Solid Angle Houdini To Arnold 5.1.0 / MacOSX + Linux

Compatibility
This release uses Arnold 5.0.2.2 and OpenVDB 4.0.0.
Binaries available for the following Houdini, Houdini FX, Houdini Indie and Houdini Education production builds:
16.0.705
16.0.736
16.5.268

and for the following platforms:
Linux x86_64 (gcc4.8)
Windows 7 x64 (vc14)
Mac OS X 10.8+ (clang7.3)

Please note that Houdini Apprentice does not support third-party renderers and thus cannot run HtoA.
Enhancements
Use explicit default color spaces in Arnold ROP (htoa#963)
Disable parameter linking with environment variable (htoa#959)
Implement standard shader UI logic (htoa#958)
Fixes
Fix for s/t wrap with <attr> expressions set in the image node filename (core#6257)
Excessive memory use with compressed UVs (core#6411)
Small polymesh memory leak at render shutdown (core#6417)
OSL raytype mismatch (core#6422)
Handle invalid characters when image tokens are resolved (core#6440)
Backfacing normals not supported in normal_map (core#6441)
Chained bumps using bump2d isn't working (core#6442)
Custom procedurals cannot create nodes before proc_init (core#6453)
Crash with linear subdivision, vertex normals, and deformation blur (code#6454)

Compatibility
This release uses Arnold 5.0.2.1 and OpenVDB 4.0.0.
Binaries available for the following Houdini, Houdini FX, Houdini Indie and Houdini Education production builds:
16.0.705
16.0.736
16.5.268

and for the following platforms:
Linux x86_64 (gcc4.8)
Windows 7 x64 (vc14)
Mac OS X 10.8+ (clang7.3)

Please note that Houdini Apprentice does not support third-party renderers and thus cannot run HtoA.
Enhancements
New sub-surface scattering algorithm: A new, more accurate way of calculating SSS has been added. Unlike the current empirical BSSRDF method based on diffusion theory, this new method actually traces below the surface with a real random walk and makes no assumptions about the geometry being locally flat. This means it can take into account anisotropic scattering like brute-force volume rendering and produces much better results around concavities and small details. It can also be substantially faster for large scattering radius (i.e. large mean free path) compared to the old method. On the other hand, the new method can be slower in dense media (i.e. small mfp), does not support sss_setname for blending two surfaces together, may require redialing materials to achieve a similar look, and is more sensitive to non-closed meshes, "mouth bags", and internal geometry potentially casting shadows. This new algorithm is exposed in the standard_surface shader via the new parameters subsurface_type (with enum values diffusion and randomwalk) and subsurface_anisotropy(Henyey-Greenstein's eccentricity g from -1.0 to +1.0). The default is to use the old empirical diffusion method in order not to break the look of existing scenes. (core#5005, core#6363)
Car paint shader: We are now shipping a dedicated shader for car paint, which can be thought of as the combination of a simplified version of the standard_surface and flakes shaders. This shader can create a wide range of car paint looks without having to connect several nodes. For example, a pearlescent effect can be easily added to both the specular and flakes layers by simply tweaking a few parameters such as specular_flip_flop and flake_flip_flop. An arbitrary number of layers of flakes can be used (flake_layers). The flakes at a deep layer are covered by the ones closer to the surface and more tinted by pigments (specified by the transmission_color parameter). A few presets are available to get started quickly (core#6238)
Subdivision frustum culling: Subdivision patches outside the view or dicing camera frustum will not be subdivided. This is useful for any extended surface that is only partially visible as only the directly visible part will be subdivided. Similarly, no subdivision work will happen if a mesh is not directly visible. This can be turned on globally by setting options.subdiv_frustum_culling true and can be turned off for specific meshes with polymesh.subdiv_frustum_ignore true. The global options.subdiv_frustum_padding adds a world space padding to the frustum that can be increased as needed to minimize artifacts from out-of-view objects in cast shadows, reflections, etc. Note that motion blur is not yet taken into account and moving objects might require some additional padding. (core#4451, core#6304, core#6336)
Improved flakes shader: The size parameter is replaced by the density parameter, which makes it easy to control the size and number of flakes. Alpha channel can be used as a mask. The new flakes shader supports non-disc shapes and 3D flakes, which are useful to render gemstone inclusions like goldstone, for example. (core#6061)
Improved shadow_matte shader: We have revamped and simplified the shader to make it easier to use, and fixed a number of long-standing issues: Indirect illumination now fills the global diffuse_indirect and specular_indirect AOVs, so we have removed the shader’s (confusingly named) indirect_diffuse and indirect_specular AOVs. Self-reflections are no longer rendered. A new specular_IOR parameter was added that controls Fresnel reflection. Parameters offscreen_color and background_type were removed. The new enum parameter background can be set to either scene_background (default) or background_color, which allows to connect a specific texture in the background_color parameter slot. Parameter alpha_mask was added to control whether the alpha must be opaque or if it has to contain the shadow mask. (core#6245, core#6248, core#6249, core#6250, core#6256)
Celullar option in triplanar shader: The triplanar shader now supports projection through Voronoi cells using the new cell parameter. The rotation angle of the projected texture for each cell can be controlled with the cell_rotate parameter. Cells can be smoothly blended using the cell_blend parameter. (core#6130)
Faster triplanar shader: Texture filtering in the triplanar shader has been improved, giving better antialiasing and up to a 2x speedup, specially when using high-resolution texture maps. (core#6187)
Option for exporting shader assignments: A new option has been created on the Archive section of the ROP called Export Shader Assignments . This determines whether to create the shader assignments when the Export Shaders option is off. The default matches the previous behaviour of always exporting the shader assignments. However now shader assignments can be turned off, for exporting ass files without shader connections if required. (htoa#954)
Transmit AOVs and Alpha: The standard_surface shader with transmission can now pass through AOVs, by enabling the transmit_aovs parameter. If the background is transparent, then the transmissive surface will become transparent so that it can be composited over another background. Light path expression AOVs will be passed through, so that for example a diffuse surface seen through a transmissive surface will end up in the diffuse AOV. Other AOVs can also be passed straight through (without any opacity blending), which can be used for creating masks for example. (core#5588)
OCIO Color families: The image node, the output color space parameters on the ROP, the render and LDR color spaces all now include a color family drop down to help select color spaces from an OCIO profile (htoa#916)
Display / view colorspace: OCIO Display/View tuples can now be selected as an output color space. (htoa#916)
Custom procedural namespaces: Procedurals can now declare a custom namespace using the new namespace parameter. This custom namespace can be used instead of the procedural name, to reference contents through absolute or relative paths. Multiple procedurals can share the same namespace by using the same custom name. Also, they can declare an empty name and they will use the global namespace. (core#6085)
Improved procedural namespace memory usage: Reduced memory used by procedural namespacing by about 28KB per procedural primitive, so that procedurals now use very little additional memory. In a scene with 100K procedurals, this gave about a 2.7GB reduction in memory use. (core#6086)
Faster AiNodeDestroy: we have optimized removal time for nodes contained in procedurals, greatly reducing the shutdown time at the end of a render in complex scenes. (core#6137)
Improved accuracy of UV coords: Mesh UV coordinates are now handled with higher numerical precision, fixing jagged artifacts that sometimes appeared when using high-resolution UDIM textures over very wide UV ranges. (core#5514)
Improved volume sampling of low-spread lights: Low-spread quad and disk lights should now produce less noise and show increased performance when participating in atmospheric scattering effects. (core#5128)
Faster opacity masks: Texture map-based opacity masks will now be faster to render. Testing indicates around 3-13% faster. (core#6153)
Accurate dielectric Fresnel: The Fresnel formula in the microfacet and microfacet refraction BSDFs now use the accurate dielectric instead of Shlick Fresnel. The old formula could give much too bright reflections for low IOR values near 1.0, like in the interface between ice and water. (core#5371)
Support for more OSL attributes: OSL shaders now support getattribute() lookups of standard camera attributes (e.g. camera:fov, camera:resolution, etc) as well as the geometry attributes geom:type, geom:name, geom:bounds, and geom:objbounds on objects. (core#6172)
Improved multi-threaded render time stats: Render times when using more than one thread did not really work. We have improved this so that render times are now much more reliable and useful and can now be confidently used to determine what parts of Arnold are the most expensive. In particular, the subdivision and displacement times will now show how much total time was used as well as what fraction of that time was spent with threads unable to do useful work. This "threads blocked" time can often be lowered by using larger buckets or the randombucket_scanning ordering. (core#6218)
maketx version info: The custom maketx binary that ships with Arnold now reports, both in the command-line and in the embedded EXR headers, that it was built specifically for "OpenImageIO-Arnold", to distinguish it from the official "OpenImageIO" one. (core#6118)
maketx color spaces: The custom maketx that ships with Arnold now supports OCIO and SynColor (when available) through the colorengine, colorconfig and colorconvert flags. See maketx --help. (core#5908)
AiMakeTx reports info messages: AiMakeTx now also prints out OIIO informational messages (generated in verbose mode, for instance) in addition to errors. (core#6148)
AiMakeTx releases input file lock sooner: AiMakeTx will now close the input texture as soon as possible instead of waiting for all the maketx jobs to finish. (core#6240)
AiMakeTx and maketx optimized flags: Both AiMakeTx and maketx now always run with the flags --monochrome-detect --opaque-detect --constant-color-detect --fixnan box3 --oiio, which can result in smaller .tx files that are faster to load and take less memory. (core#6261)
Report when textures are changed during render: The log files now report when texture modifications during a render cause a texture read error, which can happen in certain pipelines. (core#6246)
.ass metadata from compressed files: We now support loading metadata from .ass.gz files through the AiMetadataStoreLoadFromASS() API function. (core#6197)
Fixes
HtoA ZeroMQ library fix for older OSX versions (htoa#950)
Set the ginstance matte attribute from a point attribute (htoa#951)
Alpha not fully opaque in output images (core#3319)
Non-linkable light colors should have the linkable metadata disabled (core#4559)
Distant light not motion blurring direction (core#5974)
Incorrect melanin absorption values for wide gamut rendering color spaces (core#5981)
maketx fails when run on Windows with read only (mandatory) user profiles (core#6087)
Deep EXR output of light path expressions missing volumes (core#6089)
artifacts in latlong skydome_light (core#6094)
Fresnel discontinuity in diffuse term when texture is connected to specular_color (core#6095)
Subdiv: duplicate vertex index in face causes crash (core#6097)
Quad lights do not work with projected textures (core#6104)
Warn that images will be watermarked if license authorization fails (core#6116)
mesh_light crashing when provided non-mesh node (core#6124)
curvature shading differences when seen through glossy transmission/reflection (core#6128)
crash during accel construction if there are over 65k overlapping primitives (core#6135)
Indirect sample clamp not visible in AOVs (core#6136)
Light groups not supported by volume shapes and surface shader interiors (core#6138)
"A" AOV is black when 8 light AOVs are used (core#6141)
AiShaderGlobalsGetVertexUVs not working for uvsets in free render mode (core#6143)
Black AOV output due to conflicting AOV type redefinition (core#6151)
AiShaderGlobalsGetPositionAtTime, AiTraceBackground, and AiVolumeSample crash in background shading context (core#6162)
AiM4Scaling does not work in python (core#6166)
Wireframe artifacts when not in raster-space (core#6177)
MotionVector AOV empty for negative motion start / end (core#6182)
Triplanar uses wrong mipmap level for some parts of the object (core#6186)
Triplanar uses overly high res mipmaps (core#6187)
Crash when removing and recreating a procedural instance (core#6192)
counter overflow crash in big scanline EXR images (core#6202)
photometric_light filename does not support environment variable expansion (core#6219)
Metallic BSDF albedo is always white (core#6222)
Crash when removing a procedural node (core#6242)
Shadow_matte AOVs (core#6245)
Remove offscreen_color from shadow_matte (core#6248)
Remove shadow_matte background_type (core#6249)
Re-introduce background_color in shadow_matte (core#6250)
Shadow_matte self-reflections / self-shadowing (core#6251)
trace_set: crash when destroying shader (core#6254)
sss_irradiance_shader fails with closure-based shaders (core#6255)
Add alpha_mask in shadow_matte (core#6256)
Volume shader: crash when connected as atmosphere shader (core#6286)
Crash with invalid options.atmosphere and options.background shaders (core#6287)
Miscellaneous crashes (empty nurbs, empty implicit, atmosphere shadow_matte) (core#6288)
Deep Driver: write errors hang the render (core#6292)
Deep driver: append does not work with overscan renders (core#6293)
Bucket call back: user bucket coords should be snapped to bucket grid (core#6294)
Crash when saving empty ginstance with open_procs enabled (core#6295)
Camera differential evaluation can cause crashes or hangs with OSL shaders linked to camera#6313crash when 4-channel opacity texture has no alpha=0 (core#6297)
Crash on Windows when loading plugins without .dll extension (core#6331)
"host app" metadata item not readable by AiMetadataStoreLoadFromASS (core#6174)
Range shader can sometimes ouput infinite or nan (core#6258)
Support upper and mixed case versions of .ass / .ass.gz extensions (core#6303)
ASS metadata load is too slow (core#6409)
Crash when setting an array user parameter with its current value (core#6412)
Incompatible Changes
autotile disabled by default: We found that the autotile code in OIIO does not scale with high-resolution textures. In order to avoid very slow loading of untiled textures (such as JPEG) when autotile was enabled, we have now changed the autotile default setting to 0, which effectively disables it. In very rare cases, when rendering with a large number of high-resolution untiled textures, this change might degrade performance as the texture cache will blow up. The real solution is to never use untiled files, and instead convert all untiled textures to .tx tiled files. (core#5116)
maketx dependencies: The custom maketx that ships with Arnold now depends dynamically on libai.so and optionally on syncolor_shader.so, therefore to work correctly it needs to be run from its installation folder, like kick, or alternatively the new dependencies should be copied to where maketx is running from. (core#5908)
Light groups and volume shading: Just like with surface shapes, Arnold now obeys light group assignment on volume shapes and surfaces with volumetric interiors. While in many cases desirable, this can produce an unexpected change in the final image in scenes with light group assignments. (core#6138)
flakes shader: It was not easy to control the number of flakes with the size and scale parameters because they were mutually dependent. Now this can be easily done using the new density parameter. The shape of each flake has been changed from disc to Voronoi cell, which is more suitable to render inclusions of gem stones. The shader output type has been changed from RGB to RGBA to support a mask. (core#6061)
motionvector AOV: The motion vector scaling factor in the built-in motionvector AOV has changed. This was required to fix a bug that caused zero motion vectors for certain shutter positions. The output from the motion_vector shader is unchanged: it can be used as a workaround in your old scenes if you require the previous scaling. (core#6182)
shadow_matte changes: The shader AOVs indirect_diffuse and indirect_specular were removed, since the shader now fills the corresponding built-in AOVs. Parameters offscreen_color and background_type were removed. See notes in the enhancements section above.

AOV EXR Precision Output bug fix (htoa#948)
Crash when removing a procedural node (core#6242)
Removed crash when destroying trace_set shader (core#6254)
sss_irradiance_shader no longer fails with closure-based shaders (core#6255)
Crash fixed when connecting a volume shader as an atmosphere shader (core#6286)
Miscellaneous crashes fixed (empty NURBS, empty implicit, atmosphere shadow_matte) (core#6288)
driver_deepexr driver append fixed with overscan renders (core#6293)
Crash when saving empty ginstance with open_procs enabled (core#6295)
Camera differential evaluation crashes and hangs fixed for OSL shaders linked to camera (core#6297)
The range shader no longer occasionally outputs infinite or nan floats (core#6258)

Enhancements
When exporting an .ass file, the bounding box is now written as bounds metadata in the .ass file itself instead of being written as a separate .asstoc file. Exporting .asstoc files is still possible but is now disabled by default. We will remove .asstoc support entirely in a future release (htoa#910)
Fixes
Curvature shading differences when seen through glossy transmission/reflection (core#6128)
Crash during accel construction if there are over 65k overlapping primitives (core#6135)
Alpha not fully opaque in output images (core#3319)
Quad lights do not work with projected textures (core#6104)
Indirect sample clamp not visible in AOVs (core#6136)
Black AOV output due to conflicting AOV type redefinition (core#6151)
AiM4Scaling() does not work in Python (core#6166)
Crash creating motion blurred min_pixel_width inside a procedural (core#6170)

Compatibility
This release uses Arnold 5.0.1.1 and OpenVDB 4.0.0.
Binaries available for the following Houdini, Houdini FX, Houdini Indie and Houdini Education production builds:
16.0.600
16.0.633
16.0.671

and for the following platforms:
Linux x86_64 (gcc4.8)
Windows 7 x64 (vc14)
Mac OS X 10.8+ (clang7.3)

Please note that Houdini Apprentice does not support third party renderers and thus cannot run HtoA.
Enhancements
IPR Shader Translation: Adding and rewiring of surface and volume shaders is supported during IPR rendering, including previously isolated shader nodes. (htoa#924)
IPR Output Nodes: Rewiring connections to the surface and volume material output node (htoa#925)
Fixes
IPR Shader Rewiring: Fixes for rewiring of shaders during IPR rendering including node creation and undo. (htoa#907)

See also the Arnold 5.0.1.1 release notes for the full list of core enhancements and fixes.