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'

Auto-instancing in Arnold 6.0.2

Controllable auto-instancing on ASS procedurals: You can now disable the default automatic instantiation of procedurals pointing at the same ASS file with the auto_instancing parameter on each procedural or by the procedural_auto_instancing option. This workaround is sometimes useful when overriding procedural parameters with operators.

Arnold 6.0.2 release notes https://docs.arnoldrenderer.com/x/1gGvBg

So, what’s all that mean?

It means that if you load the same ass file many times, Arnold will load the ass file just one time, and then automatically create instances of that. For example, if I load an ass file three times, I will get two instances.

In previous versions, this was known as the procedural cache, and it was a global option. Now it’s called auto_instancing and you can set it on each procedural node (aka aiStandin in Maya).

In general, you want auto instancing, because instancing is more efficient than loading the same ass file over and over. But if you’re using operators to apply different looks to the same procedural, you need to turn off auto instancing. Otherwise all the procedurals will have the same look (because they all be instances of the same one procedural).

For example, with auto instancing on, I get this, even though I’ve assigned different looks to each procedural (standin):

Everything has the same look with auto instancing on

I can also tell from the Arnold log that I’m getting instances. Note that 2 are reused

| ---------------------------------------------------------
| ass file cache           
|   unique (loaded from disk)              1 (33.33%) 
|   reused (found in cache)                2 (66.67%) 
|   total referenced .ass files            3 (100.00%) 
| ---------------------------------------------------------

If I turn off auto instancing (in the procedural parameters)

then I get three different looks

In the Arnold log, that looks like this (0 reused means 0 instances)

| ---------------------------------------------------------
| ass file cache
| unique (loaded from disk) 1 (100.00%)
| reused (found in cache) 0 (0.00%)
| total referenced .ass files 1 (100.00%)
| ---------------------------------------------------------

[MtoA] Using standins with MASH dynamics


Here’s a way to use standins with MASH dynamics: instance a low-poly mesh that has its Arnold Translator set to procedural.

  • Use a low-poly primitive as a proxy for the standin
  • In the low-poly primitive Attribute Editor, set Arnold Translator to procedural, and enter the path to the ASS file in the Path box.


  • Instance that proxy object with MASH, and apply MASH dynamics

[FAQ] Can I transfer Arnold shaders to another app?

For example, can I transfer Arnold shaders from 3ds Max to Maya? Or from Houdini to Maya?

The answer is no. The Arnold plugins don’t include code for reading shader nodes from an ASS file and creating shader trees inside the host application (eg Maya, Houdini, or Cinema 4D).

You can use ASS files to take shapes and shaders from one app and render them in another app, but those shapes and shaders are never created inside the other app.

An MtoA Standin (aka an Arnold Procedural in HtoA and C4DtoA) references an ASS file, and passes that on to Arnold at render time.


The ASS file contains Arnold nodes (shapes and shaders). Those Arnold nodes are not created inside the 3D application.

Instead, the standin node is translated to an Arnold procedural node, and at render time, that procedural node is expanded, and all the Arnold shapes and shaders are created inside the Arnold scene.

[MtoA] Scripting Standin paths


To set a standin path, use the aiStandin.dso attribute:

import maya.cmds as cmds
cmds.setAttr( 'ArnoldStandInShape.dso','C:/Users/SOLIDANGLE/Documents/BlueDog.ass',  type='string' )
print cmds.getAttr( 'ArnoldStandInShape.dso' )

Here’s one way to list some of the attributes on a standin shape node, and find out what attribute to set:

import maya.cmds as cmds
ntype = cmds.nodeType( "ArnoldStandInShape" )
print cmds.attributeInfo( inherited=False, t=ntype )

for x in cmds.attributeInfo( inherited=False, t=ntype ):
    print "%s = %s" % (x, cmds.getAttr( "ArnoldStandInShape.%s" % x ))

[MtoA] Using standins to load animation sequences

To load an animated sequence into a standin:

  • In the Attribute editor for the StandInShape, under File/Frame, your Path should be something like example.####.ass (you must use four # symbols for the frame number pattern)
  • Enable Use Frame Extension
  • In the Frame box, type “= frame” without the quotation marks, and press ENTER. That creates an expression that uses the current frame number to specify which ASS file to load.

[SItoA] Expressions for standins of animated sequences

A standin can load an animation sequence, such as circling_cubes.[1..25].ass.
For example, if you open an Arnold_Standin property, click Browse, and select a sequence, Softimage automatically inserts a [Frame] token into the file name:


By default, that [Frame] token resolved to the current frame, but you can override that and use an expression to drive the sequence.

For example, if you have a 25-frame sequence of ASS files, you may want to play the 25 frames and then stop. To do that, you could use this expression:


If you wanted to continuously loop the 25 frame sequence, you could use an fmod (modulo) expression like this:

cond( fmod( Fc, 25 ) == 0, 25, fmod(Fc,25) )

And if you wanted to show just frames 10 to 25, you could use this expression:

cond( Fc<10 , 10 , MIN(Fc,25) )

If you’re not sure about these expressions, try playing with them in the Expression Editor. As you move through the timeline, watch the Current Value update in the Expression Editor.

[SItoA] Applying materials to standins

In Softimage, a standin object is the polygon mesh, hair, or point cloud with the Arnold_Standin property.

If you apply a material to a standin, that material will override any materials specified in the ASS file referenced by the standin. If you don’t want that to happen, give your material a name that starts with “Scene_Material” or “Standin_Material” (it doesn’t matter if you use uppercase or lowercase for the names).

If you do that, the standin object will use that material, but the objects in the ASS file will use the materials saved in the ASS file.

Here’s what you get in an exported ASS file. If SItoA finds a standin with a material name that starts with “scene_material” or “standin_material”, it exports a procedural node with no material.

 name standin-cube.SItoA.41000
 dso "\\server\project\Arnold_Scenes\XSI_Man.ass"
 min -4 0 -4
 max 4 0 4
  1 0 0 0
  0 1 0 0
  0 0 1 0
  0 0 0 1 

Otherwise, if you name the standin material something else, the procedural node will include a shader node for that material:

 name standin-cube.SItoA.41000
 dso "\\server\project\Arnold_Scenes\XSI_Man.ass"
 min -4 0 -4
 max 4 0 4
  1 0 0 0
  0 1 0 0
  0 0 1 0
  0 0 0 1 
 shader "Sources.Materials.DefaultLib.Test.standard.SItoA.40000.1" 
 declare procedural_shader constant ARRAY NODE
 procedural_shader "Sources.Materials.DefaultLib.Test.standard.SItoA.40000.1"