26 March 2025

HtoA 6.4.1.0 is minor feature release using Arnold 7.4.1.0.

Arnold 7.4.1.0 includes significant performance improvements, initial GPU support for Toon shading, a new HTML-based Render Stats Report, and many bug fixes.

Installation

• Download Arnold for Houdini from your Autodesk Account. See Download Arnold for more information about downloading HtoA from your Autodesk Account.

• Follow these installation instructions

Enhancements

• USD Crowd Export for classic HtoA - A new USD agent translation option was added to the new **Arnold > Shapes > Crowds** tab on Object nodes. USD crowd export uses considerably less disk space and supports per-frame or multi-frame caches. (HTOA-2658)

  

• Direct USD Render ROP Improvements - The UI logic for 'Direct USD Render' has been ported from the USD Render **LOP** (/stage) to the USD Render **ROP** (/out). (HTOA-2825)

• Render stats report: An HTML-based, interactive viewer for the Arnold render statistics. You can quickly visualize and inspect detailed render stats such as frame render time, memory usage, render time by category or by node, and texture usage. If you're using kick, add the -report flag. Arnold plugins like MtoA and HtoA include Render Report options in the Diagnostic render settings. (ARNOLD-14140)

• GPU Toon shading: Arnold GPU now includes initial support for the Toon shader and the contour filter. The Toon shader is currently limited to rendering direct lighting only (no reflections, refractions, or indirect lighting). (ARNOLD-15656, ARNOLD-15988)

• Faster procedural instancing: Arnold is faster at rendering scenes with complex hierarchies and point instancers. The new options.procedural_instancing_optimization flag usually produces speedups between 1-2x, but for sufficiently complex scenes the speedup can be significantly larger. This new optimization defaults to exhaustive mode, which gives the most speedup. Exhaustive mode may use more memory; you can switch to conservative mode to use only the optimizations that do not consume additional memory. This setting is currently only supported for batch CPU rendering. (ARNOLD-14208)

• Better scaling for parallel scene init/update: The parallel scene init and update are now faster than before, especially on many-core machines. (ARNOLD-14902, ARNOLD-15718)

• Reduced scene update time with progressive batch renders: Negative AA progressive renders no longer perform a redundant full scene update on the final pass. (ARNOLD-15974)

• Improved interactivity when using only skydome and distant lights: Scenes that contain only skydome and distant lights are now significantly more interactive, especially in scenes with many nodes. A scene with close to nine million nodes that rendered in about 1fps now allows for more than 20fps. (ARNOLD-16114)

• Light transforms in point instancer: When an instancer contains lights, it now takes their local matrix into account if instance_inherit_xform is enabled. (ARNOLD-16028)

• Improved Global Light Sampling: Rendering photometric (IES) lights and mesh lights using Global Light Sampling is now higher quality, using the same render settings and taking the same render times. (ARNOLD-13692, ARNOLD-15859)
  
  
  
  

  Before-and-after comparison: improved renders of various IES profiles (with equal render times and render settings)

  
  
  

  Before-and-after comparison: improved renders of a motion-blurred mesh light (with equal render times and render settings)

• Lights in procedural instances: When you instance a procedural, any lights created by the procedural are copied for each instance. For example, instances of ASS or USD assets with lights now render the lights properly. (ARNOLD-15657)

• Imager run once mode: Imagers can now be set to run only once, at the end of a bucket or the end of a completed final frame. This can be useful for particularly expensive imagers and can be enabled via the ImagerProperties::run_once property. (ARNOLD-12191)

• OpenColorIO 2.4.1: OpenColorIO has been updated to v2.4.1. (ARNOLD-15681)

• Improved MaterialX support: MaterialX has been upgraded to v1.38.10-OpenPBR, this new version brings support for the OpenPBR MaterialX node, improved Lama support and new nodes such as the NPR nodes. See v1.38.10, v1.38.9 release notes for details. (ARNOLD-15659)

• Update Autodesk Desktop Licensing installer Arnold now uses Autodesk Desktop Licensing installer version 15.1, which includes the following components: AdskLicensingService 11.16.6.5912, AdskLicensingAgent 5.20.12.4281, and AdskLicensingInstHelper 2.13.1.2514.

USD Enhancements

• Distant Lights normalize support : Distant lights now support the normalize attribute, so you can now disable normalize (before it was always enabled and the normalize attribute was ignored). You can use the Arnold option usd_legacy_distant_light_normalize to roll back to the previous behaviour. (usd#2176)

• USD 24.11: The USD procedural now uses USD 24.11.

• Hydra-based USD translation: When you render USD, Arnold now uses Hydra by default to handle the translation of USD to Arnold, which makes the render consistent with the Hydra render delegate usd#2248. You can roll back to the previous behavior with options.usd_legacy_translation on, or by setting the environment variable PROCEDURAL_USE_HYDRA=0. (ARNOLD-15977).

• Reduced USD memory usage: USD geometry buffers are shared with Arnold, which removes one copy of the scene in interactive mode usd#2148. As an example, rendering a high resolution USD mesh with 30 million polygons now consumes 5.5 Gb instead of 7.5 Gb to render. During batch renders, the Hydra-based USD procedural now releases the USD stage memory once the translation is done usd#2228.

API Additions

• Replace AtImagerSchedule with AtImagerProperties: The imager_prepare API has a new AtImagerProperties structure that provides more control over how an Imager should run. (ARNOLD-15774)

  Setting Imager bucket / full frame schedules will now work like so:

   imager_prepare
   {
      if (run_imager_full_frame)
         properties.schedule_full_frame = true;
      else
         properties.schedule_bucket = true;
   }

• Improved return values for device selection API: AiDeviceSelect and AiDeviceAutoSelect now return the AtDeviceSelectErrorCode enum instead of an int to better describe the error status of the API call. (ARNOLD-14652)

  AtDeviceSelectErrorCode
  SUCCESS	Device selection was successful
  FAILURE_NO_DEVICES_FOUND	No Arnold compatible devices found
  FAILURE_NAME	No matches found for options.gpu_default_names regular expression
  FAILURE_MEMORY	No matching device has free memory greater than options.gpu_default_min_memory_MB threshold
  FAILURE_INVALID_ID	Invalid device id in selection
  FAILURE_UNSUPPORTED	Unsupported device in selection
  FAILURE_UNKNOWN	Generic error

• AiProceduralExpand now updates procedurals: This API function now both initializes and updates the procedural node, along with expanding (loading) the children nodes. This ensures any custom procedural_update functions are called. (ARNOLD-15898)

• Viewport proxies for points, polymeshes, and volumes: AiProceduralViewport() now accepts non-procedural nodes such as points, polymesh, and volume, and returns viewport proxy representations. (ARNOLD-15941)

• Render stats report API: AiReportSetFileName() sets the file name so that the render creates a render stats report (an html file), AiReportGetFileName() gets the render report file name. (ARNOLD-14140)

• Cryptomatte metadata: Common cryptomatte metadata is now included in the Arnold core, so plugins do not require an mtd file for cryptomatte. (ARNOLD-15661)

Incompatible Changes

• Arnold 7.4 breaks binary compatibility. Shaders, procedurals, and other plugins compiled against older versions of Arnold are not compatible with this version of Arnold and must be updated and recompiled.

• Autodesk Network Licensing version: Arnold 7.4 switches to ARNOL_2026 licensing. (ARNOLD-15400)

  • Single-user subscriptions will work with Arnold 7.4. The plugin installers automatically register the new version. If you do not use a plugin, then after you download Arnold, open the Arnold License Manager and click Register.
  • Existing network licenses will not work with Arnold 7.4. Log on to your Autodesk account to generate your new ARNOL_2026 licenses.
  • If you do not run the installers, and you manually register Arnold, then use--pk C0PR1 --pv 2026.0.0.F with the AdskLicensingInstHelper command.

• Removed sss_use_autobump option: Use the per-object autobump_visibility flag. (ARNOLD-15782)

• Removed legacy_volume_albedo: This option is no longer available as the new volume albedo is more useful. (ARNOLD-15782)

• Removed AiMsgSetCallback(), AiMsgAddCallback(), AiMsgResetCallback(): Use AiMsgRegisterCallback() and AiMsgDeregisterCallback() instead. (ARNOLD-15782)

• Removed AiASSLoad(), AiASSWrite(), AiASSWriteWithMetadata(): Use AiSceneLoad() and AiSceneWrite() instead. (ARNOLD-15782)

• Removed deprecated versions of AiOrenNayarBSDF() and AiMicrofacetSetThinFilm(): Use the version with the same function name but additional arguments. (ARNOLD-15782)

• Removed AiLoadImage(): Use AiReadImage() instead. (ARNOLD-15782)

• Deprecated enable_new_quad_light_sampler and enable_new_point_light_sampler options: The new samplers should always be better, so these options are marked as deprecated and will be removed in a future version of Arnold. (ARNOLD-15782)

• Distant Lights normalize support : Distant lights now support the normalize attribute, so you can now disable normalize (before it was always enabled and the normalize attribute was ignored). You can use the Arnold option usd_legacy_distant_light_normalize to roll back to the previous behaviour. (usd#2176)

• Single full scene update in batch: Arnold now only does a single full scene update for render sessions in batch mode. This can cause issues for plugins allowing the modification of the scene in batch sessions, as dependencies between nodes are not tracked. Plugin developers should use interactive render sessions when modifying the scene. (ARNOLD-15974)

Bug Fixes

• HTOA-2424 - Invoking the Arnold ROP from a render COP node now works as expected in Houdini versions 20 and 20.5
• HTOA-2419 - Sick is now signed with the same entitlements as hick, this fixes a dylib loading error on OSX
• HTOA-2822 - Hick now handles AiDevice* errors explicitly
• HTOA-2866 - Fixed the cryptomatte VOP's parameter interface
• HTOA-2867 - Ramp parameters on custom nodes are now translated properly
• HTOA-2868 - Edit Arnold Material does not remove image filenames if they do not exist or have tokens
• HTOA-2870 - Edit Arnold Material does not duplicate image nodes
• ARNOLD-15723 - No visible layering when layering a dielectric on top of a diffuse Lama shader. In MaterialX, layering of a Lama dielectric node on top of a Lama diffuse node was not generating the correct appearance of the diffuse surface coated with the dielectric. The OSL layering logic in Arnold has been improved to fix this issue.
• ARNOLD-15713 - [MaterialX] Mix shader does not work as surface shader
• ARNOLD-15834 - Sampling noise is now consistent across renders
• ARNOLD-15686 - Utility AOV shader crashes Arnold if skydome has a texture
• ARNOLD-15842 - OSL shader not rebuilt when a parameter is modified
• ARNOLD-15858 - Per-light sampling performance regression fix
• ARNOLD-15892 - Procedural bounds are not updated when its child objects undergo motion transforms
• ARNOLD-15905 - Lens effect doesn't update correctly when the render resolution changes
• ARNOLD-15914 - [GPU] Arnold should throw an error when the shader stack size cannot be determined
• ARNOLD-14467 - [GPU] GPU crash during JIT compilation
• ARNOLD-15939 - Nodes created in procedural_update ignore procedural matrix
• ARNOLD-15942 - Crash when modifying a parameter that triggers procedural reloading during scene update
• ARNOLD-15943 - Procedural bounding box is not recalculated after creating or removing nodes in procedural_update, leading to clipped objects in renders or poor performance.
• ARNOLD-15948 - Crash when destroying an object node from procedural_update
• ARNOLD-15983 - OpenEXR DWAA float textures read incorrectly
• ARNOLD-15993 - Raw drivers receiving incorrect number of samples
• ARNOLD-16000 - Dependencies should not be added when enable_dependency_graph is disabled
• ARNOLD-16005 - Hang in AiProceduralViewport with ginstance nodes
• ARNOLD-16041 - [MaterialX] connections to named nodegraph outputs do not work
• ARNOLD-16050 - [MaterialX] ND_mix_surfaceshader renders black when assigned from materialx operator
• ARNOLD-15985 - [MaterialX] Parameter with "defaultgeomprop" attribute set overrides connection
• ARNOLD-15986 - [MaterialX] Crash when node with the same name exists inside a nodegraph
• ARNOLD-16053 - Triplanar produces artifacts on GPU with some NVIDIA drivers
• ARNOLD-16060 - Lights under procedurals cannot be transformed during interactive renders
• ARNOLD-16073 - Prevent instancer from setting invalid custom names to child nodes
• ARNOLD-16081 - Subdivided meshes included in cached procedurals were not always correctly identified, which might cause differences in the subdivision results
• usd#2201 - Hydra procedural should not modify the input usd stage with shutter range
• usd#2197 - MaterialX textures not applied properly in hydra through mayaHydra
• usd#2208 - Fix unnecessary allocations in the instancer and mesh
• usd#2218 - Fix Hydra warning with orthographic cameras
• usd#2219 - Fix race condition in Hydra with node names
• usd#2224 - Fix warning "HdArnoldDriverMain is already installed"
• usd#2225 - Fix crash in point instancers with missing prototypes
• usd#2232 - Fix incorrect husk render of left-handed indexed meshes with normals
• usd#2234 - Fix warning "Selected Hydra renderer doesn't support prim type 'RenderSettings'"
• usd#2239 - OpenVDB asset with explicit fieldName does not render in Hydra
• usd#1402 - Incorrect transform when exporting parented objects to USD
• usd#2227 - Hydra procedural breaks the Arnold log
• usd#2240 - Default volume shader should be assigned in the USD procedural

System Requirements

• Houdini, Houdini FX, Houdini Indie and Houdini Education
  • 19.5.805
  • 20.0.896
  • 20.5.522
  • 20.5.550
• Linux with at least glibc 2.28 and libstdc++ 9.3.1 (gcc 9.3.1) for Houdini 19.5, 20 and 20.5. This is equivalent to RHEL/CentOS 7.
• Linux with at least glibc 2.17 and libstdc++ 11.2.1 (gcc 11.2.1) for Houdini 20/20.5. This is equivalent to Rocky 8.
• macOS 11 or later
• Apple Mac models with M series chips are natively supported by HtoA
• x86-64 CPUs need to support the SSE4.1 instruction set. Apple Mac models with M series chip are supported natively mode.
• GPU rendering works on Windows and Linux only and requires an NVIDIA GPU with the Maxwell architecture or later.
  • On Linux, we recommend 560.35.03 or higher drivers.
  • On Windows, we recommend 560.76 or higher drivers.
• Optix™ denoiser requires an NVidia GPU with CUDA™ Compute Capability 5.0 and above.
• Intel OIDN GPU support is limited to:
  • MacOS
    • Apple native CPUs (M1 and newer)
  • Windows
    • Intel Xe dedicated and integrated GPUs
    • NVIDIA GPUs using Turing or newer architectures