ossim-plugins

Contains plugin libraries for extending OSSIM core functionality.

Description

Plugins extend OSSIM's core functionality with additional readers, writers, sensor models, high-level "tool" utilities, filters, and codecs. The OSSIM applications (ossim-cli, ossim-info, ossim-chipper, etc.) read a configuration file that contains a list of plugin libraries to load at run-time.

The preferences configuration file name is typically set via the environment variable OSSIM_PREFS_FILE. Alternatively, it can be specified with the -P <filename> option on most command lines.

The preferences for the plugin library specifications in the configuration file follow the format:

plugin.file1: $(OSSIM_INSTALL_PREFIX)/lib/libossim_png_plugin.so
plugin.file2: $(OSSIM_INSTALL_PREFIX)/lib/libossim_kml_plugin.so
...

Where OSSIM_INSTALL_PREFIX is the location of the OSSIM/OMAR installation on the host.

Plugin Development

To add functionality via the plugin scheme, it is necessary to perform the following steps. In this example, we're creating a plugin called Xxx that will supply a new reader, namely, ossimXxxReader

1. Copy an existing plugin directory. The "png" plugin is a good example to use as a template as it has multiple classes being registered.
2. Depending on the type of functionality you'll be adding, rename and edit the corresponding class files. For example, for our new reader, ossimXxxReader, is necessarily derived from ossimImageHandler. Refer to ossimPngReader.h/cpp as a starting point.
3. Every functional class (reader, writer, tool, etc.) needs an accompanying factory that will be registered with the application's registry system. In our example, rename ossimPngReaderFactory.h/cpp to ossimXxxReaderFactory.h/cpp and edit to reflect your reader class.
4. Rename ossimPngPluginInit.cpp to ossimXxxPluginInit.cpp and edit the file. This file contains the special symbols that will be referenced by the run-time to initialize the plugin and register the functionality with the application's object factory registries. Using Png as the starting point, this is just replacing "Png" with your plugin's name everywhere in the file. It is important to keep the names that include "Png" unique, and to leave the other symbols alone. The symbols that are expected by the run-time (exactly as is) are ossimSharedLibraryInitialize and ossimSharedLibraryFinalize.
5. Note the registration of the plugin-specific object factories with the run-time global registry in ossimSharedLibraryInitialize(). For example, the reader class is registered as

ossimImageHandlerRegistry::instance()->registerFactory(ossimXxxReaderFactory::instance());

5. Though not necessary, it is useful to also register the class list (the list of all object classes being offered by the plugin) with the code adapted from ossimPngPluginInit.cpp:

ossimXxxReaderFactory::instance()->getTypeNameList(theXxxObjList);

This has the effect of making the plugin's object classes available to a master list. You can see this list by running via the command line:

ossim-info --plugins

6. Make sure to unregister all the factories in ossimSharedLibraryFinalize().
7. Edit the CmakeLists.txt in the plugin's directory to reflect your files and dependencies.
8. Also edit the repo's top-level ossim-plugins/CMakeLists.txt to add your plugin. Note the use of cmake variables to easily enable/disable building of specific plugins from a script.

Plug and Pray

The result of the build should be a library, in our example probably named ossimXxx_plugin.so (or dll), and located in the build directory's lib subdirectory. Edit you preferences/config file to mention your plugin and run ossim-info --plugins to verify it is being loaded properly. You should see your plugin listed along with the classes it represents.