HtoA 6.5.1.0

25 March 2026

HtoA 6.5.1.0 is a feature release using Arnold 7.5.1.0. Arnold 7.5.1.0 includes a Tech Preview of Flow Render, custom AOVs for volume shaders, MIKKTSpace normal mapping support, and more.

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

Flow Render Tech Preview

HtoA 6.5.1.0 includes a Tech Preview of Flow Render, an Arnold cloud-rendering solution. We are providing 2400 minutes (per user, per month) of rendering time to Arnold users for testing and evaluating the service. Please report any feedback or issues in the Arnold forums.

See the Flow Render Tech Preview documentation for more information.

Note:

Flow Render is available for the default Houdini Qt versions ( Qt6 for Houdini 21, and Qt5 for Houdini 20.5 and Houdini 20.0)

Enhancements

Custom AOVs for volume shaders: The aov_write_rgb, aov_write_float, and aov_write_rgba shader nodes can now be used in volume shader networks, just as they already work for surface shaders. Previously, custom AOVs were silently ignored in volume contexts. The volume raymarcher now collects AOV closures at each step and integrates them using Beer-Lambert weighting, producing physically consistent AOV outputs. (ARNOLD-6548)
- Both flat (EXR, TIFF) and deep EXR outputs are supported.
- A warning is logged when aov_write_rgb nodes with conflicting blend_opacity settings are connected to both surface and volume shaders writing to the same AOV name.
- There is approximately a 16% overhead with two active volume AOVs at fine step sizes (step_size=0.01).



No Emission	Custom 'temperature' AOV mask	Beauty

New tangent_space_type option for normal_map: A new tangent_space_type parameter adds support for MIKKTSpace normal mapping. The standard mode preserves the previous behavior, while the mikk mode imports normal, tangent, and bitangent generated with MIKKTSpace in DCC tools such as 3ds Max and evaluates the normal map using MIKKTSpace for consistent shading across applications. (ARNOLD-8377)



Standard	MIKKTSpace

Faster CPU renders on Windows: CPU renders on Windows have been optimized. Most, though not all, scenes show faster render times. (ARNOLD-17549)

scene 7.5.1 speedup vs 7.5.0

OpenPBR shader ball 1.16x

gtc-robot 1.08x

Disney cloud 1.07x

ALab 1.04x

Intel jungle ruins 1.00x
Apply unit auto-scaling to linked shader parameters: Linked shader parameters are now auto-scaled to scene units when driven by links (for example, subsurface_scale from a texture), ensuring unit-dependent values retain their expected magnitudes across different scene unit settings. (ARNOLD-17176)
Custom position for nearest_points shader: The nearest_points shader has a new query_position parameter, which allows you to link to an input and customize the point where the point cloud is queried. When unlinked, sg->P (world-space shading position) is used instead. (ARNOLD-17360)
New interior_set string user data: The new interior_set attribute enables coherent scattering (such as transmission and SSS) across shapes that share the same set name. This allows smooth propagation of scattering between separate shapes, such as large ocean surfaces modeled as multiple tiles. interior_set replaces sss_setname, although sss_setname remains supported for now. If both attributes are present on the same object, interior_set takes precedence. (ARNOLD-9038)

scene	7.5.1 speedup vs 7.5.0
OpenPBR shader ball	1.16x
gtc-robot	1.08x
Disney cloud	1.07x
ALab	1.04x
Intel jungle ruins	1.00x



without `interior_set`	with `interior_set`

Qt Installer - The installer has been updated to Qt Installer Framework 4.10, the custom --extract and --silent flags present in the old installer have been replaced with the modern workflow and new command line flags:
- Silent install and Houdini package setup
  ./htoa-6.5.1.0_r34e511b_houdini-21.0.631_lin.run install --accept-licenses --confirm-command --root /opt/custom_htoa_install_dir
- Silent extraction and skip the license updater (which normally requires admin privileges)
  ./htoa-6.5.1.0_r34e511b_houdini-21.0.631_linux.run install --accept-licenses --confirm-command --root /opt/custom_htoa_install_dir ExtractOnly=true SkipLicenseUpdater=true
- Skip Flow Render install
  ./htoa-6.5.1.0_r34e511b_houdini-21.0.631_linux.run install --accept-licenses --confirm-command --root /opt/custom_htoa_install_dir InstallFlowRender=false

OSL generalized Schlick Fresnel exponent: The OSL generalized_schlick_bsdf closure now fully supports the exponent parameter for both reflection and transmission lobes. Previously, Arnold ignored the exponent (effectively always using the standard Schlick value of 5) and approximated the transmission lobe by converting F0 to a dielectric IOR via the Gulbrandsen method. Both lobes now use a proper generalized Schlick Fresnel evaluation with configurable f0, f90, and exponent. Scenes using generalized_schlick_bsdf with a non-default exponent, or with significant transmission, may produce slightly different renders. (ARNOLD-16834)
Bloom driver luminance threshold: Bloom now uses a smoothed luminance-based threshold to avoid hue shifts from per-channel clipping. (ARNOLD-17443)
Improved maketx error messages: TX conversion error messages now include the target TX filename, which makes it easier to identify the cause of the error. (ARNOLD-17044)
Include options node and driver outputs in stats JSON: Stats output now records the top-level options node and, for each image driver, an entry with the driver filename and AOV and output details, improving the metadata available for diagnostics and downstream tools. (ARNOLD-16812)
Enable CER on Linux: Added support for the Customer Error Reporting (CER) library on Linux. (ARNOLD-15747)

USD Enhancements

USD warnings and errors in Arnold logs: The USD composition warnings and errors when loading a file are now included in the Arnold log usd#2435

API Changes

volume_double_henyey_greenstein OSL support: The double Henyey-Greenstein phase function volume_double_henyey_greenstein, which corresponds to AiClosureVolumeDoubleHenyeyGreenstein, is now available in OSL. (ARNOLD-16701)

Incompatible Changes

OSL generalized_schlick_bsdf rendering change: Scenes using the OSL generalized_schlick_bsdf closure with a non-default exponent value (other than 5) or significant transmission may produce slightly different renders due to corrected Fresnel evaluation. (ARNOLD-16834)
Renamed enum AI_CLOSURE_DEBUG to AI_CLOSURE_AOV: Code referencing AI_CLOSURE_DEBUG must be updated. (ARNOLD-6548)

OSL generalized Schlick Fresnel exponent: The OSL generalized_schlick_bsdf closure now fully supports the exponent parameter for both reflection and transmission lobes. Previously, Arnold ignored the exponent (effectively always using the standard Schlick value of 5) and approximated the transmission lobe by converting F0 to a dielectric IOR via the Gulbrandsen method. Both lobes now use a proper generalized Schlick Fresnel evaluation with configurable f0, f90, and exponent. Scenes using generalized_schlick_bsdf with a non-default exponent, or with significant transmission, may produce slightly different renders. (ARNOLD-16834)
Bloom driver luminance threshold: Bloom now uses a smoothed luminance-based threshold to avoid hue shifts from per-channel clipping. (ARNOLD-17443)
Improved maketx error messages: TX conversion error messages now include the target TX filename, which makes it easier to identify the cause of the error. (ARNOLD-17044)
Include options node and driver outputs in stats JSON: Stats output now records the top-level options node and, for each image driver, an entry with the driver filename and AOV and output details, improving the metadata available for diagnostics and downstream tools. (ARNOLD-16812)
Enable CER on Linux: Added support for the Customer Error Reporting (CER) library on Linux. (ARNOLD-15747)

USD Enhancements

USD warnings and errors in Arnold logs: The USD composition warnings and errors when loading a file are now included in the Arnold log usd#2435

API Changes

volume_double_henyey_greenstein OSL support: The double Henyey-Greenstein phase function volume_double_henyey_greenstein, which corresponds to AiClosureVolumeDoubleHenyeyGreenstein, is now available in OSL. (ARNOLD-16701)

Incompatible Changes

OSL generalized_schlick_bsdf rendering change: Scenes using the OSL generalized_schlick_bsdf closure with a non-default exponent value (other than 5) or significant transmission may produce slightly different renders due to corrected Fresnel evaluation. (ARNOLD-16834)
Renamed enum AI_CLOSURE_DEBUG to AI_CLOSURE_AOV: Code referencing AI_CLOSURE_DEBUG must be updated. (ARNOLD-6548)

Bug Fixes

ARNOLD-15273 - Rare GPU artifacts with certain scene, GPU, and driver combinations
ARNOLD-17465 - Crash when a node turns invalid in IPR
ARNOLD-17498 - Fixed incorrect volume mipmap selection in certain situations
ARNOLD-17500 - Crash with invalid "idxs" user data
ARNOLD-17553 - Crashes with empty arrays in polymeshes
ARNOLD-17561 - [GPU] Matte value not inherited by child nodes
usd#2547 - Changing material terminal is not updated with Hydra 2
usd#2558 - Indexed user data shouldn't author the idxs companion to USD
usd#2563 - Incorrect region renders in Hydra when one axis is not cropped
usd#2566 - Fixed crashes when a render delegate is deleted while another is rendering
usd#2568 - Changing the root shader in IPR is not updated when it was present in the previous shading tree
usd#2572 - Arnold product types not working if drivers:parameter:aov:name is not defined
usd#2580 - String per-instance primvars cannot resolve indices
HTOA-3141 - Solaris viewport does not update for material changes
HTOA-3153 - Changing parmname in parameter vop prints an error
HTOA-3150 - Switch RGBA sourceName to color, which fixes an all black render channel in Nuke
HTOA-3127 - IPR crashes when switching frames or changing cameras
HTOA-3091 - Solaris: husk tiled render does not match full frame render
HTOA-3117 - Solaris IPR crash with looping value clips
HTOA-3192 - Delegate crash when duplicating Solaris render
HTOA-3037 - husk renders with arnold product type do not work with -o frame number variables
HTOA-2036 - OneDrive breaks package installation
HTOA-3022 - Ill-formed SdfPath warnings
HTOA-3180 - Connecting a subnet input breaks outputs in Solaris materials
HTOA-3195 - Import Materials does not set Color Space correctly
HTOA-3207 - Cannot change path on RenderProductPrim in Arnold Render Settings
HTOA-3208 - Change default thread count to -1 in render settings
HTOA-3209 - Expose cryptomatte "Object Split Instances" toggle on Arnold Render Settings
HTOA-3213 - Qt6 error on "Inspect Arnold Operators" popup
HTOA-3214 - Scene Import LOP fails for Arnold Usd procedural due to missing schema
HTOA-3220 - Fix Solaris viewport render settings defaults
HTOA-3210 - AiBegin/AiEnd are now only called once by the plugin, which improves stability when rendering in multiple scene views

System Requirements

Houdini, Houdini FX, Houdini Indie, and Houdini Education
- 20.0.896
- 20.5.684
- 21.0.631
Flow Render is available for the default Houdini Qt versions ( Qt6 for Houdini 21, and Qt5 for Houdini 20.5 and Houdini 20.0)
Linux with at least glibc 2.28 and libstdc++ 11.2.1 (gcc 11.2.1). 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.
GPU rendering and OptiX denoising work on Windows and Linux only, and require an NVIDIA GPU with the Maxwell architecture or later.
- On Linux, we recommend 590.48 or higher drivers.
- On Windows, we recommend 591.28 or higher drivers.
Intel OIDN runs on all CPUs. GPU support is limited to:
- macOS
  - Apple silicon GPUs (M1 and newer)
- Linux
  - NVIDIA GPUs with Turing, Ampere, Ada Lovelace, Hopper, and Blackwell architectures
- Windows
  - Intel Xe and Intel Arc dedicated and integrated GPUs
  - NVIDIA GPUs with Turing, Ampere, Ada Lovelace, Hopper, and Blackwell architectures
  - AMD GPUs with RDNA 2, RDNA 3, RDNA 3.5, and RDNA 4 architectures