[MAXtoA] Setting up an Object ID AOV

Here’s how to use the 3ds Max object ids for a simple Object ID AOV.

And here’s the material (aka shader tree) that writes the AOV. Use a text editor to save this as Arnold Scene Source (.ass) file, and then import it into 3ds Max (like I did in the video).

An ASS file is a plain-text file.

### exported: Thu May 21 07:53:19 2020
### from:     Arnold [991b08e9] windows icc-17.0.2 oiio-2.2.1 osl-1.11.0 vdb-4.0.0 clm- rlm-12.4.2 optix-6.7.0 2020/04/17 09:11:12
### host app: MAXtoA (2021) 3ds Max 23.0.915.2021 

 name /Write_Object_ID_AOV
 aov_input /gBufID_Switch
 aov_name "object_id"
 declare nodeName constant STRING
 nodeName "Material #43"

 name /gBufID_Switch
 index /gBufID
 input1 1 0 0 1
 input2 0 1 0 1
 input3 0 0 1 1

 name /gBufID
 attribute "gBufID"

Updating procedural file paths with string_replace

This is actually pretty cool…you can use an operator to update file paths before an ASS file or ABC file is loaded, then use another operator to touch the geometry loaded by that procedural.

For example, suppose at render time you want to replace trex_proxy.abc with trex.abc. You can easily do that with a string replace operator:

  • *.(@node==’alembic’) selects all Alembic procedural nodes
  • Match matches any file name that ends with “_proxy.abc”
  • Replace replaces “_proxy.abc” with “.abc”

And that all happens before the abc file is loaded.

We can see this in the Arnold log (Debug verbosity). First the string replace operator is applied; then after the abc file is loaded, a set parameter operator is applied to the nodes loaded from the abc file.

| initializing 16 nodes …
| [operators] init op: 'aiStringReplace1'
| [operators] cook op: 'aiStringReplace1' | node: '/aiStandIn/aiStandInShape'
| [proc] /aiStandIn/aiStandInShape: loaded 1 nodes (1 objects, 0 shaders)
| [operators] init op: 'TRex:tRexShape_aiSetParameter1'
| [operators] cook op: 'TRex:tRexShape_aiSetParameter1' | node: '/TRex:tRex/TRex:tRexShape'

How to use wireframe shader for alpha

You can use the wireframe shader to drive the opacity of a standard_surface.

The wireframe shader itself doesn’t support opacity, and neither do the flat, utility, and ambient_occlusion shaders. They are all color shaders (RGB). In Arnold, only shaders that return closures can support opacity.

In the more general case, where the opacity isn’t coming from the same shader, you can use the emission for the color:

The case of Maya and the missing ProductInformation.pit file

One of the strange thing about supporting Arnold at Autodesk is that you have to be a guru-level licensing expert on Autodesk licensing (not RLM, but Autodesk licensing).

In this case, Maya 2019 would silently fail at startup. Sometimes you’d see the splash screen, but then that would just disappear.

  • There was nothing in the Adlm.log
  • No MayaAdlm log was created
  • The MayaCLM log had only this:
    ERROR: checkoutLicense: Failed to authorize license
  • The clm log did have this:
    JsProductLicenseFact.loadSelectedProductInfoKey - Unable to get selected product key due to unknown problem.

TIP All these log files are in the Temp folder.

  • On Windows, look in %LOCALAPPDATA%\Autodesk\Logs
  • On OSX, look in $TMPDIR
  • On Linux, look in /var/tmp

Process Monitor confirmed that the ProductInformation.pit file was missing:

ProductInformation.pit is an all-important file used by the licensing infrastructure. Every Autodesk product must be registered in that pit file.

If ProductInformation.pit is missing, or corrupted, then everything stops working.

[macOS] Rendering xgen procedurals in another app

Or: how to fix the node xgen_procedural” is not installed problem.

For example, suppose you want to render XGen hair in CINEMA 4D on macOS.

  • From MtoA, export your XGen hair to an ass file.
  • Set DYLD_LIBRARY_PATH before you start your application (for example, open a Terminal, set DYLD_LIBRARY_PATH there, and start CINEMA 4D the command line)
    export DYLD_LIBRARY_PATH=/Applications/Autodesk/maya2018/plug-ins/xgen/lib:/Applications/Autodesk/maya2018/Maya.app/Contents/MacOS
  • Load the ass file into CINEMA 4D with an Arnold Procedural.
  • Set the Plugin Search Path to the MtoA procedurals folder

backdoor Arnold: setting visibility

If you want to control object visibility with operator, or the Arnold Python API, or by editing an ASS file, you need to understand Arnold’s visibility parameter.

In the UI, the object visibility options look like a bunch of separate parameters:


but in Arnold, all those options are stored in one visibility parameter.

For example, if an object is visible to the camera (primary visibility), and to transmission (both diffuse and specular), then that’s visibility 13.

Why 13? Because 13 = 1 + 4 + 8

  • camera rays = 1
  • diffuse transmission rays = 4
  • specular transmission rays = 8

The Arnold visibility parameter tells Arnold the rays to which the object is visible.

  • visibility 0 means the object isn’t visible to any rays
  • visibility 255 means the object is visible to all rays
  • visibility 253 means the object isn’t visible to shadow rays (so the object does not cast shadows).

Here’s the decimal values for all the different ray types.

To figure out the visibility, just add up the values for the rays you want.

Ray typeDecimal Value
Camera (Primary Visibility)1
Shadow (Casts Shadows)2
Diffuse transmission4
Specular transmission8
Diffuse reflection32
Specular reflection64
SSS (subsurface)128

kicking ass with debug shading

You can use kick to render with debug shading. Here’s the flags to use:

  • -is to ignore the shaders assigned to the shapes
    When you ignore shaders, a default utility shader is used to render the scene.
  • -sm sets the shade mode ( ndoteye lambert flat ambocc plastic metal)
  • -cm sets the color mode (color ng ns n bary uv u v dpdu dpdv p prims uniformid wire polywire obj edgelength floatgrid reflectline bad_uvs nlights id bumpdiff pixelerror)

Here’s some examples that show how to kick with different types of debug shading (I’ve used the Arnold Render View debug shading modes for these examples)


Debug shading kick flags
Basic: disable all shaders in the scene, switching to a gray ‘ndoteye’ shader; a very fast shading mode.
kick -is


kick -is -sm notdoteye
Lighting: renders the scene with a white lambert shader
kick -is -sm lambert
Occlusion: use ambient occlusion shading
kick -is -sm ambocc
Wireframe: displays geometry as a wireframe
 kick -is -cm polywire


kick -is -sm ndoteye -cm polywire
Normal: visualizes the normal vector (between 0 and 1, in tangent space)
 kick -is -sm flat -cm n
UV: displays the coordinates of the primary UV set (red=U, green=V)
 kick -is -sm flat -cm uv
Primitive ID: displays random colors based on the per-primitive (triangle, curve) index
 kick -is -sm flat -cm prims
Barycentric: displays intra-primitive parametric coordinates (barycentric for triangles, parametric length, and width for curve segments)
 kick -is -sm flat -cm bary
Object: displays random colors based on the per-object ID
kick -is -sm flat -cm obj