Enhancements

• Improved progressive and adaptive sampling: Better progressive sampling patterns lead to increased noise convergence rate and faster adaptive sampling termination. (#8447)
• clip_geo shader: The new clip_geo shader will clip out all geometry against the shapes it has been assigned to. You can limit the objects that are affected with trace sets, and optionally choose a specific shader for the intersection surfaces. Note that self-intersecting geometry or intersecting clipping objects can cause artifacts. (#7274)
• AOV read shaders: The new aov_read_* shaders let you access built-in AOVs in the AOV shader context or custom AOVs in the regular shading context. They can be useful to reformat built-in AOVs, for example to output normal maps derived from the built-in N AOVs when baking to texture. (#7787)
• Alembic Layering: The alembic procedural now supports Alembic layering, where extra Alembic files specified via the new layers parameter will override properties on top of the Alembic file. (#8467)
• Alembic Arnold Properties: The alembic procedural will now set Arnold parameters from arbGeomParams if they match any Arnold parameter name and type. (#7577)
• Alembic Material Attribute: The alembic procedural has a new material_attribute optional parameter that defines the Alembic property that contains the material names, which will be used to create the per polygon material assignments. (#8535)
• Support for the MaterialX standard library: The materialx operator now supports look development using nodes and node graphs in the MaterialX standard library. Shading models defined using the PBR library are generated using the MaterialX Arnold OSL shader generator. (#8453)
• Disable assignment expressions: Assignments in the set_parameter operator can be now be individually disabled. (#7182)
• Individual parallel init selection for procedurals: You can now tag individual procedural types for serial or parallel intialization. All built-in procedurals are tagged for parallel initialization, but custom procedurals are serial by default, unless their parallel_init boolean metadata has been set. This is useful to tag non-thread safe procedural without disabling parallel initialization altogether. (#7271)
• Autodesk analytics program: Arnold can now optionally send anonymous statistics data back to Autodesk for analysis. This will help us optimize Arnold for the workflows and systems that are commonly used. This service can be opted-in either by calling the AiPromptADPDialog() API, through the kick -ADP flag, or through other Autodesk applications that support this, such as Maya. For the moment this is only available for OSX. (#8433)
• oiiotool: oiiotool is now distributed with Arnold. (#8501)
• Improved round corners: round_corners no longer generates non physical normals, this prevents excessive darkening near objects' silhouettes. (#8365)
• Faster individual node destruction: Destroying individual nodes with AiNodeDestroy() is now substantially faster in scenes with many global-scope nodes. Individually destroying 500k ginstance nodes went from 50s to about 2s. (#8343)
• Faster scene creation and destruction on Windows: Poor Windows performance when creating and destroying scenes has been greatly improved so that performance is almost as fast as Linux and OSX. For instance, the "releasing resources" step at the end of a render with 29k curves objects went from 45s to 4s. (#4399)

GPU enhancements

• Open Shading Language: Initial support for OSL has been added to the GPU. Like on the CPU you can mix and match OSL and compiled shaders. Note that not all closure and shading ops are supported yet, refer to the  supported features and known limitations docs for details. (#5975)
• OpenVDB Volumes: Initial OpenVDB support, note that this initial implementation is likely to use more memory on the GPU than on the CPU. (#7051)
• Volume displacement: The volume displacement and padding logic has been implemented on GPU. (#7606)
• Lights compatibility : Added support for light portals, cylinder lights, disk lights, and low light threshold. (#8531, #8540, #8557, #8551)
• Standard Surface compatibility: Added support for Randomwalk v2 SSS and transmission_dispersion. (#8253, #8253)
• Less noise in indirect samples: Better stratification now reduces the noise in indirect samples, for example with refractive surfaces. (#8615)
• Better GPU cache pre-population ETA: More accurate GPU cache prepopulation time remaining estimate. (#8361)

API additions

• Earlier listing of built-in AOVs: Built-in AOVs and LPEs are now also listed by the AtAOVIterator, no longer requiring an AiRender() call. (#8431)
• New functions for custom procedurals: Custom procedurals can now define new functions procedural_update and procedural_finish (equivalent to node_update and node_finish). Those custom functions (if present) will be executed during scene update and during procedural destruction, respectively (#7961).
• AiPromptADPDialog(): Call AiPromptADPDialog() in order to opt-in or out of the Autodesk analytics program (ADP). Currently ADP is only implemented for OSX, so for other platforms this is a no-op. There is a known limitation that only allows this function to work if the Arnold libraries are in the same path as the host process. (#8433)

Incompatible changes

• Trace sets: a trace set is now an optional named set of objects to ray trace against or to exclude. Geometry with no trace set is no longer hit by all inclusive trace sets. The one exception are procedurals, which are always traced against when they have no trace set, so that their contents can be considered. (#8358)

• complex_ior shader deprecated: complex_ior is only usable with the deprecated standard shader and has been deprecated too. (#8457)
• Object transform removed from Alembic procedural: The alembic procedural node's object_transform matrix parameter is no longer used and has been removed, and should instead be baked into the procedural's transform if a matrix offset is required. (#8148)
• Crashes with mismatched AiMalloc/AiFree: A gentle reminder that AiMalloc needs to be paired with AiFree or crashes can occur. This was already the case on Linux and now this is also the case on Windows. (#4399)
• Adaptive sampling criterion removed: Removed the AA_adaptive_criterion option, we now use the best criterion. (#8556)
• Random token is now a function: The random token in operator expression assignments is now a function and needs trailing brackets: random(). (#7761)
• JSON log file periods: The JSON log file was using periods for ".ass parsing" and "accel. building" which can cause issues with some JSON readers. We've removed the periods and incremented the JSON log version to 3. (#8357)

Bug fixes