A C++ SDK would then limit people to writing an add-on in C++. With ActiveX, they could write an add-on in Python, Java, Visual BASIC, C++, C, Delphi, .NET, C#, Perl, etc. That's a lot more developers you're targetting. For me, a C++ SDK would be fine since I know C++. But I'll bet most of the would-be plug-in developers would be using Visual Basic, Python, and Java because those languages are geared more for hobbyist programmers (ie, the kind who tend to write plug-ins). What's the use of having a plug-in architecture for both Windows and Mac if you've got very, very few developers writing plug-ins (because they have to know both C++, and Poser's "C++ SDK" which no doubt would have a lot less docs and examples than ActiveX, if Poser's user manual is any indication)?