Based on my knowledge (which I'm still a novice, so take that for what it's worth), GPL v3.0 is fairly protective license. Although it has limits when it comes to liability, it has clauses that protect you from misappropriation and anyone who uses your rendering engine must include the very same license, and also acknowledge you as the author as per its Copyright Notice requirement. This license also has a Source Code Availability requirement, which requires that the source code for your engine be made available openly.

My concern is, are GPL-family licenses good enough to let contributors involve with this project? Because I heard that GPL licenses are like viruses and someone hates them.

Or maybe dual licenses are good enough if one of them is Apache License (for personal use only)? This is what VGG License 2.0 does.

I can see your concern with GPL-family being like a virus. Indeed, it is very restrictive and contributors may not want to get involved due to the logistical overhead of trying to add features to/improve it.

Addressing your concern about not being paid for your work, this is an age-old problem with open source. It's hard to make money off a product that is freely available to download. A good rule of thumb is to make users feel like they _want_ to give you money, instead of making them feel like they _have_ to give you money.

Some options you have if you want to keep it open source but still make money:

1. Do the dual licensing approach, and use one open source license of your choice, and one commercial license to allow businesses to use your engine in their proprietary software. 2. Donations link in the open source software 3. Paid technical support/Freemium model 4. Paid training programs to train users to become an expert with your engine 5. Early access subscription model: create a system where users can pay a monthly subscription fee in order to get new features earlier than official release.

> Because I heard that GPL licenses are like viruses and someone hates them.

That's just old Microsoft FUD. The Linux kernel is GPL and approximately zero people refuse to run it because of its license.

The VGG License 2.0 is pretty confusing. It offers two licenses: VGG License 1.0 for commercial use (with royalties) and Apache License 2.0 for personal use only. But if personal use doesn’t involve making money, why add the Apache License? The original VGG License 1.0 could cover both personal and non-commercial use without royalties. Disregarding whether the royalty fee makes sense, the dual licensing setup just seems unnecessary and complicates things.

Dual licenses, or triple licenses are common in some open source software. Maybe more rules should be presented to make the dual licensing more clear?

Yes these licenses are common, but the dual licensing with essentially the same effect as a single license seems fishy and suggests there might be something hidden. Just my two cents.

I don't think it's a scam, but it's just not open source, that is perfectly fine.

It's very hard to make money with open source projects. Why do you want to make it open source? Why not just source available?

Ask a lawyer, but I think you should remove the Apache for non comertial option. The Apache licence has no such option. Also someone can copy your project with the plain Apache licence for free, and then someone else can use the Apache licence and make a proffit.

Thanks for your advice. I've reverted the license back to 1.0, adjusted the royalty rate, and made the royalty disclaimer more apparent in case someone says it contains a HIDDEN royalty :)

As a small team, we don't expect to make great money just by the royalty. We choose to open source this project for many good reasons. But the only reason of adding royalty is that we want to protect our work from being stolen by some other random guy.

1. This is a question for your lawyer. If you don't have a lawyer, then it doesn't matter what license you pick because you don't have a lawyer to enforce it.

2. Any license you pick will irritate some people.

My advice, picking a license is pretend work. Something to do instead of building. YAGNI. Good luck.

Great advice. Thank you so much!