Houdini 19.5 Nodes TOP nodes

Block Begin Feedback TOP node

Starts a feedback loop. TOP nodes within the block execute serially, optionally looping for each incoming work item.

On this page
Since 17.5


A TOPs Feedback Loop block lets you run a sequence of serially executed steps for multiple iterations.

A TOP network already behaves somewhat like a parallel loop: it runs as many work items at the same time as possible based on the scheduler settings. So there’s no need for a typical loop construct since “repeating the same action with different inputs” is just how the network works.

Sometimes, however, you want to run a series of steps serially rather than in parallel, and use the output of previous work items as input for subsequent work items. For simple simulations this is already handled by the ROP Fetch node, which is able to create batches that run as a single job, one frame at a time. For more complicated use cases, such as looping that spans multiple nodes, or where the size of the feedback loop isn’t fully known, you can use a feedback loop.

In a feedback loop block, the network runs the work items node by node, with later work items depending on the previous work items, forcing them to execute serially. Then when all work in an iteration is done, if the block specifies more than one iteration, it loops back to the start and executes the next loop.


Depending on the settings, a feedback loop block can also run multiple serial loops in parallel.

For example, imagine an RBD simulation where a jar is being filled with marbles, one handful at a time. The entire process could be run as a single simulation, however marbles at the bottom of the jar might become unstable and the numbered of simulated objects would keep growing. One way to manage this is to run the RBD simulation for the first handful of marbles and use its results as a static object in the second simulation. The result of the second sim and first sim combined would be static objects in the third sim, and so on. You can do this in TOPs using a feedback loop with a ROP Geometry in the loop block.

(Feedback loops are also used to implement command server chains, where the commands must be sent to the server sequentially, one at a time.)

How to

To...Do this

Create a feedback loop block

  1. In a TOP network editor, press ⇥ Tab and choose “For-Loop with Feedback”.

    This tool puts down a Block Begin Feedback and a Block End Feedback node.

  2. Select the Begin node. In the parameter editor, choose how to specify the number of iterations:

    • The default is to run the number of iterations specified in the Iterations parameter. If the Begin node has upstream items, the loop runs iterations times for each incoming item.

      For example, you might have a loop that iteratively builds up a scene over the certain number of iterations (such as the “dropping marbles in a jar” example from the overview). You could then repeat the loop for each item generated by a Wedge node you wire into the Begin node.

    • If the Begin node has static work items, you can turn on Iterations from upstream items. This sets the number of iterations to be the number of upstream items. This repeats the sequential series of steps within the block once for each input item.

      For example, you could use a File Pattern node to list all of the files in a directory, and a loop block to iterate over the files and append the contents to a temporary file.

  3. If you Begin node generates items dynamically, you must also turn on Use dynamic partitioning on the End node.

  4. Wire nodes between the start and end nodes to make them part of the loop.

    Houdini draws a border around the nodes in the block to help you visualize it.

Cook parallel “side tasks” based off work items in the loop

If you wire out from a node inside the loop to a processor outside the loop (that is, not connected to the loop’s end node), the work items in that processor will generate based on the in-loop work items, but will be scheduled normally in parallel.

This can be useful for “side work” based on items in the loop but not required by the loop. For example, if the loop involves generating and manipulating images, you might generate thumbnails of the images outside the loop.

Tips and notes

  • You can use any processor node in a feedback loop. However, currently you cannot use a dynamic partitioner or mapper inside a feedback loop. You can use static partitioners, if the partitions only contain work items from the same loop iteration. If work items from different iterations are somehow partitioned together, the partition node will report an error.

  • You should color the start and end nodes of a block the same to make their relationship clear. The default nodes put down by the For-Loop tool are colored orange, but you can change the node colors. This is especially useful to distinguish nested loops.

    The border around the block takes on the color of the end node.

  • The begin node is a processor that generates loop iteration work items.

  • Each work item depends on the previous item from the same loop, and has attributes to identify the iteration and loop number.

    The feedback end node is a partitioner which partitions work items based on the loop iteration they're associated with. This is useful because the nodes in the feedback loop are free to fan out into as many additional items as needed, and the partitioner will collect them. The second loop iteration item in the begin node depends on the partition for the first loop iteration, and so on. If the loop begin is generating work items dynamically, the feedback end node must be set to use dynamic partitioning.

  • You can wire a node from outside the feedback block into a node inside the block that has multiple inputs.


The $HH/help/files/pdg_examples/top_feedbackhda and $HH/help/files/pdg_examples/top_feedbacksim examples show how you can use this node to cook geometry that is based on the output of a previous iteration.

TOP Attributes

The names of these attributes can be configured using parameters on the node.



The loop iteration number. This attribute can be an array of values when using nested feedback loops, since the iteration number at each level is preserved. The loop iteration value for the outer most loop is stored in loopiter[0], the next level is stored in loopiter[1], and so on. In this case, you would use loopiter[loopnum] to get the current loop iteration.



Tracks which loop the work item is associated with. This attribute is relevant when generating multiple independent loops in the same feedback begin node, for example by driving the feedback begin node with a Wedge node.



The total number of iterations in the current loop.


If you have nested loops, you may want to give the loopiter/loopsize attributes different custom names at each nested level to avoid having to deal with arrays.


Generate When

Determines when this node will generate work items. You should generally leave this set to “Automatic” unless you know the node requires a specific generation mode, or that the work items need to be generated dynamically.

All Upstream Items are Generated

This node will generate work items once all of the input nodes have generated their work items.

All Upstream Items are Cooked

This node will generate work items once all of the input nodes have cooked their work items.

Each Upstream Item is Cooked

This node will generate work items each time a work item in an input node is cooked.


The generation mode is selected based on the generation mode of the input nodes. If any of the input nodes are generating work items when their inputs cook, this node will be set to Each Upstream Item is Cooked. Otherwise, it will be set to All Upstream Items are Generated.

Iterations from Upstream Items

Set the number of iterations based on the number of incoming static work items, instead of the Iterations parameter.


If the Begin node has upstream items, the loop runs this number of times for each incoming item.

Copy Inputs For

Determines how input files should be copied onto loop items. By default, upstream files are copied onto all input files, however it’s also possible to only copy input files onto the first iteration or none of the loop iterations.

No Iterations

Upstream input files are not copied to the outputs of any loop iteration items

First Iteration

Upstream input files are copied to the output file list only for the first loop iteration

All Iterations

Upstream input files are copied to the output file list of all iterations.

Cook Loops Sequentially

When this toggle is enabled, loops created by this node cook to completion one at a time. Otherwise, when this toggle is off, independent loops are able to cook in parallel.

Feedback Attributes

When on, the specified attributes are copied from the end of each iteration onto the corresponding work item at the beginning of the next iteration. This occurs immediately before the starting work item for the next iteration cooks.


The attribute(s) to feedback can be specified as a space-separated list or by using the attribute pattern syntax. For more information on how to write attribute patterns, see Attribute Pattern Syntax.

Feedback Output Files

When on, the output files from each iteration are copied onto the corresponding work item at the beginning of the next loop iteration. The files are added as outputs of that work item, which makes them available as inputs to work items inside the loop.

Loop Attribute Names

These parameters can be used to customize the names of the work item attributes created by this node.


The name of the attribute containing the work item’s iteration number.

Number of Iterations

The name of the attribute containing the total iteration count.

Loop Number

The name of the attribute that stores the loop number.


FeedbackBegin Example for Block Begin Feedback TOP node

This example demonstrates how to create a feedback loop.

FeedbackModes Example for Block Begin Feedback TOP node

This example demonstrates the different ways upstream files are copied onto feedback begin loop iterations.

See also

TOP nodes