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Disclaimer: I am not a lawyer, nor do I play one on TV. Referring to #744, mentioning the zlib License as a license that applies to libjpeg-turbo is confusing, and it isn't actually necessary, since the IJG License subsumes the terms of the zlib License in the context of the libjpeg API library and associated programs. This was presumably understood to be the case by Miyasaka-san when he chose the zlib License for the first libjpeg SIMD extensions. The libjpeg/SIMD web site (https://cetus.sakura.ne.jp/softlab/jpeg-x86simd/jpegsimd.html) states (translated from Japanese): "The terms of use of this SIMD enhanced version of IJG JPEG software are subject to the terms of use of the original version of IJG JPEG software." Detailed analysis of the zlib License terms: This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software. This text is an almost literal subset of the warranty disclaimer text in the IJG License. The IJG License states everything above with only slight differences in wording, and it further clarifies that the user assumes all risk as to the software's quality and accuracy and that vendors of commercial products based on the software must assume all warranty and liability claims. Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions: This is semantically the same as the permission text in the IJG License, since "use, copy, modify, and distribute this software (or portions thereof) for any purpose, without fee" covers "use" for "any purpose, including commercial applications" as well as alteration and redistribution. 1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required. The IJG License requirement that "If any part of the source code for this software is distributed, then this README file must be included, with this copyright and no-warranty notice unaltered; and any additions, deletions, or changes to the original files must be clearly indicated in accompanying documentation" (Clause 1), as well as the requirement that "If only executable code is distributed, then the accompanying documentation must state that 'this software is based in part on the work of the Independent JPEG Group'" (Clause 2), satisfies the requirement of Clause 1 of the zlib License. 2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software. Since Clause 1 of the IJG License applies only to the distribution of source code, the copyright headers in the source code are effectively "accompanying documentation" in that case. This is why we ensure that the copyright headers of individual source files indicate the year(s) in which modifications were made by each contributor. Doing so satisfies the requirements of both Clause 2 of the zlib License and Clause 1 of the IJG License. 3. This notice may not be removed or altered from any source distribution. Clauses 2 and 3 of the zlib License apply only to the source code that bears that license. Thus, as applied to the software as a whole, those requirements of the inbound zlib License are compatible with the outbound IJG License as long as the IJG License does not contradict them (which it doesn't.) NOTE: To be clear, existing source code that bears the zlib License cannot literally be re-licensed under the IJG License, since that would violate Clause 3 of the zlib License. However, when considering the terms under which the overall library is made available, the IJG License effectively subsumes the terms of the zlib License. https://www.gnu.org/licenses/license-compatibility.en.html is a thorough, albeit somewhat GPL-biased, discussion of license compatibility.
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libjpeg-turbo Licenses
libjpeg-turbo is covered by two compatible BSD-style open source licenses:
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The IJG (Independent JPEG Group) License, which is listed in README.ijg
This license applies to the libjpeg API library and associated programs, including any code inherited from libjpeg and any modifications to that code. Note that the libjpeg-turbo SIMD source code bears the zlib License, but in the context of the overall libjpeg API library, the terms of the zlib License are subsumed by the terms of the IJG License.
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The Modified (3-clause) BSD License, which is listed below
This license applies to the TurboJPEG API library and associated programs, as well as the build system. Note that the TurboJPEG API library wraps the libjpeg API library, so in the context of the overall TurboJPEG API library, both the terms of the IJG License and the terms of the Modified (3-clause) BSD License apply.
Complying with the libjpeg-turbo Licenses
This section provides a roll-up of the libjpeg-turbo licensing terms, to the best of our understanding. This is not a license in and of itself. It is intended solely for clarification.
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If you are distributing a modified version of the libjpeg-turbo source, then:
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You cannot alter or remove any existing copyright or license notices from the source.
Origin
- Clause 1 of the IJG License
- Clause 1 of the Modified BSD License
- Clauses 1 and 3 of the zlib License
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You must add your own copyright notice to the header of each source file you modified, so others can tell that you modified that file. (If there is not an existing copyright header in that file, then you can simply add a notice stating that you modified the file.)
Origin
- Clause 1 of the IJG License
- Clause 2 of the zlib License
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You must include the IJG README file, and you must not alter any of the copyright or license text in that file.
Origin
- Clause 1 of the IJG License
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If you are distributing only libjpeg-turbo binaries without the source, or if you are distributing an application that statically links with libjpeg-turbo, then:
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Your product documentation must include a message stating:
This software is based in part on the work of the Independent JPEG Group.
Origin
- Clause 2 of the IJG license
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If your binary distribution includes or uses the TurboJPEG API, then your product documentation must include the text of the Modified BSD License (see below.)
Origin
- Clause 2 of the Modified BSD License
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You cannot use the name of the IJG or The libjpeg-turbo Project or the contributors thereof in advertising, publicity, etc.
Origin
- IJG License
- Clause 3 of the Modified BSD License
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The IJG and The libjpeg-turbo Project do not warrant libjpeg-turbo to be free of defects, nor do we accept any liability for undesirable consequences resulting from your use of the software.
Origin
- IJG License
- Modified BSD License
- zlib License
The Modified (3-clause) BSD License
Copyright (C)2009-2023 D. R. Commander. All Rights Reserved.
Copyright (C)2015 Viktor Szathmáry. All Rights Reserved.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
- Neither the name of the libjpeg-turbo Project nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS", AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
Why Two Licenses?
The zlib License could have been used instead of the Modified (3-clause) BSD License, and since the IJG License effectively subsumes the distribution conditions of the zlib License, this would have effectively placed libjpeg-turbo binary distributions under the IJG License. However, the IJG License specifically refers to the Independent JPEG Group and does not extend attribution and endorsement protections to other entities. Thus, it was desirable to choose a license that granted us the same protections for new code that were granted to the IJG for code derived from their software.