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.
Because the PAD() macro can only handle powers of 2, this is a necessary
restriction (and a documented one, except in the case of
tjCompressFromYUV()-- oops.) Failing to check the 'pad' argument
caused tjBufSizeYUV2() to return bogus results if 'pad' was less than 1
or otherwise not a power of 2. tjEncodeYUV3() and tjDecodeYUV()
effectively treated a 'pad' value of 0 as unpadded, but that was subtle
and undocumented behavior. tjCompressFromYUV() did not check whether
'pad' was a power of 2, so the strides passed to
tjCompressFromYUVPlanes() would have been incorrect if 'pad' was not a
power of 2. That would not have caused tjCompressFromYUV() to overrun
the source buffer, as long as the calling application allocated the
buffer based on the return value of tjBufSizeYUV2() (which computes the
strides in the same manner as tjCompressFromYUV().) However, if the
calling application attempted to initialize the source buffer using
correctly-computed strides, then it could have overrun its own
buffer in certain cases or produced incorrect JPEG images in others.
Realistically, there is no reason why an application would want to pass
a non-power-of-2 'pad' value to a TurboJPEG API function, so this commit
is about user-proofing the API rather than fixing any known issue.
libjpeg-turbo has never supported non-ANSI C compilers. Per the spec,
ANSI C compilers must have locale.h, stddef.h, stdlib.h, memset(),
memcpy(), unsigned char, and unsigned short. They must also handle
undefined structures.
+ document that tjFree() accepts NULL pointers without complaint.
Effectively, it has had that behavior all along, but the API does not
guarantee that tjFree() will be implemented with free() behind the
scenes, so it's best to formalize the behavior.
Including the license templates was confusing to some, since it made
it appear as if the copyright year and author were unspecified for the
libjpeg-turbo source. Thus, rather than include the zlib License
template, link to that template on opensource.org. For the Modified BSD
License, include a roll-up of copyright years and authors, since the
terms of that license require the text of it to be included in product
documentation for binary distributions without accompanying source code.
LICENSE.md is included in the binary distributions as well, so it
doesn't make much sense to refer to license headers in source files that
aren't necessarily going to be there.
The IJG README file has been renamed to README.ijg, in order to avoid
confusion (many people were assuming that that was our project's README
file and weren't reading README-turbo.txt) and to lay the groundwork for
markdown versions of the libjpeg-turbo README and build instructions.
