DRC 94c64ead85 Various doc tweaks ...

- "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.

2024-06-24 22:11:43 -04:00

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