In libjpeg-turbo 2.1.x and prior, the WITH_12BIT CMake variable was used
to enable 12-bit JPEG support at compile time, because the libjpeg API
library could not handle multiple JPEG data precisions at run time. The
initial approach to handling multiple JPEG data precisions at run time
(7fec5074f9) created a whole new API,
library, and applications for 12-bit data precision, so it made sense to
repurpose WITH_12BIT to allow 12-bit data precision to be disabled.
e8b40f3c2b made it so that the libjpeg API
library can handle multiple JPEG data precisions at run time via a
handful of straightforward API extensions. Referring to
6c2bc901e2, it hasn't been possible to
build libjpeg-turbo with both forward and backward libjpeg API/ABI
compatibility since libjpeg-turbo 1.4.x. Thus, whereas we retain full
backward API/ABI compatibility with libjpeg v6b-v8, forward libjpeg
API/ABI compatibility ceased being realistic years ago, so it no longer
makes sense to provide compile-time options that give a false sense of
forward API/ABI compatibility by allowing some (but not all) of our
libjpeg API extensions to be disabled. Such options are difficult to
maintain and clutter the code with #ifdefs.
The Gordian knot that 7fec5074f9 attempted
to unravel was caused by the fact that there are several
data-precision-dependent (JSAMPLE-dependent) fields and methods in the
exposed libjpeg API structures, and if you change the exposed libjpeg
API structures, then you have to change the whole API. If you change
the whole API, then you have to provide a whole new library to support
the new API, and that makes it difficult to support multiple data
precisions in the same application. (It is not impossible, as example.c
demonstrated, but using data-precision-dependent libjpeg API structures
would have made the cjpeg, djpeg, and jpegtran source code hard to read,
so it made more sense to build, install, and package 12-bit-specific
versions of those applications.)
Unfortunately, the result of that initial integration effort was an
unreadable and unmaintainable mess, which is a problem for a library
that is an ISO/ITU-T reference implementation. Also, as I dug into the
problem of lossless JPEG support, I realized that 16-bit lossless JPEG
images are a thing, and supporting yet another version of the libjpeg
API just for those images is untenable.
In fact, however, the touch points for JSAMPLE in the exposed libjpeg
API structures are minimal:
- The colormap and sample_range_limit fields in jpeg_decompress_struct
- The alloc_sarray() and access_virt_sarray() methods in
jpeg_memory_mgr
- jpeg_write_scanlines() and jpeg_write_raw_data()
- jpeg_read_scanlines() and jpeg_read_raw_data()
- jpeg_skip_scanlines() and jpeg_crop_scanline()
(This is subtle, but both of those functions use JSAMPLE-dependent
opaque structures behind the scenes.)
It is much more readable and maintainable to provide 12-bit-specific
versions of those six top-level API functions and to document that the
aforementioned methods and fields must be type-cast when using 12-bit
samples. Since that eliminates the need to provide a 12-bit-specific
version of the exposed libjpeg API structures, we can:
- Compile only the precision-dependent libjpeg modules (the
coefficient buffer controllers, the colorspace converters, the
DCT/IDCT managers, the main buffer controllers, the preprocessing
and postprocessing controller, the downsampler and upsamplers, the
quantizers, the integer DCT methods, and the IDCT methods) for
multiple data precisions.
- Introduce 12-bit-specific methods into the various internal
structures defined in jpegint.h.
- Create precision-independent data type, macro, method, field, and
function names that are prefixed by an underscore, and use an
internal header to convert those into precision-dependent data
type, macro, method, field, and function names, based on the value
of BITS_IN_JSAMPLE, when compiling the precision-dependent libjpeg
modules.
- Expose precision-dependent jinit*() functions for each of the
precision-dependent libjpeg modules.
- Abstract the precision-dependent libjpeg modules by calling the
appropriate precision-dependent jinit*() function, based on the
value of cinfo->data_precision, from top-level libjpeg API
functions.
By default, libjpeg-turbo 1.3.x and later have enabled the in-memory
source/destination manager functions from libjpeg v8 when emulating the
libjpeg v6b or v7 API/ABI, which has allowed operating system
distributors to provide those functions without adopting the
backward-incompatible libjpeg v8 API/ABI.
Prior to libjpeg-turbo 1.5.x, it made sense to allow users to disable
the in-memory source/destination manager functions at build time and
thus retain both backward and forward API/ABI compatibility relative to
libjpeg v6b or v7. Since then, however, we have introduced several new
libjpeg API functions that break forward API/ABI compatibility, so it no
longer makes sense to allow the in-memory source/destination managers to
be disabled. libjpeg-turbo only claims to be
backward-API/ABI-compatible, i.e. to allow applications built against
libjpeg or an older version of libjpeg-turbo to work properly with the
current version of libjpeg-turbo.
@rpath is only supported with 10.5 and later deployment targets.
libjpeg-turbo hasn't supported 10.4 "Tiger" since prior to 1.4, but I
still sometimes use the 10.4 SDK to test PowerPC code in a Snow Leopard
VM.
When attempting to configure an iOS/ARM build with Xcode 7.2 and CMake
2.8.12, I got the following errors:
CMake Error at CMakeLists.txt:560 (add_library):
Attempting to use MACOSX_RPATH without CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG
being set. This could be because you are using a Mac OS X version less
than 10.5 or because CMake's platform configuration is corrupt.
(x 3)
CMake Error at sharedlib/CMakeLists.txt:38 (add_library):
Attempting to use MACOSX_RPATH without CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG
being set. This could be because you are using a Mac OS X version less
than 10.5 or because CMake's platform configuration is corrupt.
(x 3)
Upgrading to CMake 3.x (tried 3.0 and 3.1) got rid of the errors, but
the resulting shared libs still did not use @rpath as expected. Note
also that CMake 3.x (at least the two versions I tested) does not
automatically set the MACOSX_RPATH property as claimed. I could find
nothing in the release notes for later CMake releases to indicate that
either problem has been fixed. What I did find was this little nugget
of code in the Darwin platform module:
f6b93fbf3a/Modules/Platform/Darwin.cmake (L33-L36)
This sets CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG="-Wl,-rpath," only if you
are running OS X 10.5 or later. It makes no such check for iOS, perhaps
because shared libraries aren't much of a thing with iOS apps. In any
event, this commit simply sets CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG if it
isn't set already, and that fixes all of the aforementioned problems.
With rare exceptions ...
- Always separate line continuation characters by one space from
preceding code.
- Always use two-space indentation. Never use tabs.
- Always use K&R-style conditional blocks.
- Always surround operators with spaces, except in raw assembly code.
- Always put a space after, but not before, a comma.
- Never put a space between type casts and variables/function calls.
- Never put a space between the function name and the argument list in
function declarations and prototypes.
- Always surround braces ('{' and '}') with spaces.
- Always surround statements (if, for, else, catch, while, do, switch)
with spaces.
- Always attach pointer symbols ('*' and '**') to the variable or
function name.
- Always precede pointer symbols ('*' and '**') by a space in type
casts.
- Use the MIN() macro from jpegint.h within the libjpeg and TurboJPEG
API libraries (using min() from tjutil.h is still necessary for
TJBench.)
- Where it makes sense (particularly in the TurboJPEG code), put a blank
line after variable declaration blocks.
- Always separate statements in one-liners by two spaces.
The purpose of this was to ease maintenance on my part and also to make
it easier for contributors to figure out how to format patch
submissions. This was admittedly confusing (even to me sometimes) when
we had 3 or 4 different style conventions in the same source tree. The
new convention is more consistent with the formatting of other OSS code
bases.
This commit corrects deviations from the chosen formatting style in the
libjpeg API code and reformats the TurboJPEG API code such that it
conforms to the same standard.
NOTES:
- Although it is no longer necessary for the function name in function
declarations to begin in Column 1 (this was historically necessary
because of the ansi2knr utility, which allowed libjpeg to be built
with non-ANSI compilers), we retain that formatting for the libjpeg
code because it improves readability when using libjpeg's function
attribute macros (GLOBAL(), etc.)
- This reformatting project was accomplished with the help of AStyle and
Uncrustify, although neither was completely up to the task, and thus
a great deal of manual tweaking was required. Note to developers of
code formatting utilities: the libjpeg-turbo code base is an
excellent test bed, because AFAICT, it breaks every single one of the
utilities that are currently available.
- The legacy (MMX, SSE, 3DNow!) assembly code for i386 has been
formatted to match the SSE2 code (refer to
ff5685d5344273df321eb63a005eaae19d2496e3.) I hadn't intended to
bother with this, but the Loongson MMI implementation demonstrated
that there is still academic value to the MMX implementation, as an
algorithmic model for other 64-bit vector implementations. Thus, it
is desirable to improve its readability in the same manner as that of
the SSE2 implementation.
This builds upon the existing GNUInstallDirs module in CMake but adds
the following features to that module:
- The ability to override the defaults for each install directory
through a new set of variables (`CMAKE_INSTALL_DEFAULT_*DIR`).
Before operating system vendors began shipping libjpeg-turbo, it was
meant to be a run-time drop-in replacement for the system's
distribution of libjpeg, so it has traditionally installed itself
under /opt/libjpeg-turbo on Un*x systems by default. On Windows, it
has traditionally installed itself under %SystemDrive%\libjpeg-turbo*,
which is not uncommon behavior for open source libraries (open source
SDKs tend to install outside of the Program Files directory so as to
avoid spaces in the directory name.) At least in the case of Un*x,
the install directory behavior is based somewhat on the Solaris
standard, which requires all non-O/S packages to install their files
under /opt/{package_name}. I adopted that standard for VirtualGL and
TurboVNC while working at Sun, because it allowed those packages to be
located under the same directory on all platforms. I adopted it for
libjpeg-turbo because it ensured that our files would never conflict
with the system's version of libjpeg. Even though many Un*x
distributions ship libjpeg-turbo these days, not all of them ship the
TurboJPEG API library or the Java classes or even the latest version
of the libjpeg API library, so there are still many cases in which it
is desirable to install a separate version of libjpeg-turbo than the
one installed by the system. Furthermore, installing the files under
/opt mimics the directory structure of our official binary packages,
and it makes it very easy to uninstall libjpeg-turbo.
For these reasons, our build system needs to be able to use
non-GNU-compliant defaults for each install directory if
`CMAKE_INSTALL_PREFIX` is set to the default value.
- For each directory variable, the module now detects changes to
`CMAKE_INSTALL_PREFIX` and changes the directory variable accordingly,
if the variable has not been changed by the user.
This makes it easy to switch between our "official" directory
structure and the GNU-compliant directory structure "on the fly"
simply by changing `CMAKE_INSTALL_PREFIX`. Also, this new mechanism
eliminated the need for the crufty mechanism that previously did the
same thing just for the library directory variable.
How it should work:
- If a dir variable is unset, then the module will set an internal
property indicating that the dir variable was initialized to its
default value.
- If the dir variable ever diverges from its default value, then the
internal property is cleared, and it cannot be set again without
unsetting the dir variable.
- If the install prefix changes, and if the internal property
indicates that the dir variable is still set to its default value,
and if the dir variable's value is not being manually changed at the
same time that the install prefix is being changed, then the dir
variable's value is automatically changed to the new default value
for that variable (as determined by the new install prefix.)
- The directory variables are now always cached, regardless of whether
they were set on the command line or not. This ensures that they can
easily be examined and modified after being set, regardless of how they
were set.
This was made possible by the introduction of the aforementioned
`CMAKE_INSTALL_DEFAULT_*DIR` variables.
- Improved directory variable documentation (based on descriptions at
https://www.gnu.org/prep/standards/html_node/Directory-Variables.html)
- The module now allows "<DATAROOTDIR>" to be used as a placeholder in
relative directory variables.
It is replaced "on the fly" with the actual path of
`CMAKE_INSTALL_DATAROOTDIR`.
This should more closely mimic the behavior of the old autotools build
system while retaining our customizations to it, and it should retain
the behavior of the old CMake build system.
Closes#124
- Replace CMAKE_SOURCE_DIR with CMAKE_CURRENT_SOURCE_DIR
- Replace CMAKE_BINARY_DIR with CMAKE_CURRENT_BINARY_DIR
- Don't use "libjpeg-turbo" in any of the package system filenames
(because CMAKE_PROJECT_NAME will not be the same if building LJT as
a submodule.)
Closes#122
CMAKE_INSTALL_RPATH has to be set before the targets are defined (oops.)
This also explicitly turns on MACOSX_RPATH for the shared libraries
(which is the default with newer versions of CMake but not with 2.8.x.)
The old autotools/libtool build system hard-coded the install name
directory of the OS X shared libraries to libdir, which meant that any
executable that linked against those libraries would also be hard-coded
to look for the libjpeg-turbo libraries in that directory. @rpath makes
the OS X version of libjpeg-turbo behave like the Linux version, in the
sense that the executables under /opt/libjpeg-turbo/bin will
automatically pick up the libraries under /opt/libjpeg-turbo/lib* by
default, but other executables won't unless they are linked with -rpath.
