Houdini-Niagara Plug in

Install and setup for Houdini-Niagara

On this page

Installation
Getting Started
Modules
- Default Attributes
- Custom Attributes
Houdini Point Cache Asset
- Special Attributes
Houdini-Niagara Data Interface
- Data Interface

The Houdini Niagara plugin can be found in your Houdini install directory, next to the Houdini Engine plugin: HoudiniXX.Y\engine\unreal\5.Z\HoudiniNiagara. You can also download the Houdini Niagara plugin at SideFX Houdini Niagara GitHub page.

The latest version supports Unreal 5.4. Similar to the Houdini Engine plugin, the Houdini Niagara plugin’s source code is intended to be compatible with all previous version of UE5. That plugin can be found under the FX category, and requires the Niagara plug-in to be activated.

This plugin adds an additional type of asset: Houdini Point Cache Assets that contains various point attributes exported from Houdini. It also provides the custom Houdini-Niagara Data interface for parsing and processing those assets in Niagara.

Installation ¶

Download the latest release from GitHUb.
Unzip the downloaded file.
Copy the HoudiniNiagara folder into _Your_Unreal_Project/Plugins_ folder.
To confirm the plugin is installed, load your project and navigate to Edit ▸ Plugins.
Search for Houdini Niagara in the Project section of the Plugins window.

You can also build the plugin from source by following the instructions on the GitHub page.

Getting Started ¶

The plugin includes a configured emitter, 1HoudiniNiagaraBasic1, that can be used as the basis for most Niagara Systems that utilise an Houdini Point Cache. The required modules for reading an Houdinin Point Cache can be found below.

For a quick introduction to the workflow and various assets:

Modules ¶

Module	Description
Init Houdini Point Cache	Emitter Spawn Group - This module tracks spawning info and ensures unique point ID’s for each emitter
Spawn Particles from Houdini Point Cache	Emitter Update Group - This module reads an Houdini Point Cache and spawns particles based on their age attribute relative to `Emitter.LoopedAge` SpawnGroup can be set if multiple caches need to be accessed in the same emitter
Sample Spawned Houdini Point Cache	Particle Spawn Group - This module reads an Houdini Point Cache and sets the default attributes in the Houdini namespace. It only set the attribute for each particle once at spawn time. The Set Parameter module can then be used to set attributes like `Particles.Position` from `Particles.Houdini.Position` SetLifetime sets `Particles.Lifetime` based on the `life` attribute (if it exists) SetVelocity sets `Particles.Houdini.Velocity` based on the `v` attribute (if it exists)
Sample Houdini Point Cache	Particle Update Group - This module reads an Houdini Point Cache and updates the default attributes in the Houdini namespace based on `Emitter.LoopedAge` The Set Parameter module can then be used to set attributes like `Particles.Position` from `Particles.Houdini.Position` UseCustomAge when on allows the user to specify a custom attribute to control the time index used for reading point cache attributes

Default Attributes ¶

The sample modules will attempt to read the following attributes from an Houdini Point Cache. If an attribute does not exist the particle attribute will be set to 0.

Particles.Houdini.Position is set from P
Particles.Houdini.Normal is set from N
Particles.Lifetime is set from life
Particles.Houdini.Color is set from Cd
Particles.Houdini.Alpha is set from Alpha
Particles.Houdini.Impulse is set from impulse
Particles.Houdini.Force is set from force
Particles.Houdini.Type is set from type
Particles.Houdini.Velocity is set from v
Particles.Houdini.Pscale is set from pscale
Particles.Houdini.Orient is set from orient

Custom Attributes ¶

Custom attributes can be read by creating your own module.

This video shows how to do that.

Houdini Point Cache Asset ¶

You can import .hbjson, .hjson and .hcsv files generated by the Niagara ROP in Houdini. hcsv has been deprecated in favor of the ascii and binary json file formats. The plugin supports float, vector (including color) and int values.

If vector or tuple values are exported, they will be expanded upon being parsed by the plug-in (so P will become P.x, P.y, P.z).

If you select a Houdini Point Cache Asset, it shows you the following properties on the file:

This can be useful for checking the header info of the file - number of points, number of frames, etc. It will also list the array of attributes stored in the file.

Hover over a Houdini Point Cache Asset in the Content Browser to display the following information on the available data:

Special Attributes ¶

The header of the point cache file contains the names of all the attributes exported. Certain attributes have built-in functions for getting their values:

Position values should be stored as P
Time values should be stored as time. The Niagara ROP will add this attribute automatically or it can be set by the user.
Point IDs should be stored as id
Normal values should be stored as N
Velocity values should be stored as v
Color values should be stored as Cd
Alpha values should be stored as Alpha
Life values should be stored as life
Type values can be stored as type
Impulse values should be stored as impulse

If you want to update point position over time, Point IDs and time values are required.

The spawn time for a given point in the file is calculated in the following way:

if the asset has an age attribute, the spawn time is calculated by comparing age to time.
if the asset does not have age, when time==0, a particle will be spawned.

If the asset has a life attribute, this will directly set the lifetime of a particle. This can be disabled on the sample modules and set in the Emitter stack as well.

The type for a given point is set to its first type value. If the file doesn’t contain type values, all points will have a type set to 0.

Houdini-Niagara Data Interface ¶

To access the point cache data contained in the Houdini Point Cache Asset, the custom Houdini-Niagara Data Interface provided by the plug-in should be used.

The Data Interface can be accessed by adding a Houdini Point Cache Info parameter to your module/emitter/system in Niagara, and selecting a Houdini Point Cache Asset for it.

All the functions that get attributes from an Houdini Point Cache Asset require:

Attribute	Description
`Houdini Point Cache Info`	A reference to the Houdini Point Cache Asset
`PointID`	A unique particle ID, usually `Particles.Houdini.NID`
`SampleIndex`	An integer value of the index in a series of attribute values over time. Use the Get Sample Indexes for Point at Time or Get Last Sample Index function to get this value
`Time`	The current time of the emitter, usually `Emitter.LoopedAge`

Data Interface ¶

The Data Interface will expose the following functions:

Function	Description
Get Sample Indexes for Point at Time	Returns the sample indexes for a given point at a given time. The previous index, next index and weight can then be used to Lerp between values
Get Last Sample Index at Time	Returns the index of the last sample in the point cache that has a time value lesser or equal to the Time parameter
Get Point IDs to Spawn at Time	Returns the count and point IDs of the points that should spawn for a given time value. Refer to the Spawn Particles From Houdini Point Cache module for an implementation.
Get Number Of Attributes	Return the number of attributes in the cache
Get Number Of Points	Return the number of points in the cache
Get Color	Returns the color value for a given Sample Index in the point cache
Get Float Value	Returns the float value in the point cache for a given Sample Index and Attribute Index
Get Float Value by String	Returns the float value in the point cache for a given Sample Index and Attribute name
Get Impulse	Return the impulse value for a given Sample Index in the point cache
Get Normal	Return the normal value for a given Sample Index in the point cache. The returned Normal vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Alpha at Time	Returns the linearly interpolated alpha for a given point at a given time
Get Point Color at Time	Returns the linearly interpolated color for a given point at a given time
Get Point Impulse at Time	Returns the linearly interpolated impulse for a given point at a given time
Get Point Life	Returns the life value for a given point when spawned. The life value is either calculated from the alive attribute or is the life attribute at spawn time
Get Point Life at Time	Returns the remaining life for a given point in the point cache at a given time
Get Point Normal at Time	Returns the linearly interpolated normal for a given point at a given time. The returned vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Position at Time	Returns the linearly interpolated position for a given point at a given time. The returned Position vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Quat Value at Time	Returns the linearly interpolated quaterion value in the specified attribute index for a given point at a given time The 1DoHoudiniToUnrealConversion1 parameter indicates if the vector4 should be converted from Houdini’s coordinate system to Unreal’s
Get Point Quat Value at Time by String	Returns the linearly interpolated Quat value in the specified attribute (by name) for a given point at a given time. The `DoHoudiniToUnrealConversion` parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s
Get Point Type	Returns the integer type value for a given point when spawned
Get Point Type at Time	Returns the integer type value for a given point at a given time
Get Point Value at Time by String	Returns the linearly interpolated float value in the specified attribute (by name) for a given point at a given time
Get Point Vector4 Value at Time	Returns the linearly interpolated Vector4 value in the specified Attribute Index for a given point at a given time. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Vector4 Value at Time by String	Returns the linearly interpolated Vector4 value in the specified attribute (by name) for a given point at a given time. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Vector Value at Time	Returns the linearly interpolated Vector value in the specified Attribute Index for a given point at a given time.The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Vector Value at Time by String	Returns the linearly interpolated Vector value in the specified attribute (by name) for a given point at a given time. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Point Vector Value at Time Ex	Returns the linearly interpolated Vector value in the specified attribute index for a given point at a given time The `DoSwap` parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s The `DoScale` parameter decides if the Vector value should be converted from meters (Houdini) to centimeters (Unreal)
Get Point Vector Value at Time Ex by String	Returns the linearly interpolated Vector value in the specified attribute (by name) for a given point at a given time The `DoSwap` parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s The `DoScale` parameter decides if the Vector value should be converted from meters (Houdini) to centimeters (Unreal)
Get Point Velocity at Time	Returns the linearly interpolated velocity for a given point at a given time. The returned vector is converted from Houdini’s coordinate system to Unreal’s
Get Position	Returns the position value for a given sample index in the point cache file. The returned Position vector is converted from Houdini’s coordinate system to Unreal’s
Get Position and Time	Returns the position and time values for a given Sample Index in the point cache. The returned Position vector is converted from Houdini’s coordinate system to Unreal’s
Get Quat Value	Returns a Quat in the point cache for a given Sample Index and Attribute Index.
Get Quat Value by String	Returns a Quat in the point cache for a given Sample Index and Attribute Name The DoHoudiniToUnrealConversion parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s
Get Time	Returns the time value for a given Sample Index in the point cache
Get Vector4 Value	Returns a Vector4 in the point cache for a given Sample Index and Attribute Index. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Vector4 Value by String	Returns a Vector4 in the point cache for a given Sample Index and Attribute name. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Vector Value	Returns a Vector in the point cache for a given Sample Index and Attribute Index. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Vector Value by String	Returns a Vector in the point cache for a given Sample Index and Attribute name. The returned Vector is converted from Houdini’s coordinate system to Unreal’s
Get Vector Value Ex	Returns a Vector3 in the point cache for a given Sample Index and Attribute Index The DoSwap parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s The DoScale parameter decides if the Vector value should be converted from meters (Houdini) to centimeters (Unreal)
Get Vector Value by String	Returns a Vector3 in the point cache for a given Sample Index and Attribute name The DoSwap parameter indicates if the vector should be converted from Houdini’s coordinate system to Unreal’s The DoScale parameter decides if the Vector value should be converted from meters (Houdini) to centimeters (Unreal)
Get Velocity	Returns a Vector3 in the point cache for a given Sample Index and Attribute name. Returns the velocity value for a given Sample Index in the point cache. The returned velocity vector is converted from Houdini’s coordinate system to Unreal’s