Launched at SIGGRAPH 2014, Arnold for Houdini (or HtoA) provides a tight bridge to the Arnold renderer from within the standard Houdini interface, in a way that is familiar to both Houdini users and Arnold users in Maya or Softimage. It also enables smooth lighting workflows between Houdini and other DCC applications, since setups can be exported and shared.

Features
All Arnold cameras (perspective, orthographic, spherical, fisheye) with depth of field and advanced shutter controls.
All Arnold lights (point, distant, spot, quad, disk, cylinder, skydome, mesh) with light filters and accurate viewport representation.
Custom Arnold shading network context with a comprehensive list of 106 shaders and utilities.
Atmospheric and background effects.
Volume rendering with support for OpenVDB and particles.
Polymeshes, curves, points with support for displacement and bump mapping and subdivision.
Procedurals, with sample Alembic and mandelbulb implementations.
Accurate motion blur (transform, deform, velocity, acceleration), overridable per object.
Geometry attributes translation as user data.
Optional Arnold properties for objects and cameras.
Render to AOVs, in single or separate files.
Support all Houdini rendering contexts (render region, mplay, render viewer, render COP, batch), with support for AOVs and clickable buckets.
Interactive rendering (IPR) allows parameter changes to be rapidly previewed without interrupting your work.
Instancing.
Multi-camera renders.
DeepEXR support.

Fixes:

Invalid houdini.env after installation on Windows (htoa#683)
Several versions of Houdini can run the same version of HtoA without re-installing (htoa#572)
Cannot override the output filename with hrender (htoa#721)
Fixed erroneous default PNG pixel format (htoa#720)
Cannot specify the number of threads for interactive or batch renders (htoa#256)
Fix memory leaks in color_convert and matrix_multiply_vector shaders (htoa#723)
Wrong bounds in .asstoc when hou.RopNode.render() called for a single frame (htoa#726)
Display log messages in the status bar of Houdini when the Arnold universe is not active (htoa#497)
Cannot export frame as integer user data for procedurals such as Yeti (htoa#728)
Render metadata should have short names for maximum OpenEXR compatibility (core#4954)
Bounding box given by options.curved_motionblur is clipped (core#4997)