Background

==========

libjpeg-turbo is a JPEG image codec that uses SIMD instructions to accelerate

baseline JPEG compression and decompression on x86, x86-64, Arm, PowerPC, and

MIPS systems, as well as progressive JPEG compression on x86 and x86-64

systems.  On such systems, libjpeg-turbo is generally 2-6x as fast as libjpeg,

all else being equal.  On other types of systems, libjpeg-turbo can still

outperform libjpeg by a significant amount, by virtue of its highly-optimized

Huffman coding routines.  In many cases, the performance of libjpeg-turbo

rivals that of proprietary high-speed JPEG codecs.

libjpeg-turbo implements both the traditional libjpeg API as well as the less

powerful but more straightforward TurboJPEG API.  libjpeg-turbo also features

colorspace extensions that allow it to compress from/decompress to 32-bit and

big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java

interface.

libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated

derivative of libjpeg v6b developed by Miyasaka Masaru.  The TigerVNC and

VirtualGL projects made numerous enhancements to the codec in 2009, and in

early 2010, libjpeg-turbo spun off into an independent project, with the goal

of making high-speed JPEG compression/decompression technology available to a

broader range of users and developers.

License

=======

libjpeg-turbo is covered by three compatible BSD-style open source licenses.

Refer to [LICENSE.md](LICENSE.md) for a roll-up of license terms.

Building libjpeg-turbo

======================

Refer to [BUILDING.md](BUILDING.md) for complete instructions.

Using libjpeg-turbo

===================

libjpeg-turbo includes two APIs that can be used to compress and decompress

JPEG images:

- **TurboJPEG API**

  This API provides an easy-to-use interface for compressing and decompressing

  JPEG images in memory.  It also provides some functionality that would not be

  straightforward to achieve using the underlying libjpeg API, such as

  generating planar YUV images and performing multiple simultaneous lossless

  transforms on an image.  The Java interface for libjpeg-turbo is written on

  top of the TurboJPEG API.  The TurboJPEG API is recommended for first-time

  users of libjpeg-turbo.  Refer to [tjexample.c](tjexample.c) and

  [TJExample.java](java/TJExample.java) for examples of its usage and to

  <http: documentation="" libjpeg-turbo.org=""> for API documentation.</http:>

- **libjpeg API**

  This is the de facto industry-standard API for compressing and decompressing

  JPEG images.  It is more difficult to use than the TurboJPEG API but also

  more powerful.  The libjpeg API implementation in libjpeg-turbo is both

  API/ABI-compatible and mathematically compatible with libjpeg v6b.  It can

  also optionally be configured to be API/ABI-compatible with libjpeg v7 and v8

  (see below.)  Refer to [cjpeg.c](cjpeg.c) and [djpeg.c](djpeg.c) for examples

  of its usage and to [libjpeg.txt](libjpeg.txt) for API documentation.

There is no significant performance advantage to either API when both are used

to perform similar operations.

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 ten 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 */

    JCS_EXT_RGBA  /* red/green/blue/alpha */

    JCS_EXT_BGRA  /* blue/green/red/alpha */

    JCS_EXT_ABGR  /* alpha/blue/green/red */

    JCS_EXT_ARGB  /* alpha/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 libjpeg implementation

that does not support them will result in a "Bogus input colorspace" error.

Applications can trap this error in order to test whether run-time support is

available for the colorspace extensions.

When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the

X byte is undefined, and in order to ensure the best performance, libjpeg-turbo

can set that byte to whatever value it wishes.  If an application expects the X

byte to be used as an alpha channel, then it should specify `JCS_EXT_RGBA`,

`JCS_EXT_BGRA`, `JCS_EXT_ABGR`, or `JCS_EXT_ARGB`.  When these colorspace

constants are used, the X byte is guaranteed to be 0xFF, which is interpreted

as opaque.

Your application can check for the existence of the alpha channel colorspace

extensions at compile time with:

    #ifdef JCS_ALPHA_EXTENSIONS

[jcstest.c](jcstest.c), located in the libjpeg-turbo source tree, demonstrates

how to check for the existence of the colorspace extensions at compile time and

run time.

libjpeg v7 and v8 API/ABI Emulation

-----------------------------------

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 were

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 not

as widely used as v6b, enough programs (including a few Linux distros) made

the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs

in libjpeg-turbo.  It should be noted, however, that this feature was added

primarily so that applications that had already been compiled to use libjpeg

v7+ could take advantage of accelerated baseline JPEG encoding/decoding

without recompiling.  libjpeg-turbo does not claim to support all of the

libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all

cases (see below.)

By passing 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 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.

### Support for libjpeg v7 and v8 Features

#### Fully supported

- **libjpeg API: IDCT scaling extensions in decompressor**

  libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,

  1/2, 5/8, 3/4, 7/8, 9/8, 5/4, 11/8, 3/2, 13/8, 7/4, 15/8, and 2/1 (only 1/4

  and 1/2 are SIMD-accelerated.)

- **libjpeg API: Arithmetic coding**

- **libjpeg API: In-memory source and destination managers**

  See notes below.

- **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**

- **cjpeg: `-rgb` option**

- **jpegtran: Lossless cropping**

- **jpegtran: `-perfect` option**

- **jpegtran: Forcing width/height when performing lossless crop**

- **rdjpgcom: `-raw` option**

- **rdjpgcom: Locale awareness**

#### Not supported

NOTE:  As of this writing, extensive research has been conducted into the

usefulness of DCT scaling as a means of data reduction and SmartScale as a

means of quality improvement.  Readers are invited to peruse the research at

<http: about="" smartscale="" www.libjpeg-turbo.org=""> and draw their own conclusions,</http:>

but it is the general belief of our project that these features have not

demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.

- **libjpeg API: DCT scaling in compressor**

  `cinfo.scale_num` and `cinfo.scale_denom` are silently ignored.

  There is no technical reason why DCT scaling could not be supported when

  emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see

  below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and

  8/9 would be available, which is of limited usefulness.

- **libjpeg API: SmartScale**

  `cinfo.block_size` is silently ignored.

  SmartScale is an extension to the JPEG format that allows for DCT block

  sizes other than 8x8.  Providing support for this new format would be

  feasible (particularly without full acceleration.)  However, until/unless

  the format becomes either an official industry standard or, at minimum, an

  accepted solution in the community, we are hesitant to implement it, as

  there is no sense of whether or how it might change in the future.  It is

  our belief that SmartScale has not demonstrated sufficient usefulness as a

  lossless format nor as a means of quality enhancement, and thus our primary

  interest in providing this feature would be as a means of supporting

  additional DCT scaling factors.

- **libjpeg API: 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.

### What About libjpeg v9?

libjpeg v9 introduced yet another field to the JPEG compression structure

(`color_transform`), thus making the ABI backward incompatible with that of

libjpeg v8.  This new field was introduced solely for the purpose of supporting

lossless SmartScale encoding.  Furthermore, there was actually no reason to

extend the API in this manner, as the color transform could have just as easily

been activated by way of a new JPEG colorspace constant, thus preserving

backward ABI compatibility.

Our research (see link above) has shown that lossless SmartScale does not

generally accomplish anything that can't already be accomplished better with

existing, standard lossless formats.  Therefore, at this time it is our belief

that there is not sufficient technical justification for software projects to

upgrade from libjpeg v8 to libjpeg v9, and thus there is not sufficient

technical justification for us to emulate the libjpeg v9 ABI.

In-Memory Source/Destination Managers

-------------------------------------

By default, libjpeg-turbo 1.3 and later includes the `jpeg_mem_src()` and

`jpeg_mem_dest()` functions, even when not emulating the libjpeg v8 API/ABI.

Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8

API/ABI emulation in order to use the in-memory source/destination managers,

but several projects requested that those functions be included when emulating

the libjpeg v6b API/ABI as well.  This allows the use of those functions by

programs that need them, without breaking ABI compatibility for programs that

don't, and it allows those functions to be provided in the "official"

libjpeg-turbo binaries.

Those who are concerned about maintaining strict conformance with the libjpeg

v6b or v7 API can pass an argument of `-DWITH_MEM_SRCDST=0` to `cmake` prior to

building libjpeg-turbo.  This will restore the pre-1.3 behavior, in which

`jpeg_mem_src()` and `jpeg_mem_dest()` are only included when emulating the

libjpeg v8 API/ABI.

On Un*x systems, including the in-memory source/destination managers changes

the dynamic library version from 62.2.0 to 62.3.0 if using libjpeg v6b API/ABI

emulation and from 7.2.0 to 7.3.0 if using libjpeg v7 API/ABI emulation.

Note that, on most Un*x systems, the dynamic linker will not look for a

function in a library until that function is actually used.  Thus, if a program

is built against libjpeg-turbo 1.3+ and uses `jpeg_mem_src()` or

`jpeg_mem_dest()`, that program will not fail if run against an older version

of libjpeg-turbo or against libjpeg v7- until the program actually tries to

call `jpeg_mem_src()` or `jpeg_mem_dest()`.  Such is not the case on Windows.

If a program is built against the libjpeg-turbo 1.3+ DLL and uses

`jpeg_mem_src()` or `jpeg_mem_dest()`, then it must use the libjpeg-turbo 1.3+

DLL at run time.

Both cjpeg and djpeg have been extended to allow testing the in-memory

source/destination manager functions.  See their respective man pages for more

details.

Mathematical Compatibility

==========================

For the most part, libjpeg-turbo should produce identical output to libjpeg

v6b.  The one exception to this is when using the floating point DCT/IDCT, in

which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the

following reasons:

- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so

  slightly more accurate than the implementation in libjpeg v6b, but not by

  any amount perceptible to human vision (generally in the range of 0.01 to

  0.08 dB gain in PNSR.)

- When not using the SIMD extensions, libjpeg-turbo uses the more accurate

  (and slightly faster) floating point IDCT algorithm introduced in libjpeg

  v8a as opposed to the algorithm used in libjpeg v6b.  It should be noted,

  however, that this algorithm basically brings the accuracy of the floating

  point IDCT in line with the accuracy of the accurate integer IDCT.  The

  floating point DCT/IDCT algorithms are mainly a legacy feature, and they do

  not produce significantly more accuracy than the accurate integer algorithms

  (to put numbers on this, the typical difference in PNSR between the two

  algorithms is less than 0.10 dB, whereas changing the quality level by 1 in

  the upper range of the quality scale is typically more like a 1.0 dB

  difference.)

- If the floating point algorithms in libjpeg-turbo are not implemented using

  SIMD instructions on a particular platform, then the accuracy of the

  floating point DCT/IDCT can depend on the compiler settings.

While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood it is

still using the same algorithms as libjpeg v6b, so there are several specific

cases in which libjpeg-turbo cannot be expected to produce the same output as

libjpeg v8:

- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8

  implements those scaling algorithms differently than libjpeg v6b does, and

  libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.

- When using chrominance subsampling, because libjpeg v8 implements this

  with its DCT/IDCT scaling algorithms rather than with a separate

  downsampling/upsampling algorithm.  In our testing, the subsampled/upsampled

  output of libjpeg v8 is less accurate than that of libjpeg v6b for this

  reason.

- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or

  "non-smooth") chrominance upsampling, because libjpeg v8 does not support

  merged upsampling with scaling factors > 1.

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 accurate integer forward DCT

whenever encoding images with a JPEG quality of 98 or higher.

Memory Debugger Pitfalls

========================

Valgrind and Memory Sanitizer (MSan) can generate false positives

(specifically, incorrect reports of uninitialized memory accesses) when used

with libjpeg-turbo's SIMD extensions.  It is generally recommended that the

SIMD extensions be disabled, either by passing an argument of `-DWITH_SIMD=0`

to `cmake` when configuring the build or by setting the environment variable

`JSIMD_FORCENONE` to `1` at run time, when testing libjpeg-turbo with Valgrind,

MSan, or other memory debuggers.