Reichart it is a starting point but not "the definitive guide", it seems there is some information spread on: the original R3 documentation, Saphirion's GitHub, the demo script, some forum and Q&A on StackOverflow, Nick Antonaccio Guide's (!!!) and after you read everything you must consult the source code to understand which is the right statement. Also, it seems we lack of software which uses R3GUI to look at its source. So, apart from the demo script I was not able to find anything.
Pekr
GiuseppeC: my question is - whatever you try to develop - why would you choose R3? Original R3 GUI (Carl's version) is non-finished, hence gone. As for Saphirion, I might be mistaken, but - their tools are still mostly R2 (custom RebGUI based?). I wonder, development wise, when, for the last time, whoever touched R3 GUI at all? I don't want to provide wrong assumption here, it is just how I understand the situation, of course lacking any internal information. Henrik e.g. found GUI quite complicated and wanted to start new GUI effort. Does Cyphre work on R3-GUI? Not sure, but I doubt it.
Asfor other R3 efforts - Atronix guys have their own View version, it seems to be OpenGL accelerated. Not sure R3-GUI is going to work with their View version. HostilFork and his Ren-C - he's not much interested in GUI at all, but he kind of cooperates with Atronix and guys are considering switching to Ren-C R3 variant.
So - for me, GUI wise, I would consider 3 options - R2, Red, Ren-C - whatever guys come-up with ...
GiuseppeC
Pekr, I am concentrating on R3-GUI Saphirion version as it is an evolution/change from CARL R3-GUI. So we Have R2, RED, R3-GUI Saphirion.
Also I don't know where to find ANY info about atronix accellerated opengl version, or ANY R3-GUI Saphirion chances since 2013-4 as they are no published anywhere.
*chances *changes
Pekr
As for Atronix - their GUI might not be VID anymore. As for their View engine, I think, that in the end, they might release it, dunno though. You would have to ask them ....