Thank you so much. I plan to do practical add-ons for common people that are useful for modeling, not really large projects. I don’t know the price tag yet but before pricing I will see what others sell in my markets and set it not too high or low. We all want good competence.
Thank you so much for your advice and help.
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Don’t be driven away by license problems and protection of I.P. The only reason you should have to use a GPL python / closed C++ is you need that C++ part because you need to accomplish something that Blender can’t. I don’t know what kind of esoteric addon you mean to produce, but 90% of cases can be covered with Blender sdk. Only if you want to do something ultra-pro, hi-level you may need I.P protection. But probably that’s a bit out of Blender market…
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I will use gpl and see. Like the saying “wherever you go, do as you see” 
Honestly, I’d just sell the add-on, source-code and all. Secure a registered copyright before you release a single copy of anything. (Costs $35 in the USA, strictly on-line, and takes effect immediately.) Include the required proprietary notices in your documents and in all files. Very plainly state your legal position in all advertisements and other things so that everyone is fully informed what they’re buying, what they’ll get and under what terms.
You are legally entitled to license your materials any way that you can persuade someone else to buy them. But – think carefully about what will make your plug-in most useful to your happy customers. I think that likely includes “source code.” Which they will possess if they want or need it, but are not allowed to disclose nor to fashion into so-called “derivative works” because you, the lawful owner, said they couldn’t. If they "just need to tweak this one wee thing" to use your tool in their project, they can do it.
Do everything you can think of to make it easier for them to do their work using your helpful tool. Make it more than “worth the money.” Maybe they’ll tell all their friends how cool your tool is and how happy they were that they bought a copy! Take care of your customers and they’ll take care of you. None of them are actually the slightest bit interested in stealing. Trust them.
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That is what we do. As we two are artists, we always try to give what we would like to find when we do a product. And yes you are right, if you just treat customers as you would like to be treated, they return the same, and support you.
In all the markets we were, we always had good relationship because of that, and with Blender we will do the same. I can say one of the best things is the appreciation of the people who get the products, and that is why we try to do the best we can, that is no more or less that giving something useful and beautiful. Just as one expects to get as a customer. More than trust, we respect them, and appreciate them.
Thank you everyone in this thread, for the advice and the insight. The first things I knew about Blender from inside was when I reviewed some books by Allan Brito (who is a great writer and Blender supporter) at my site. I have used it for some time, but now that I know the community better I also see that the people is nice and great too. I think that there will be very good things with such people
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You can make a part of your addon closed source (via the thin gpl interface method you mentioned), but honestly I’m more likely to buy it if I get the source so I can fix it for myself when inevitably it becomes out of date and unsupported.
But it depends on the specifics. I was happy to buy QuadRemesher because it is miles ahead of any open source addon, and simply sends the data over by writing it to an .fbx, which generally means the closed source part will never break while the python part is fixable.
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Thank you!
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There is a difference between a GPL compatible license (Apache2, MIT, GPL, etc) and GPL license.
It lists the WTFPL as compatible 
It has been a while since I saw it. For those who don’t know it:
https://directory.fsf.org/wiki/License:WTFPL-2
That is a very clear license text
Now, jokes apart: if a license that allows more uses than the GPL like the WTFPL or not so radical is combined with GPL content, how does it work? People can use the GPL content in its terms, and the the other licensed code for anything? Does being compatible mean that they keep separate, and not all becomes GPL? Or all becomes WTFPL? I suppose they keep each content and its rights separate, isn’t it?
A complete valid license if you want to start a project using it, for example. I wish you the best of luck to achieve with it a project as successful as Blender, where GPL is a fundamental part which developers take into account when deciding to get on board of the project.
Here a Blender developer talking about combination of GPL and other more permissive and compatible licenses (I guess):
Individual parts on their own may be licensed under varying licenses. Just like Cycles is licensed as Apache 2, or libraries by others that Blender uses and are licensed in various ways.
The important point is when they are distributed in a combined work (the definition of “combined work” is a discussion on its own), that has to be done in a GPLv2+ compliant manner. Since parts of Blender are not license compatible with GPLv2 (e.g. Cycles’ Apache 2), but only GPLv3, any combination of Blender with these parts has to be distributed under GPLv3.
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Don’t do that. It’s going to take a heavy-weight lawyer just to understand.
Do as Ton said:
https://twitter.com/tonroosendaal/status/1135229214607773696
In the end you only have to distribute the GPL python part separated from the C++ closed source part.
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Yes I was just curious. Thank you very much.
Yes, people must be careful when they write things as irrefutable facts (like what Luca Rood wrote), when they are not:
That is something unfair to say when I in the thread above I showed how some of the Blender developers who contributed the most with code to the project, decided in the beginning to contribute with Blender mainly because they felt their work was protected by the GPL license. There are many who contributed primarily for the GPL. And now that Blender has become very popular thanks to the work of all those developers, that others come with particular intentions to want to circumvent the GPL, is inadmissible.
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What i meant by that is that there’s some who are enthusiastic about FLOSS and some who aren’t
edit* just changed my post, i realise it’s maybe sound a bit rude, it was not the goal 
Bests
There are some people who are quick to label FLOSS people as fundamentalists, freedom idealists, or fanboys. What we want is simply that third parties respect the chosen development model and the rules imposed by the license. The success of the chosen model depends on the respect of these rules for all the participants involved. If the rules are broken, trust is broken. There in that thread I had shared a very interesting interview with Linus Torvalds where he talks about these things. FLOSS people do not pursue a common good or save the world, nor are they communists. They want to have the free choice of a development model for their project and that this model is respected by all participants to ensure that the system is fair and reliable:
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See? that’s what i meant by that… 
you are putting the FLOSS ideology in front of the blender product itself 
While not caring about the potentially huge benefit of allowing bridges with non FLOSS compatible software would bring to both the blender product and to all the millions the users.
I was looking a Houdini presentation the other night and look
it’s really sad
Apparently you have not understood anything of what I have written. Ideology? I have tried anyway to explain to you that it is not about ideology, but about the rules and licenses chosen for the development model of the project. Licenses are not ideology.
What must be put first, first of all, is the compliance and respect of the rules and licenses, in all areas, even with proprietary software and their licenses.
There have been millions of contributors involved in Blender throughout its history, many of whom would never have been if it weren’t for the project’s license, and this is not ideology. It is for this reason that (legally) Blender cannot change the license arbitrarily and unilaterally. If the license prevents third parties from doing something, it is what it is and must be followed. If this, according to some, harms Blender product, it could be a worthy argument. But still Blender and GPL is what it is, and if the project is harmed by it, it is something that will have to be dealt with because, as I have said, the license cannot be changed unilaterally, nor can it be decided unilaterally that some will be able to comply with it but others don’t. In any case, the Blender project will have to deal with this or any other limitation, and look for the best ways to be successful by ensuring that everyone complies with the license. And from what I have been able to see, Blender has not done badly at all.
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“FLOSS ideology” is Ton’s ideology and therefore Blender’s. He nearly always includes the Free Software Foundation when speaking about Blender and the license(see the Blender Dev Talk link a few posts up for multiple comments by Ton).
The potential benefits of closed source extensions have already been recognized and discarded long ago. They were around when Blender’s source was purchased from NaN and open sourced, it’s not like they’re something that were invented after Blender was released to the public for free. They’re not really worth asking for or arguing about because it’s literally impossible for Blender to change the license now. You’d have to start a new software project with a new license to have it and that wouldn’t be Blender anymore. That’s just the reality of the situation.
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Apparently you have not understood anything of what I have written. Ideology? I have tried anyway to explain to you that it is not about ideology, but about the rules and licenses chosen for the development model of the project. Licenses are not ideology.
What must be put first, first of all, is the compliance and respect of the rules and licenses, in all areas, even with proprietary software and their licenses.
There have been millions of contributors involved in Blender throughout its history, many of whom would never have been if it weren’t for the project’s license, and this is not ideology. It is for this reason that (legally) Blender cannot change the license arbitrarily and unilaterally. If the license prevents third parties from doing something, it is what it is and must be followed. If this, according to some, harms Blender product, it could be a worthy argument. But still Blender and GPL is what it is, and if the project is harmed by it, it is something that will have to be dealt with because, as I have said, the license cannot be changed unilaterally, nor can it be decided unilaterally that some will be able to comply with it but others don’t. In any case, the Blender project will have to deal with this or any other limitation, and look for the best ways to be successful by ensuring that everyone complies with the license. And from what I have been able to see, Blender has not done badly at all.
“FLOSS ideology” is Ton’s ideology and therefore Blender’s. He nearly always includes the Free Software Foundation when speaking about Blender and the license(see the Blender Dev Talk link a few posts up for multiple comments by Ton).
The potential benefits of closed source extensions have already been recognized and discarded long ago. They were around when Blender’s source was purchased from NaN and open sourced, it’s not like they’re something that were invented after Blender was released to the public for free. They’re not really worth asking for or arguing about because it’s literally impossible for Blender to change the license now. You’d have to start a new software project with a new license to have it and that wouldn’t be Blender anymore. That’s just the reality of the situation.
Indeed thanks for these amazing response
your enthusiasm over this subject is proving my point
cheers