IJ-OpenCV is a Java library allowing the conversion between ImageJ1 and OpenCV objects (images, rois…). The github page of IJ-OpenCV contains all the information to download and install this library. The wiki section of the repository will help you to get started, and a set of examples are also provided.
There are several benefits provided by the communication of ImageJ and OpenCV obtained thanks to the IJ-OpenCV library. From the ImageJ perspective, this tool is enhanced with several computer vision and machine learning algorithms, avoiding the re-implementation of methods and the connection with several external libraries. From the OpenCV point of view, this library is improved with a simple-to-use GUI and with the functionality to manage regions of interest.
IJ-OpenCV has been built on top of SciJava Common using SciJava converters.
Several ImageJ plugins have been developed using this library and can be easily downloaded and installed using the ImageJ Update site.