This is a minor feature release, using Arnold and Arnold-USD (6b712999). It is fully binary compatible with 4.0.* versions. 

It brings improved Solaris support including support for rendering curves, and fixes issues with alembic packed primitive translation.


  1. Download Arnold for Houdini.
  2. Follow these installation instructions.


Autodesk Network Licensing in Arnold 6 requires new license files with an updated 2020 version. Please follow the instructions on this page to generate your license file. More info about Arnold 6 licensing can be found here.

Compatible Houdini Versions 

Binaries available for the following Houdini, Houdini FX, Houdini Indie and Houdini Education production builds:

  • 17.0.506
  • 17.5.460
  • 18.0.532
  • 18.0.566

Please note that Houdini Apprentice does not support third-party renderers and thus cannot run HtoA.


System Requirements


  • OSX 10.11 or later
  • Windows 7 or later, with the Visual Studio 2015 redistributable
  • Linux with at least glibc 2.12 and libstdc++ 3.4.13 (gcc 4.4.7). This is equivalent to RHEL/CentOS 6
  • CPUs need to support the SSE4.1 instruction set
  • Optix™ denoiser requires an NVidia GPU with CUDA™ Compute Capability 5.0 and above
  • Arnold GPU works on Linux and Windows and requires an NVIDIA GPU of the Turing, Volta, Pascal, or Maxwell architecture. We recommend using the 450.57 or higher drivers on Linux and 451.77 or higher on Windows. See Getting Started with Arnold GPU for more information.


  • htoa#1419 Support spotlight shaping parameters in Solaris
  • htoa#1422 Support mat context in Force Materials and Assign Material
  • htoa#1425 Add Arnold parameters for Render Vars LOP
  • htoa#1427 Add GPU settings to render settings LOP
  • htoa#1445 Add rim_light_tint to toon shader
  • htoa#1447 Update RenderVar parameter naming conventions
  • htoa#1453 Refactor Solaris LOP UI code
  • htoa#1459 Get GPU device list for Solaris render settings LOP
  • htoa#1472 Dynamic creation of render geometry settings
  • htoa#1481 Add preset for AA_inv_density to AOV list
  • htoa#1514 Create curve parameters for the render geometry settings LOP
  • htoa#1515 Increased default texture maximum memory

  • Automatic file path remapping: String parameter values can now be automatically modified according to the OS Arnold is running on. For example, this is useful when the artist workstations are running Windows but the renderfarm nodes are running Linux. Configuration is done with a JSON file that contains the strings to search and replace for each OS. This configuration file can be selected through the ARNOLD_PATHMAP environment variable.  (#9311)  For example:

           "this_path/": "some/other_path/windows/"
          "this_path/": "some/other_path/mac/"
          "this_path/": "some/other_path/linux/"
  • Rim light control for the Toon shader: A new rim_light_tint float parameter was added to the toon shader to tint the color of the rim light using the base color of the surface. See the full documentation. (#9003)

  • Performance improvements on Windows with many cores and texture-heavy scenes: Several performance improvements have been made, particularly for those running Windows, using many cores, tiff based textures, and/or reading many textures/sec. Expected speedups can be anywhere from a few percent to orders of magnitude faster on many-core Windows machines. (#9515, #9685, #9686, #9695, #9709, #9761) For a medium complexity scene that uses exr based .tx textures we get the following speedups on Windows :

    AMD 3970X (64 threads)439s397s1.11x
    AMD 3990X (128 threads)379s267s1.42x
    AMD 3995WX (128 threads)351s276s1.27x
    Intel Xeon 8280 x 2 (112 threads)356s337s1.06x
    Intel i9-9980XE (36 threads)748s720s1.04x

    For a texture heavy stress scene (8GB of tiff based .tx texture tiles end up being read in) we get the following speedups on Windows:

    AMD 3970X (64 threads)2002s9.9s202x
    AMD 3990X (128 threads)2103s10.1s208x
    AMD 3995WX (128 threads)2130s9.0s237x
    Intel Xeon 8280 x 2 (112 threads)1585s11.2s142x
    Intel i9-9980XE (36 threads)1145s16.8s  68x
  • String parameter path metadata: A new path metadata can be assigned to string parameters to give specific hints about how they will be handled. Currently available values are: filefolder and folderlist. Arnold will expand environment variables, apply the remapping, and handle backslashes on string parameters with the metadata. (#4969)
  • AA_adaptive_threshold in log: The options.AA_adaptive_threshold used is now output in the log file. (#9841)
  • Upgraded OSL: Arnold now uses OSL 1.11.6 bringing multiple optimizations and bug fixes, see the full list of changes. (#8917)

GPU Enhancements 

  • Global AOV shaders: The aov_shader option is now supported on GPU. This option defines a list of shaders that the renderer will evaluate after the regular surface shader. With this, it's possible to add shaders to set specific AOVs without modifying the original shader tree. See the option documentation. (#9394)
  • Initial support for attributes in OSL:  You can now retrieve user data bound to shapes via the getattribute function or via a lockgeom parameter attribute. This initial implementation still has some limitations when compared to the CPU implementation. Queryable attributes are currently restricted to those bound to the current shaded shape and attributes of the active render camera. Querying node parameters is also not yet supported. (#9666)
  • Initial support for tracing in OSL:  You can now use the trace function to trace rays in OSL shaders along with the getmessage function to retrieve information regarding any intersected object. This initial implementation has the restriction that you cannot yet query parameters on an intersected object. (#9797)

USD Enhancements 

  • UsdRender schema: UsdRenderSetting, UsdRenderProduct and UsdRenderVar are now supported in the procedural. (usd#453)
  • Improved Solaris Primvar support: Primvars with arrays of a single element are converted to non-array user data in Arnold. This improves primvar support in Houdini Solaris. (usd#456, usd#463, usd#478)

  • Curve support in the render delegate: Curves now render in the Arnold render delegate (usd#483)

Incompatible Changes 

  • AiMsg* changes: Changes have been made to how logging behaves. Now the log session is for the life of the Arnold process instead of just between AiBegin/AiEnd pairs. If messages are not showing up or too many messages show up, you might need to set the logging flags and other options before calling AiBegin. Another change is that you can now set the log filename at any point in time, each time AiMsgSetLogFileName is called the previous file will be closed and the new file will be created. See the API documentation for the full set of functions. (#9575)
  • Increased default texture maximum memory: The options.texture_max_memory_MB default value has been raised from 2048MB to 4096MB. (#9846)

Bug Fixes

  • htoa#1478 IPR regression fix for reconnecting shaders
  • htoa#1480 Object Merge with Alembic and Polysoup fix

  • htoa#1484 Show mat context vops in the operator path selection menu filters
  • htoa#1485 Fix alembic packed primitive translation
  • htoa#1486 Fix multiple retimed packed alembics
  • htoa#1500 Instancer LOP uses the Cd attribute
  • htoa#1509 Include graph target operator menu fix
  • htoa#1518 Expose Archive tab for Houdini Indie

  • #9828 AiMetaDataGet/Set functions crash with a NULL node entry
  • #9746 [Alembic] sampled faceset data causes crash
  • #9816 [Alembic] Velocities data on curves not applied correctly
  • #9515 Allow IPR to properly scale past 64 cores on Windows
  • #9809 Allow maketx to use either single dash or double dashes for arguments
  • #9684 Arnold ignores non-linear output color spaces on images wider than 8 bit
  • #9020 Back slashes in strings get changed to forward slashes when queried or written to .ass
  • #9672 Better duplicate output detection
  • #9749 BVH with 3+ motion keys is missing AiProfileBlock
  • #9575 Crash when calling AiMsg while the Arnold session is ending
  • #9725 [GPU] Arnold prematurely runs out of memory with on demand textures
  • #9748 [GPU] Crash in GPU prepare when failing to compile programs
  • #9794 [GPU] fov not set when using custom perspective camera
  • #9852 [GPU] GPU crash with multiple shaders connected to the UV camera post_bake
  • #9580 [GPU] Incorrect "Insufficient device memory" error when using NVLink
  • #9692 [GPU] Incorrect result when calling user data shaders from uv_camera.post_bake
  • #9771 [GPU] OptiX denoiser incorrectly accumulates feature buffers in progressive mode with multiple filters
  • #9774 [GPU] OSL: Don't throw exceptions when getattribute is called
  • #9636 [GPU] OSL texture crash when toggling devices
  • #9202 [GPU] Precision issue in bump mapping
  • #9532 [GPU] Random crash during the GPU prepare stage
  • #9801 [GPU] "Unable to load Optix library" error on Tesla and TCC GPUs on Windows
  • #8956 kick logs show superfluous plugin loading messages before official log start
  • #9758 Make smart opaque robust to numerical precision error
  • #7934 mesh_light with degenerate triangles has poor performance
  • #9522 [OSL] Arnold can fail to find stdosl.h when the path to Arnold contains specific characters on Windows
  • #9840 [OSL] Crash when SIP is enabled on MacOS 10.14 when using OSL
  • #9607 Parsing errors in ass file parameters should abort scene loading
  • #9659 Potential memory leak when using _triggers_reinitialize metadata
  • #9753 Prevent texture cache from being smaller than 2GB
  • #9863 Print license error details at warning level
  • #9845 Python binding for AiParamGetType returns c_int instead of c_uint8
  • #9762 Slow build with many ginstances of many-prim objects
  • #9595 Slow node update of VDB volumes on Windows and Linux even when volume did not change
  • #9610 Spurious render message when using -default_nodes in kick
  • #9644 Using AOV write shaders increases texture usage
  • #9638 Wrong exterior illumination with portal lights in interior_exterior mode
  • usd#463 Texture coordinates of texcoord2f type are not read correctly


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