eg. You have 5 variations of wood splinters and you're emitting them from a snapping plank.
Instead of using the slower copy stamp with random point values, you can use a ForEach loop and a Switch node..and of course a point wrangle.

Connect the objects to the Switch node. In a point wrangle, give each point an (random) integer value (eg "instanceID") ranging from 0 to the number of objects you have.
Make a ForEach network and connect the Point wrangle into the top. Create a null node directly after the start of the Foreach Begin. This will be a "temporary holder" node. It will carry the instanceID value to the Switch node. I've called it PT
The clever bit happens in the Switch node. In the parameter window, click on the cog wheel and add a Spare Input. This will create a new dialog box called Spare Input 0. Type in:

../PT

This will point the spare input to our temporary null & its instanceID value.
In the Select Input dialog box, type

point(-1, 0, "instanceID", 0)

here we are telling the Switch node to get a point attribute from input -1 (which is the spare dialog....which is pointing to the PT temporary null..). The attribute in question is instanceID.. NICE.
A purple arrow going from PT to Switch should have appeared now. This represents that link we've just made.

All that is left to do is to create a Copy To Points node & put the Switch output into the objects input & plug PT into the points input. Finally plug the output of all of that into the Foreach End.