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Houdini Forum Notes



So this page is going to be answers to random questions you may have about Houdini. You might see me occasionally lurk the Houdini SideFx Forums, and see that I have answered some questions there. Feel free to help out or say Hi back! :)

The answers and questions on this page are responses that I have read over in pages on the Houdini forum, and figured it might be helpful if more people had answers to those questions. I'll probably update this page as I find more cool facts there. 

Here we go.

Helpful Hint for Group SOPs

I'm inside a SOP Solver and trying to convert from points to primitives. The Group create node no longer seems to have a convert tab. 


-Use a group promote SOP.


Helpful Hint for Vellum vs FEM

FEM is physically based and should provide similar results regardless of the tet resolution, however it's much slower especially with collisions

Vellum is much faster but you will get very different results with different resolutions, which is especially apparent on the geo with varying sizes of tets.

 Vellum has more complex effects and it offers more types of constraints to mix and match.

There is more differences in how they deal with fracturing, embedding, etc. So overall it doesn't hurt to get familiar with both and pick accordingly.

Helpful Hint for Random Orientation on Instanced Objects

What is the simplest way to Randomize Orientation on instanced objects? I have points scattered on a grid and want to randomize them so my instanced objects have a more organic look. 


- Attribute Randomize SOP
- Attribute Name: orient
- Distribution: Direction Or Orientation
- Dimensions: 4
- (optional) Cone Angle: whatever you prefer


Helpful Hint for Using Acceleration Blur


You need geo time samples in Mantra to sample the acceleration attribute over time.


You can set ”Velocity Blur“ or ”Acceleration Blur“  deformation blur to an object or sim. When this is set to ”Acceleration Blur“, you can use the Geo time samples property to set the number of acceleration samples.


Creating High resolution Clouds


The edges of my clouds continue to look blocky, even though I have increased my rendering samples.


-The problem is due to the offset. Too large an offset has exhausted float precision by 5 significant figures, resulting in an aliased look. Drop the offset value down a few orders of magnitude, and it will appear to be high resolution again.

Pre-Rolling your Simulation

How do you add pre-roll to a simulation when you are using a DOP net?

-The creation frame must be before or on the start frame of the DOP network. Change the start frame of the DOP network to be which either your desired range of frames to be.


What the Heck is an Edge Group?

See above question.

-Edge groups are defined by point pairs.
-You can use expandedgegroup() to get array of point pairs from existing edge group.
-Or setedgegroup() to add/remove point pair to a group, plus other edge functions.

-Since edges are technically not an element of a geometry, they will not show up in spreadsheet. They don't have edgenums and can't hold attributes, therefore the point pair representation.

-Each point pair defines exactly one edge.
-Houdini calls them half-edges, and you can also query them using vex.

Collisions in Vellum

Is there is a way to exclude some points on my dynamic object to collide with the colliders?
and make it dynamic? For example, I have a groom or a cloth with interception problems, I would like to deactivate the collisions of those strands/mesh in some frames

-you can set i@disableself and/or i@disableexternal attributes on individual vellum points to 1 to exclude those from collisions

 -You can also set them to 2 to initially exclude them, but once the solver is able to resolve them, it will turn collisions back on automatically.

unexpected end of .hip file

I was working on a fire sim when the file crashed. I tried opening it up again and received the message "Error: Unexpected end of .hip file.

-Your file is dead. Hope you have a backup. If not turn on incremental backup

Finding the Vertexes associated with a Point

I'm currently having issues with verts. How would i be able to import or find out the neighboring verts within a vertex VOP?.

-I don't know what the VOP equivalent is if there even is one, but you can get this information from a wrangle using pointvertices() given a point, or vertexprev() and vertexnext() if you're looking for neighboring vertices.
MOPs (Motion Operators for Houdini):


Volume Rendering Issue

My density attribute based on my setup is a vector, and is based off another attribute. It's also not rendering.

-Example: Because your density attribute is a vector since you created it from Cd(example attribute)
just Bind Export density float attribute directly from your vop instead

Is there a place where I can download Houdini versions between 9 and 13?

I have an old hip-file that I am trying to inspect. Is there a place where I can download Houdini versions between 9 and 13 (for Windows)?


Extrude that is compile compliant

I am looking to extrude open edges but the extrude nodes: polyextrude, extrudevolume and extrude-old do not work with my compile nodes. Looking for an alternative away to extrude and cap open edges.

-Transform or peak A, merge with original A, skin, polycap.

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