*******************************************************************************
**     Background
*******************************************************************************

libjpeg-turbo is a derivative of libjpeg that uses SIMD instructions (MMX,
SSE2, etc.) to accelerate baseline JPEG compression and decompression on x86
and x86-64 systems.  On such systems, libjpeg-turbo is generally 2-4x as fast
as the unmodified version of libjpeg, all else being equal.

libjpeg-turbo was originally based on libjpeg/SIMD by Miyasaka Masaru, but
the TigerVNC and VirtualGL projects made numerous enhancements to the codec in
2009, including improved support for Mac OS X, 64-bit support, support for
32-bit and big-endian pixel formats (RGBX, XBGR, etc.), accelerated Huffman
encoding/decoding, and various bug fixes.  The goal was to produce a fully
open-source codec that could replace the partially closed-source TurboJPEG/IPP
codec used by VirtualGL and TurboVNC.  libjpeg-turbo generally achieves 80-120%
of the performance of TurboJPEG/IPP.  It is faster in some areas but slower in
others.

In early 2010, libjpeg-turbo spun off into its own independent project, with
the goal of making high-speed JPEG compression/decompression technology
available to a broader range of users and developers.


*******************************************************************************
**     License
*******************************************************************************

The TurboJPEG/OSS wrapper, as well as some of the optimizations to the Huffman
encoder (jchuff.c) and decoder (jdhuff.c), were borrowed from VirtualGL, and
thus any distribution of libjpeg-turbo that includes those files must, as a
whole, be subject to the terms of the wxWindows Library Licence, Version 3.1.
A copy of this license can be found in this directory under LICENSE.txt.  The
wxWindows Library License is based on the LGPL but includes provisions that
allow the Library to be statically linked into proprietary libraries and
applications without requiring the resulting binaries to be distributed under
the terms of the LGPL.

The rest of the source code, apart from TurboJPEG/OSS and the Huffman codec
optimizations, falls under a less restrictive, BSD-style license (see README.)
You can choose to distribute libjpeg-turbo, as a whole, under this BSD-style
license by simply removing TurboJPEG/OSS and replacing the optimized jchuff.c
and jdhuff.c with their unoptimized counterparts from the libjpeg v6b source.


*******************************************************************************
**     Using libjpeg-turbo
*******************************************************************************

=============================
Replacing libjpeg at Run Time
=============================

If a Unix application is dynamically linked with libjpeg, then you can replace
libjpeg with libjpeg-turbo at run time by manipulating LD_LIBRARY_PATH.
For instance:

  [Using libjpeg]
  > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg
  real  0m0.392s
  user  0m0.074s
  sys   0m0.020s

  [Using libjpeg-turbo]
  > export LD_LIBRARY_PATH=/opt/libjpeg-turbo/{lib}:$LD_LIBRARY_PATH
  > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg
  real  0m0.109s
  user  0m0.029s
  sys   0m0.010s

NOTE: {lib} can be lib, lib32, lib64, or lib/64, depending on the O/S and
architecture.

System administrators can also replace the libjpeg sym links in /usr/{lib} with
links to the libjpeg-turbo dynamic library located in /opt/libjpeg-turbo/{lib}.
This will effectively accelerate every application that uses the libjpeg
dynamic library on the system.

The libjpeg-turbo SDK for Visual C++ installs the libjpeg-turbo DLL
(jpeg62.dll, jpeg7.dll, or jpeg8.dll, depending on whether it was built with
libjpeg v6b, v7, or v8 emulation) into c:\libjpeg-turbo[64]\bin, and the PATH
environment variable can be modified such that this directory is searched
before any others that might contain a libjpeg DLL.  However, if a libjpeg
DLL exists in an application's install directory, then Windows will load this
DLL first whenever the application is launched.  Thus, if an application ships
with jpeg62.dll, jpeg7.dll, or jpeg8.dll, then back up the application's
version of this DLL and copy c:\libjpeg-turbo[64]\bin\jpeg*.dll into the
application's install directory to accelerate it.

The version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
Visual C++ requires the Visual C++ 2008 C run-time DLL (msvcr90.dll).
msvcr90.dll ships with more recent versions of Windows, but users of older
Windows releases can obtain it from the Visual C++ 2008 Redistributable
Package, which is available as a free download from Microsoft's web site.

NOTE:  Features of libjpeg that require passing a C run-time structure, such
as a file handle, from an application to libjpeg will probably not work with
the version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
Visual C++, unless the application is also built to use the Visual C++ 2008 C
run-time DLL.  In particular, this affects jpeg_stdio_dest() and
jpeg_stdio_src().

Mac applications typically embed their own copies of the libjpeg dylib inside
the (hidden) application bundle, so it is not possible to globally replace
libjpeg on OS X systems.  If an application uses a shared library version of
libjpeg, then it may be possible to replace the application's version of it.
This would generally involve copying libjpeg.*.dylib from libjpeg-turbo into
the appropriate place in the application bundle and using install_name_tool to
repoint the dylib to the new directory.  This requires an advanced knowledge of
OS X and would not survive an upgrade or a re-install of the application.
Thus, it is not recommended for most users.

=======================
Replacing TurboJPEG/IPP
=======================

libjpeg-turbo is a drop-in replacement for the TurboJPEG/IPP SDK used by
VirtualGL 2.1.x and TurboVNC 0.6 (and prior.)  libjpeg-turbo contains a wrapper
library (TurboJPEG/OSS) that emulates the TurboJPEG API using libjpeg-turbo
instead of the closed-source Intel Performance Primitives.  You can replace the
TurboJPEG/IPP package on Linux systems with the libjpeg-turbo package in order
to make existing releases of VirtualGL 2.1.x and TurboVNC 0.x use the new codec
at run time.  Note that the 64-bit libjpeg-turbo packages contain only 64-bit
binaries, whereas the TurboJPEG/IPP 64-bit packages contained both 64-bit and
32-bit binaries.  Thus, to replace a TurboJPEG/IPP 64-bit package, install
both the 64-bit and 32-bit versions of libjpeg-turbo.

You can also build the VirtualGL 2.1.x and TurboVNC 0.6 source code with
the libjpeg-turbo SDK instead of TurboJPEG/IPP.  It should work identically.
libjpeg-turbo also includes static library versions of TurboJPEG/OSS, which
are used to build VirtualGL 2.2 and TurboVNC 1.0 and later.

========================================
Using libjpeg-turbo in Your Own Programs
========================================

For the most part, libjpeg-turbo should work identically to libjpeg, so in
most cases, an application can be built against libjpeg and then run against
libjpeg-turbo.  On Unix systems (including Cygwin), you can build against
libjpeg-turbo instead of libjpeg by setting

  CPATH=/opt/libjpeg-turbo/include
  and
  LIBRARY_PATH=/opt/libjpeg-turbo/{lib}

({lib} = lib32 or lib64, depending on whether you are building a 32-bit or a
64-bit application.)

If using MinGW, then set

  CPATH=/c/libjpeg-turbo-gcc[64]/include
  and
  LIBRARY_PATH=/c/libjpeg-turbo-gcc[64]/lib

Building against libjpeg-turbo is useful, for instance, if you want to build an
application that leverages the libjpeg-turbo colorspace extensions (see below.)
On Linux and Solaris systems, you would still need to manipulate
LD_LIBRARY_PATH or create appropriate sym links to use libjpeg-turbo at run
time.  On such systems, you can pass -R /opt/libjpeg-turbo/{lib} to the linker
to force the use of libjpeg-turbo at run time rather than libjpeg (also useful
if you want to leverage the colorspace extensions), or you can link against the
libjpeg-turbo static library.

To force a Linux, Solaris, or MinGW application to link against the static
version of libjpeg-turbo, you can use the following linker options:

  -Wl,-Bstatic -ljpeg -Wl,-Bdynamic

On OS X, simply add /opt/libjpeg-turbo/lib/libjpeg.a to the linker command
line (this also works on Linux and Solaris.)

To build Visual C++ applications using libjpeg-turbo, add
c:\libjpeg-turbo[64]\include to the system or user INCLUDE environment
variable and c:\libjpeg-turbo[64]\lib to the system or user LIB environment
variable, and then link against either jpeg.lib (to use the DLL version of
libjpeg-turbo) or jpeg-static.lib (to use the static version of libjpeg-turbo.)

=====================
Colorspace Extensions
=====================

libjpeg-turbo includes extensions that allow JPEG images to be compressed
directly from (and decompressed directly to) buffers that use BGR, BGRX,
RGBX, XBGR, and XRGB pixel ordering.  This is implemented with six new
colorspace constants:

  JCS_EXT_RGB   /* red/green/blue */
  JCS_EXT_RGBX  /* red/green/blue/x */
  JCS_EXT_BGR   /* blue/green/red */
  JCS_EXT_BGRX  /* blue/green/red/x */
  JCS_EXT_XBGR  /* x/blue/green/red */
  JCS_EXT_XRGB  /* x/red/green/blue */

Setting cinfo.in_color_space (compression) or cinfo.out_color_space
(decompression) to one of these values will cause libjpeg-turbo to read the
red, green, and blue values from (or write them to) the appropriate position in
the pixel when compressing from/decompressing to an RGB buffer.

Your application can check for the existence of these extensions at compile
time with:

  #ifdef JCS_EXTENSIONS

At run time, attempting to use these extensions with a version of libjpeg
that doesn't support them will result in a "Bogus input colorspace" error.

=================================
libjpeg v7 and v8 API/ABI support
=================================

With libjpeg v7 and v8, new features were added that necessitated extending the
compression and decompression structures.  Unfortunately, due to the exposed
nature of those structures, extending them also necessitated breaking backward
ABI compatibility with previous libjpeg releases.  Thus, programs that are
built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is
based on the libjpeg v6b code base.  Although libjpeg v7 and v8 are still not
as widely used as v6b, enough programs (including a few Linux distros) have
made the switch that it was desirable to provide support for the libjpeg v7/v8
API/ABI in libjpeg-turbo.  Although libjpeg-turbo can now be configured as a
drop-in replacement for libjpeg v7 or v8, it should be noted that not all of
the features in libjpeg v7 and v8 are supported (see below.)

By passing an argument of --with-jpeg7 or --with-jpeg8 to configure, or an
argument of -DWITH_JPEG7=1 or -DWITH_JPEG8=1 to cmake, you can build a version
of libjpeg-turbo that emulates the libjpeg v7 or v8 API/ABI, so that programs
that are built against libjpeg v7 or v8 can be run with libjpeg-turbo.  The
following section describes which libjpeg v7+ features are supported and which
aren't.

libjpeg v7 and v8 Features:
---------------------------

Fully supported:

-- cjpeg: Separate quality settings for luminance and chrominance
   Note that the libpjeg v7+ API was extended to accommodate this feature only
   for convenience purposes.  It has always been possible to implement this
   feature with libjpeg v6b (see rdswitch.c for an example.)

-- cjpeg: 32-bit BMP support

-- jpegtran: lossless cropping

-- jpegtran: -perfect option

-- rdjpgcom: -raw option

-- rdjpgcom: locale awareness


Fully supported when using libjpeg v7/v8 emulation:

-- libjpeg: In-memory source and destination managers


Not supported:

-- libjpeg: DCT scaling in compressor
   cinfo.scale_num and cinfo.scale_denom are silently ignored.
   There is no technical reason why DCT scaling cannot be supported, but
   without the SmartScale extension (see below), it would only be able to
   down-scale using ratios of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and 8/9,
   which is of limited usefulness.

-- libjpeg: SmartScale
   cinfo.block_size is silently ignored.
   SmartScale is an extension to the JPEG format that allows for DCT block
   sizes other than 8x8.  It would be difficult to support this feature while
   retaining backward compatibility with libjpeg v6b.

-- libjpeg: IDCT scaling extensions in decompressor
   libjpeg-turbo still supports IDCT scaling with scaling factors of 1/2, 1/4,
   and 1/8 (same as libjpeg v6b.)

-- libjpeg: Fancy downsampling in compressor
   cinfo.do_fancy_downsampling is silently ignored.
   This requires the DCT scaling feature, which is not supported.

-- jpegtran: Scaling
   This requires both the DCT scaling and SmartScale features, which are not
   supported.

-- Lossless RGB JPEG files
   This requires the SmartScale feature, which is not supported.


*******************************************************************************
**     Performance pitfalls
*******************************************************************************

===============
Restart Markers
===============

The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
in a way that makes the rest of the libjpeg infrastructure happy, so it is
necessary to use the slow Huffman decoder when decompressing a JPEG image that
has restart markers.  This can cause the decompression performance to drop by
as much as 20%, but the performance will still be much greater than that of
libjpeg.  Many consumer packages, such as PhotoShop, use restart markers when
generating JPEG images, so images generated by those programs will experience
this issue.

===============================================
Fast Integer Forward DCT at High Quality Levels
===============================================

The algorithm used by the SIMD-accelerated quantization function cannot produce
correct results whenever the fast integer forward DCT is used along with a JPEG
quality of 98-100.  Thus, libjpeg-turbo must use the non-SIMD quantization
function in those cases.  This causes performance to drop by as much as 40%.
It is therefore strongly advised that you use the slow integer forward DCT
whenever encoding images with a JPEG quality of 98 or higher.