Coin is an OpenGL-based, 3D graphics library that has its roots in the Open Inventor 2.1 API, which Coin still is compatible with.
If you are not familiar with Open Inventor, it is a scene graph based, retained mode, rendering and model manipulation, C++ class library, originally designed by SGI. It quickly became the de facto standard graphics library for 3D visualization and visual simulation software in the scientific and engineering community after its release. It also became the basis for the VRML1 file format standard. Several books exists on the subject of Open Inventor, most notably The Inventor Mentor, and The Inventor Toolmaker, both highly recommended for those who want to learn how to use Open Inventor.
Coin is based on the API of this library, but was developed from scratch independently before SGI Open Inventor became open source. It does not share any code with SGI Open Inventor, other than through random coincidences guided by the Open Inventor API design. Coin reached the goal of Open Inventor 2.1 compatibility in the autumn of the year 2000, and has since then been extended with a huge set of additional features, ranging from 3D sound support to GLSL shader support, additional file formats like VRML97, and a large number of internal changes for keeping up with the newer, more optimized OpenGL rendering techniques that were not available in the earlier days.
Another term you might find when reading about Coin is "Coin3D", which is the term we use on the larger group of libraries that all fall under the same license as Coin. Coin is the core of Coin3D.
Coin is one of three Open Inventor implementations. All implementations are source code compatible across the Open Inventor 2.1 API. Source code compatibility means you can write software for a given API and at build time choose to use any implementation of that API. Which one should not matter - the software should function the same way with all of them.
The Open Inventor implementations are not Application Binary Interface compatible with each other. This means it will not work to first build a piece of software against one implementation of an API and then later replace the run-time library with another implementation of the API. Some libraries have ABI compatibility in this way (OpenGL and Mesa, Motif and Lesstif) but those libraries are C libraries, and do not have to worry about different memory footprints for objects and different entry orders in the virtual function tables amongst other things.
Coin is binary compatible with itself. Each release of the Coin library has a version number that consists of three digits. They are called "major", "minor", and "micro" version numbers respectively. Coin 1.2.3 has major 1, minor 2 and micro 3.
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All releases with the same major and minor version number have the exact same API and ABI. Such releases are called patch level releases, and only consist of bugfixes, documentation updates and updates to the packaging.
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All releases with the same major number are upwards binary compatible. Such releases are called minor releases, and are releases made to add new extensions to the library API.
Upwards compatibility means that applications linked with one version of Coin will not have to be rebuilt if you or the end-user of your application install a newer version of Coin with the same major number and a minor number that is greater than the library the software was initially linked with. Let's say an application you have distributed was based on Coin 3.0.1 and then Coin 3.1.0 is released, your users can safely upgrade their Coin library without needing a new version of your application.
This also works the other way for most platforms, as long as the software does not actually use any of the extensions that have been introduced after the release of the library you downgrade to. This can be a bit tricky to get right (sometimes you might reference new functionality in an indirect way), so if you really need to have backwards compatibility like this, the best thing is to link with an early version of Coin in the particular major number series (e.g. Coin 3.0.0) and then release the software with the newest version of Coin (e.g. the future Coin 3.1.2). Then, the end-user can safely downgrade the Coin-library on his side without any extra hassle. The Coin API documentation will clearly state for which minor release an extension was introduced, so you can stay away from those functions if it is important, and so you can document with your software what the lowest acceptable Coin library version is.
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Releases with different major numbers are not compatible with each other. They are called major releases, and break compatibility with the other major releases of the library on purpose. The purpose of major releases is to clean up the library API (as for any evolving project, this has to be done once in a while) and change the fundamentals where they can be improved upon.
Run-time libraries with different major versions can safely coexist on a system at once, so installing Coin 3.0.0 does not mean that you must scrap all the Coin 3.*-based applications you may have. On Windows, the DLLs are named differently for each major release, so there will be no mixups between coin3.dll and coin2.dll. On UNIX systems, the application will at load-time look for a shared library named libCoin.so.#, where # will be indicative of the library ABI version. Different major releases will have different # numbers, so the files do not conflict.
However, there can only be one Software Development Kit for Coin available at once. A Software Development Kit constitutes the header files and linktime libraries. The declarations in the Coin 1.0 header files will conflict with the declarations needed for producing code for Coin 2.0, and the same with both of those versus Coin 3.0. When you install the Coin 3.x SDK, it will overwrite the headers for Coin 2.x unless you take special measures to prevent it. You can therefore only develop software against the version of Coin you installed most recently. To circumvent this limitation, you need to do some trickery with include and libdir paths. This should not be too difficult for a seasoned software developer.
Coin started out, back in 1995, as a scene graph rendering library for VRML 1.0 scenes. It was originally based on SGI's Qv library for parsing files in the VRML 1.0 format. After years of extending this humble beginning with new functionality like VRML1 and VRML2 rendering and export, the library was in late 1997 in dire need of a fundamental redesign.
On the surface, the API looked quite like Open Inventor already. The concepts used by Open Inventor are also often mentioned as good design methodologies in many software engineering books, and some of our developers had some experience with the library in advance and found it incredibly convenient. At the same time as we were contemplating a rewrite, the Free Software Movement got some great buzz going, and we saw the golden opportunity to homestead our library as the Free Software alternative to Open Inventor. We therefore decided to go for the rewrite, and after a short period coined the name Coin.
As luck would not have it, as soon as we went to beta status with Coin for SIGGRAPH 2000, SGI also decided to release their Open Inventor as Free Software. It soon became apparent though, that SGI Open Inventor was released to mainly be kept in maintenance mode. This made us confident that continuing the Coin development would still be well worth it.
The development of Coin was in the beginning primarily done on Linux and IRIX systems, but is now mostly developed under Linux, Windows with Cygwin, and Mac OS X systems.
Many people have contributed through the years to the success of Coin, be it in the form of patches, problem reports, or other kinds of feedback to the core Coin developer team. The file THANKS tries to credit all those helpful souls. Our apologies to those who have been forgotten.
Coin has historically been released quite infrequently. To try to improve on this, we decided in 2007 to switch from feature-based release cycles to date-based release cycles, aiming for a new release every six months. What we haven't switched is the decision to let the version number be decided based on ABI compatibility with earlier Coin versions.
See the file NEWS for the summary of changes, the file RELNOTES for a more verbose description of the more significant updates, and the file ChangeLog for the detailed source code update list.
See the file INSTALL for installation instructions, and all the other README.* files for platform-specific notes.
In 2019 a new major version was released, 4.0.0 which included some additional API changes to improve conformance to Open Inventor 2.1 API.
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OpenGL and Open Inventor are trademarks of SGI Inc.
