Commit 9ef0d03e1e5f0140636215afe627954b81602c38
2024-01-26T10:50:26
LICENSE.md: zlib License clarifications 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.