OpenEXR

OpenEXR is an image format developed by Industrial Light and Magic. Is it freely available as source code here and1is supported by Houdini.

Configuration

Various OpenEXR options are supported by modifying either the MantraROP (for transitory settings) or the FBoptions file (for studio-wide defaults).

Houdini supports the following features of OpenEXR:

Layers - Houdini supports all multiple image layers at all supported bit depths.
DataWindow - a bounding box can be saved from COPs indicating where the useful data in the image is. Mantra does NOT currently generate this information (as of Houdini 9.1).
Metadata - an abitrary string may be encoded

To make Houdini work best with Nuke:

Set compression to zips - single scanline zip. piz (wavelet compression) causes slowdowns when accessing multiple layers since Nuke processes a scanline at a time - as opposed to tiles/buckets as COPs does.
Layers are read correctly into Nuke with .red, .green, .blue and .r, .g, .b, extensions.

This is best done in the FBoptions file.

As of Houdini 7.0, EXR support is limited to 4 channels but supports the float and half DataTypes.