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This release uses Arnold and OpenVDB 3.0.0.

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

  • 14.0.335
  • 14.0.291
  • 14.0.258

and for the following platforms:

  • Linux x86_64 (gcc4.4)
  • Windows 7 x64 (vc11)
  • Mac OS X 10.8+

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


The full release notes of Arnold are available here.

  • Faster cutout opacity mapping for textured mapped opacity mask renders, such as for tree leaves. Tests with production scenes have shown up to 25x speedups. This flag also toggles a fix for more accurate renders, where previously the object would be rendered more transparent when further away from the camera. When using this option, noise/flicker can occur on far away geometry, similar to when rendering far away high res geometry. If this happens, it is best to use a high amount of AA samples (and correspondingly lower amount of diffuse/glossy/light samples) to reduce flickering in animation. The fast opacity flag is enabled for anything that is connected to any RGB parameter called "opacity", so custom shaders that have an RGB opacity parameter will benefit from this.

  • Multiple scattering for volumes: Indirect light in volumes now supports an arbitrary number of bounces instead of being fixed to one bounce. It is now possible to render volumes such as clouds for which multiple scattering has a large influence on their appearance. The new Volume Depth parameter sets the number of bounces, defaulting to 0. The default value of Volume Indirect samples has been changed to 2. 

  • Support for deep volume output: Volumes are now visible in deep renders, but note that older "atmosphere" shaders and volumetric mattes are not supported yet. Previously, volumetric samples were composited with the next surface sample. Now, in case you want to implement your own deep driver, volumetric samples are available as independent samples to raw drivers or filters. In order to query the end of a volumetric sample you can request the new built-in float Zback AOV channel.

  • Per light volume contribution: A Volume contribution scaling parameter was added to lights, similar to the existing Diffuse and Specular parameters.

  • New EXR compression modes: Four additional compression modes, B44, B44A, DWAA and DWAB have been added to the OpenEXR driver. B44 is lossy for half data and stores 32-bit data uncompressed. B44A is an extension to B44 where areas of flat color are further compressed. DWAA and DWAB correspond to JPEG like compression from DreamWorks Animation. Note that DWAA and DWAB require the reading program to be compatible with OpenEXR 2.2 which is not yet widespread, Nuke 9.x will not read them.

  • Faster UDIMs: UDIMs accessed through the image shader node now internally use texture handles, which helps improve multi-threading performance.

  • OIIO improvements: Upgraded to OIIO to 1.5.15. 16-bit textures are now stored as 16-bit in the texture cache instead of as 32-bit floats. This halves the amount of memory 16-bit textures use.

  • Spaces are now allowed in EXR metadata names.

  • New Merge Vertex Attributes property on objects to allow the merging of coinciding, identical data for vertex attributes. This allows proper interpolation of UVs when subdividing for example and can also induce significant memory savings as duplicated data is factorized among the vertices. Note that this feature has an approximate 15% translation performance overhead and thus is disabled by default. (#506)

  • You can now set a User Option string on geometry, lights, cameras and ROPs. This allows you to set Arnold parameters on the generated nodes with the ASS file syntax. (#278)
  • Added option to create intermediate directories for output images and ASS files. (#577)
  • Added Python commands to refresh single textures and have them update in the IPR. See textureInvalidate()skydomeTextureInvalidate() and universeCacheFlush() in the htoa.ipr module for details. (#469)
  • The licensing status of  Arnold is now cached on startup to avoid the timeouts at each render in trial mode when you don't have a license. (#571)
  • Added volume and volume_opacity in built-in AOVs drop down menu. (#579)

  • New Diagnostics menu entry to test for version mismatch between the build and runtime Houdini and Arnold versions, which is a common installation error to troubleshoot. More tests will be added in the future. (#352)
  • The Python module now exposes the versions of Houdini and Arnold that were used to build HtoA along with some other compile-time information (#352)
  • Fixed installer PATH on Windows that caused the wrong Arnold version to load in MtoA when Maya was loading the HoudiniEngine plugin with HtoA installed with different version of Arnold (#575)
  • Restrict shape IDs to unsigned 24-bit integers as Nuke converts them to floats (#578)
  • Fixed Arnold properties such as subdivision, displacement or visibility not being set on master instances (#573)
  • OpenVDB plugin in procedural hangs on exit (csp#73)
  • Fixed Arnold Volume grid selection drop-down menus on Windows (#569)
  • Fixed crash when point velocity blur was enabled on instances but the motion blur was disabled (#580)
  • Restored Houdini 13 compatibility for Arnold Volume HDA (#583)
  • Degenerate primitives such as curves with a single point are now filtered out before sending to Arnold (#584)
  • Fixed regression where Force Phantom for objects was not enforced (#573)
  • Fixed bounding box calculation for OpenVDB files that prevented grids with internal transformations to render (#570)
  • Fallback to the standard geo_velocityblur when the Arnold specific ar_mb_velocity_enable property cannot be found on the object to toggle velocity blur (#587)
  • Fixed wrong velocity motion blur on point instances when the number of transform keys is greater than 2 (#582)
  • Fixed intermittent crash when translating procedurals on OSX (#547)
Incompatible changes


  • Crash with volumes when the P AOV is enabled and an object is behind or inside (core#4724)
  • Crash when using deep volumes without an RGBA AOV (core#4725)
  • Wrong camera in AOV definitions when exporting ASS files with custom camera shaders (htoa#595)
  • Fixed regression that zeroed procedural bounding boxes in contents mode (htoa#347)
  • Crash with empty folder metadata, notably for some of the alShaders (htoa#590)
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