- "bits per component" = "bits per sample"
Describing the data precision of a JPEG image using "bits per
component" is technically correct, but "bits per sample" is the
terminology that the JPEG-1 spec uses. Also, "bits per component" is
more commonly used to describe the precision of packed-pixel formats
(as opposed to "bits per pixel") rather than planar formats, in which
all components are grouped together.
- Unmention legacy display technologies. Colormapped and monochrome
displays aren't a thing anymore, and even when they were still a
thing, it was possible to display full-color images to them. In 1991,
when JPEG decompression time was measured in minutes per megapixel, it
made sense to keep a decompressed copy of JPEG images on disk, in a
format that could be displayed without further color conversion (since
color conversion was slow and memory-intensive.) In 2024, JPEG
decompression time is measured in milliseconds per megapixel, and
color conversion is even faster. Thus, JPEG images can be
decompressed, displayed, and color-converted (if necessary) "on the
fly" at speeds too fast for human vision to perceive. (In fact, your
TV performs much more complicated decompression algorithms at least 60
times per second.)
- Document that color quantization (and associated features), GIF
input/output, Targa input/output, and OS/2 BMP input/output are legacy
features. Legacy status doesn't necessarily mean that the features
are deprecated. Rather, it is meant to discourage users from using
features that may be of little or no benefit on modern machines (such
as low-quality modes that had significant performance advantages in
the early 1990s but no longer do) and that are maintained on a
break/fix basis only.
- General wordsmithing, grammar/punctuation policing, and formatting
tweaks
- Clarify which data precisions each cjpeg input format and each djpeg
output format supports.
- cjpeg.1: Remove unnecessary and impolitic statement about the -targa
switch.
- Adjust or remove performance claims to reflect the fact that:
* On modern machines, the djpeg "-fast" switch has a negligible effect
on performance.
* There is a measurable difference between the performance of Floyd-
Steinberg dithering and no dithering, but it is not likely
perceptible to most users.
* There is a measurable difference between the performance of 1-pass
and 2-pass color quantization, but it is not likely perceptible to
most users.
* There is a measurable difference between the performance of
full-color and grayscale output when decompressing a full-color JPEG
image, but it is not likely perceptible to most users.
* IDCT scaling does not necessarily improve performance. (It
generally does if the scaling factor is <= 1/2 and generally doesn't
if the scaling factor is > 1/2, at least on my machine. The
performance claim made in jpeg-6b was probably invalidated when we
merged the additional scaling factors from jpeg-7.)
- Clarify which djpeg switches/output formats cannot be used when
decompressing lossless JPEG images.
- Remove djpeg hints, since those involve quality vs. speed tradeoffs
that are no longer relevant for modern machines.
- Remove documentation regarding using color quantization with 16-bit
data precision. (Color quantization requires lossy mode.)
- Java: Fix typos in TJDecompressor.decompress12() and
TJDecompressor.decompress16() documentation.
- jpegtran.1: Fix truncated paragraph
In a man page, a single quote at the start of a line is interpreted as
a macro.
Closes#775
- libjpeg.txt:
* Mention J16SAMPLE data type (oversight.)
* Remove statement about extending jdcolor.c. (libjpeg-turbo is not
quite as DIY as libjpeg once was.)
* Remove paragraph about tweaking the various typedefs in jmorecfg.h.
It is no longer relevant for modern machines.
* Remove caveat regarding systems with ints less than 16 bits wide.
(ANSI/ISO C requires an int to be at least 16 bits wide, and
libjpeg-turbo has never supported non-ANSI compilers.)
- usage.txt:
* Add copyright header.
* Document cjpeg -icc, -memdst, -report, -strict, and -version
switches.
* Document djpeg -icc, -maxscans, -memsrc, -report, -skip, -crop,
-strict, and -version switches.
* Document jpegtran -icc, -maxscans, -report, -strict, and -version
switches.
- Restore GIF read/compressed GIF write support from jpeg-6a and
jpeg-9d.
- Integrate jpegtran -wipe and -drop options from jpeg-9a and jpeg-9d.
- Integrate jpegtran -crop extension (for expanding the image size) from
jpeg-9a and jpeg-9d.
- Integrate other minor code tweaks from jpeg-9*
The sentence:
"Indeed, one of the original reasons for developing this free software
was to help force convergence on common, interoperable format standards
for JPEG files."
might be seen to imply that JPEG 2000 and JPEG XR are not interoperable
with themselves, although it is certainly the case that those formats
are not interoperable with each other, nor with
ITU T.81 | ISO/IEC 10918. They are also certainly not as common as
ITU T.81 | ISO/IEC 10918, and (as an example) popular web browsers will
not display JPEG 2000 files.
The sentence in question was originally referring to proprietary,
non-standard formats and was meant to provide historical context.
libjpeg was originally released prior to the adoption of JFIF as an
official standard, so it encouraged adoption of JFIF as a de facto
standard by providing, under a business-friendly free software license,
a library for reading and writing images in that format.
This commit merges the following paragraph from the latest libjpeg
release:
https://github.com/libjpeg-turbo/ijg/blob/jpeg-9c/README#L222-L229
which takes into account the fact that JFIF is now an official ISO/ITU-T
standard. I also included the ISO/IEC document number for the JFIF spec
(jpeg-9c included only the ITU-T rec number.)
This commit also heavily wordsmiths the "FILE FORMAT WARS" section.
In jpeg-7 and later, this section has become somewhat impolitic,
referring to JPEG 2000 and JPEG XR as "faulty technologies" and
"momentary mistakes." The original intent of this section, which was
introduced in jpeg-5 and refined in jpeg-6
(https://github.com/libjpeg-turbo/ijg/blob/jpeg-5/README#L317-L338,
https://github.com/libjpeg-turbo/ijg/blob/jpeg-6b/README#L335-L367)
was to highlight the problem of JPEG file format divergence that existed
in the 1990s prior to the adoption of JFIF as an official ISO/ITU-T
standard. That problem is fortunately no longer a problem, thanks in
part to the existence of libjpeg. I have attempted to preserve Tom's
intent of using this section to describe which file formats the code is
compatible with and why it isn't compatible with some file formats
bearing the name "JPEG." Such modifications always put our project in a
very awkward position, because we are not the IJG and do not claim to
be, but it is still necessary for us to modify the IJG README file from
time to time to eliminate obsolete information while attempting to
remain as neutral as possible.
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.
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.
