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Icon sets are often released under software licenses. For example, I use the Numix icon set, which is GPLv3, on my laptop.

Suppose I wanted to use these icons to create a derivative non-software artwork, such as an illustration.

What licenses can I apply to the artwork?

Must it be GPL (which to my understanding is not really meant for non-software works), or is there a license that is suitable for non-software work and is compatible with GPLv3?

And if I have to release the derivative artwork as GPLv3, what licensing restrictions apply if I include it in a larger piece of work - does this mean that if e.g. I put the illustration inside a book, the book is "infected" by GPL and must also be GPLed?

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    I think that using the GPL for artwork (or fonts or docs or data or content in general) is a poor choice because of the confusion it creates and that you rightfully experience: the GPL has been designed as a software license, not a content license. Numix also sells its artwork so they might have picked this license to maintain some quid pro quo? I would either stay away from these icons or contact the authors (enter a ticket on their Github issue tracker). – Philippe Ombredanne May 15 '16 at 8:16
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    Just to extend the comment: imagine some piece of code is released under the OFL (designed for fonts) or the GFDL (designed for docs): I would would completely puzzled and scratching my head about what could be done with this code given the mismatch of the licensing terms with what is actually licensed. – Philippe Ombredanne May 15 '16 at 8:28
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If you consider your artwork to be a derivative of a GPL-licensed icon set, then the rules are quite simple: the derivative work, as a whole, must be licensed under the GPL. This is explicitly stated in the GPL FAQ:

Am I required to claim a copyright on my modifications to a GPL-covered program?

You are not required to claim a copyright on your changes. In most countries, however, that happens automatically by default, so you need to place your changes explicitly in the public domain if you do not want them to be copyrighted.

Whether you claim a copyright on your changes or not, either way you must release the modified version, as a whole, under the GPL (if you release your modified version at all).

This is also mentioned in the FAQ regarding license compatibility:

What does it mean to say a license is “compatible with the GPL?”

It means that the other license and the GNU GPL are compatible; you can combine code released under the other license with code released under the GNU GPL in one larger program.

All GNU GPL versions permit such combinations privately; they also permit distribution of such combinations provided the combination is released under the same GNU GPL version. The other license is compatible with the GPL if it permits this too.

GPLv3 is compatible with more licenses than GPLv2: it allows you to make combinations with code that has specific kinds of additional requirements that are not in GPLv3 itself. Section 7 has more information about this, including the list of additional requirements that are permitted.

Note the proviso: "they also permit distribution provided the combination is released under the same GNU GPL version".

This continues to apply to further derivative works. Placing the illustration in a book probably doesn't make the book a derivative work though.

Of course, you may license your own modifications using any license compatible with the original work; for the GPL, this includes GPL-compatible software licenses and licenses for other works highlighted in green in the FSF's list. CC-BY 4 is compatible and seems quite appropriate. If it makes sense, this would allow recipients of your work to replace the original work (icon set) with another, which may result in a different license for the new combined work. This possibility applies to you in any case since you're the copyright holder of your own modifications.

As to your specific circumstances, things are a bit more complicated because the exact licensing situation of the Numix icon set is unclear. I haven't looked into it in detail, but I did notice that the icon set includes the Humanity icons, which are licensed under the GPLv2 (only); if the combined work is considered a derivative, then there's a problem, because versions 2 and 3 of the GPL are mutually incompatible.

There may be a different way of analysing things though: you stated that your artwork is a derivative of the icons, but if you're integrating the icons into an illustration, the resulting work may be an aggregate rather than a derived work. Depending on the importance of the icons in the illustration, it's possible that fair use may come into it, and that you don't actually need a license to use the icons — in which case the icon set's license wouldn't have an impact on your licensing possibilities.

All in all, I'd seriously consider consulting a specialised attorney (or your book's publisher's legal department, if you have a publisher lined up).

  • "the derivative work, as a whole, must be licensed under the GPL" that is true; but it does not have to be released only under such a licence. – MadHatter May 13 '16 at 12:13
  • @MadHatter I don't agree, but I don't think there's much point debating it further — I've asked the FSF for clarification. – Stephen Kitt May 13 '16 at 12:31
  • Fair enough! I wait with bated breath. – MadHatter May 13 '16 at 12:32
  • @MadHatter let me get this straight — if I've understood correctly, by your reasoning I can create a derivative work of some GPL software, license my contributions under the BSD license (which is GPL-compatible), and release the whole work under "BSD or GPL"? So then I can release it as a binary only, with no obligation to distribute the source? – Stephen Kitt May 13 '16 at 12:35
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    @MadHatter OK, that makes sense, although it doesn't help the OP really. Care would have to be taken to avoid falling foul of sections 7 and 10 of the GPLv3 though. What's confusing about your answer is the use of "or" in the second paragraph, since it's not "or", it's "and"... – Stephen Kitt May 13 '16 at 13:14
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If you make a derivative work of a GPL work, then the one downstream licensing requirement that the GPLv3 makes of you is in 5(c):

You must license the entire [modified] work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.

This means that -- regardless of whatever else you do -- you must make your derivative work available under the GPL. You may also, separately, license your own contributions to the derivative work under some other GPL-compatible license(s), so that if the GPL components are completely and cleanly removed (such that the work is no longer a derivative of the GPL components) then the GPL no longer has any effect on the work, and only your license remains.

Note that section 5 of the GPL requires you to include the "preferred form for making modifications" (i.e., the "source code"). If you used image-editing software like Photoshop or the GIMP (or even PowerPoint), this requirement probably compels you to include a copy of the original file when distributing the finished product. This can get a bit awkward if you're distributing actual physical prints of the image, but in that case your distribution appears to be "in, or embodied in, a physical product" under section 5, and you can accompany the work with either the source files or a written offer for the source (but note that this compels you to honor requests for the source code for at least three years).

As for whether a book that included such an image would need to be licensed under the GPL, this comes down to whether the use creates a derivative work or a collective work, and whether copyright law allows the license of the smaller work to place any restrictions on the larger work in this particular context. Unfortunately, the GPL has limited case law, so there are quite a few unknowns about what boundaries its copyleft provisions can and cannot cross. If I wanted to do this, I would certainly consult an experienced attorney.


To address MadHatter's answer, section 5(c) of the GPL says "This License gives no permission to license the work in any other way" which seems to suggest that if your work has a GPL component, you must license it exclusively under the GPL -- to do otherwise appears to be operating outside of the license. The license says that "it does not invalidate such permission if you have separately received it," but this almost certainly refers to case where you receive the original work under more than one license: if you receive a work under the GPL and then later under the BSD license, this clause merely clarifies that your more permissive BSD-granted rights are in no way affected by having previously received a separate GPL license grant.


If I am wrong, and you can in fact license the modified work under another more permissive license concurrently with the GPL, then the fact remains that a downstream recipient of your work who wants to redistribute it further must also include the GPL in the list of licenses she distributes it under.

For example: Alice downloads a GPL-licensed work Foo and uses it to create a derivative work Bar. GPLv3 5(c) (or GPLv2 2(b)) requires that she license the entire work under the GPL and include the source code (the "preferred from for making modifications", which may or may not be the normal work itself, for an image) when distributing the work, per section 5 of the GPLv3.

MatHatter's argument here is that Alice could also license the work under a more permissive license like the MIT license. I suppose that's true, based on my reading of the license text. However, there is a substantial catch; read on.

Bob receives Alice's modified work Bar and wants to distribute it further. Normally, under a dual-licensing scheme, Bob can choose to receive Bar under either the MIT license or the GPL. However, because Bar is a work derived from Foo, the GPL on Foo requires him to license the modified work Bar under the GPL (in addition to, I suppose, whatever other GPL-compatible licenses he likes).

As you can see, licensing a work derived from a GPL-licensed work under anything other than the GPL is not much different from licensing your component contributions under a GPL-compatible license and letting the entire work be under the GPL only.

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Firstly, IANAL/IANYL. That said, your question accepts that you have created a derivative work of the input icon set, which is handy, because it means we don't need to address that whole thorny issue.

The GPL requires that derivative work be released under the GPL or a more-permissive licence. So you could release the whole work under GPLv3. Or, since the FSF say that CC Attribution 4.0 is GPLv3-compatible, you could release the derivative work under those terms. The FSF regard this as "a non-copyleft free license that is good for art and entertainment works, and educational works", so that might meet your requirements ("a license that is suitable for non-software work and is compatible with GPLv3"). As you know, what you can't do is redistribute your derivative work under more restrictive terms than GPLv3.

If you include your derivative work (the illustration) in a book, we re-engage the derivative works issue. I don't know if the book is then a derivative work of your illustration, or merely an aggregation of your illustration with a lot of other content. I don't think anyone else really knows, either, because I don't think any higher courts have ruled on the issue. If pushed, I would lean towards an aggregation approach.

If the book is a derivative work, and you picked a copyleft licence for your illustration, then the book, too, must be distributed on the terms of the licence you choose. If this is GPLv3, the FSF says that

anyone publishing the book on paper would have to either include machine-readable “source code” of the book along with each printed copy, or provide a written offer to send the “source code” later

This is one reason why the FSF doesn't recommend using GPLv3 for books. If the book is mere aggregation, then this analysis applies only to your illustration. If you picked a non-copyleft licence for your illustration, then the book's publisher need only honour the terms of that (which for CC-BY 4.0, aren't onerous).

Note that whatever licence you pick, and however you decide the derivative/aggregation issue, you would still need to honour the requirements of GPLv3 with regard to the conveyance of the Numix icon set. Anyone attempting to convey verbatim or modified copies of your illustration, or the book, would need to do likewise (unless (a) you had chosen a non-copyleft licence, and (b) the modifications had completely removed the GPLv3'ed material).

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    According to the links in your answer, the combination of CC-BY 4.0 and GPL can be released under GPL, but it's not clear to me that the combination can be released under CC-BY 4.0. I think it's only one-way compatibility. – ff524 May 13 '16 at 9:51
  • No, CC-BY 4.0 and the GPLv3 are fully-compatible (from the second link in my answer). CC-BY-SA 4.0 is one-way compatible with GPLv3 (same place, next licence down), and that one-way compatibility is in the wrong direction, which is why I did not suggest it as a possible licence for you. The first link in my answer is where the FSF clarify that works which derive from GPLed content can be released under any fully-compatible licence (though the original GPL content is of course still covered by GPL). – MadHatter May 13 '16 at 9:53
  • Note the colour key at the top of the page. The green border to the left of the CC-BY 4.0 section is for "Free licenses, compatible with the GNU GPL". – MadHatter May 13 '16 at 10:02
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    Yes, but it doesn't distinguish between 1-way and 2-way compatibility. For example Apache v2 is listed as compatible with GPLv3, but it's really only one-way compatible. Their FAQ says that they consider a license to be compatible with GPL if the combination of GPL and that license can be distributed under GPL. – ff524 May 13 '16 at 10:04
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    I think this answer confuses some concepts - the GPL doesn't allow you to release a derivative work under a more-permissive license: you can't just go and take away the protections of the GPL by making it MIT, for instance. Nor does it allow you to make it more restrictive, by keeping back the source, for instance. You must continue to share it in the way it has been shared already: this is the concept of copyleft. – Tim Malone May 13 '16 at 12:00
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The GPL is intended primarily as a software license. However, it is worded so that it can apply equally well to non-software works such as icons1. From the definitions section of the license:

"The Program" refers to any copyrightable work licensed under this License.

The license refers to the covered work as "the Program", but that doesn't necessarily mean computer program - it can equally mean the icons.

They also cover discrepancies about code:

The "source code" for a work means the preferred form of the work
for making modifications to it.  "Object code" means any non-source
form of a work.

So again, they're not limiting the license to actual computer code; the source code of a covered work is simply the form in which the work is easiest to modify. For images, that may well be the images themselves.


So, how do you use GPL icons in non-software artwork? In the same way you'd apply GPL to a derivative program, you must apply GPL to any derivative artwork you make, whether it has to do with software or not.


1 This does not necessarily make it a good choice; there are much better licenses for artwork around, primarily the CC licenses.

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