- is it just open source (meaning you can read the source, but you cannot use it for other (closed source/commercial) products, such as MySql http://www.mysql.com/about/legal/licensing/index.html), or Open Source as in http://opensource.org/docs/osd (meaning that you release the source and have no restrictions on its use)?
No restrictions at all. The
PCL license allows you to develop open and closed source, free and commercial products with PCL. As long as you respect the four conditions in the PCL license, you are free to release your PCL-based product under any license of your choice.
- Will it be possible (technically and legally) to use PCL without PixInsight (for instance with some other image processing software)?
PCL requires a running PixInsight Core application for most of its image processing functionality (even more in version 2.0), and it is obviously necessary for all module-related and GUI functionality. However you can link the PCL static library with an application and, as long as you don't use any functionality requiring the low-level API, it should work. In fact, we use PCL in some code maintenance command-line utilities, and the three auxiliary applications that implement PI's update system are also linked to PCL in this way. The PCL license is no legal obstacle.
- Is the intention to be able to use PCL with GPLed software - without disclosing the source of PI?
I really am not sure. Since the PixInsight Core application is proprietary / closed source, and PCL depends on it, you probably cannot use anything infected with GPL in a PixInsight module, even if you release your source code. However this is a complex case, and the opinion of somebody with legal knowledge (and experience in software licensing issues) would be very welcome. For example, there is no problem with GPLed software on Windows. But of course, Windows is extremely important for the FSF while a project like PI simply does not exist.
- Is the intention to encourage other developers to contribute to PCL?
Absolutely! This is one of my *main* intentions