Hair rendered using XGen (image courtesy of Jonas Tornqvist).

MtoA supports Maya's XGen, a powerful geometry instancing tool. There are some current limitations when using XGen with MtoA. These are as follows:  

Tutorials about working with XGen can be found here. You can find information on how to render .ass files with XGen here.

Arnold Output Settings

To show the Arnold output settings: 

  1. In the XGen Editor, go to the Preview/Output tab.
  2. In the Output Settings section, select Arnold Renderer in the Renderer list.

Arnold Settings

Render Mode

Live allows XGen to query patches and guide animation directly from Maya. This does not support Motion Blur and will try to reuse information the preview generated.

Batch Render mode will always read geometry and animation from files. Batch render mode is always on when doing a batch render. You must first export the patches and guide animation to caches. Use this mode to make sure your caches are properly exported before doing a batch render.


Apply default Arnold XGen Hair Shader to the current Description. The shader reacts to root_colortip_color and back_color XGen custom shader parameters.

Arnold Curve Settings

Min Pixel Width - Arnold's minimum pixel width for curves. More information here.

Mode - Curve Primitives can be rendered as ribbon or thickTo get 'oriented' facing splines that do not face the camera, you must disable Face Camera in the XGen Primitives tab. 

Arnold Motion Blur Settings 

Choose between Global Settings (uses the MtoA motion blur Render Settings), On or Off. 

Motion Blur and Splines

It is important that you use the following steps when rendering motion blur with XGen splines.

  • Ensure that your scene is saved.
  • With the geometry and XGen description selected, go to 'File>Export Patches for Batch render'.
  • Enable motion blur in the XGen description or in the Render Settings (remember to choose Global settings in the XGen Description).
  • Save the scene again.

Arnold Advanced Settings

When using 'export patches for batch rendering', XGen will create an .abc file that is stored in the same folder as the scene. 'Use Aux Render Patch' enables you to explicitly choose the file that you want to use, for example, a previously exported .abc file.

Use Aux Render Patch

Enable/disable Auxiliary Render Patch.

Auxiliary Render Patch

File path used for the exported .abc file.

Spline Opacity 

To change the Opacity of XGen splines, you must first select the XGen description and then disable Ai Opaque within the Extra Attributes of the Attribute Editor. 

Spline Color

To obtain color information from XGen splines, such as those found in the XGen Presets, you will need to add the following steps.

A scene that demonstrates this process can be downloaded here.

  1. Go to the XGen window (Preview/Output tab) and make a note of the custom shader parameter names. 

2. Assign a shader to the XGen description.

3. Connect an aiUserDataColor shader to the color attribute of the shader that you want to use (e.g. root color).

aiUserDataColor shader connected to standard_hair attributes

4. Add the name of the attribute to the Color Attr Name attribute of the aiUserDataColor shader. In this case it is 'root_color'.

XGen hair color rendered using aiUserDataColor

Curve Implicit UVs

Implicit UVs on curves (root-to-tip) can be enabled in the Ramp's Extra attributes.

When disabling CurveImplicit UVs the ramp will be applied over the real UV coordinates

UVs are supported in "XGen splines". This is an XGen shape (available in Maya 2017), found under Generate->Create interactive groom splines.