path: root/usage.txt

NOTE:  This file was modified by The libjpeg-turbo Project to include only
information relevant to libjpeg-turbo and to wordsmith certain sections.

USAGE instructions for the Independent JPEG Group's JPEG software
=================================================================

This file describes usage of the JPEG conversion programs cjpeg and djpeg,
as well as the utility programs jpegtran, rdjpgcom and wrjpgcom.  (See
the other documentation files if you wish to use the JPEG library within
your own programs.)

If you are on a Unix machine you may prefer to read the Unix-style manual
pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.


INTRODUCTION

These programs implement JPEG image encoding, decoding, and transcoding.
JPEG (pronounced "jay-peg") is a standardized compression method for
full-color and gray-scale images.


GENERAL USAGE

We provide two programs, cjpeg to compress an image file into JPEG format,
and djpeg to decompress a JPEG file back into a conventional image format.

On Unix-like systems, you say:
	cjpeg [switches] [imagefile] >jpegfile
or
	djpeg [switches] [jpegfile]  >imagefile
The programs read the specified input file, or standard input if none is
named.  They always write to standard output (with trace/error messages to
standard error).  These conventions are handy for piping images between
programs.

On most non-Unix systems, you say:
	cjpeg [switches] imagefile jpegfile
or
	djpeg [switches] jpegfile  imagefile
i.e., both the input and output files are named on the command line.  This
style is a little more foolproof, and it loses no functionality if you don't
have pipes.  (You can get this style on Unix too, if you prefer, by defining
TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.)

You can also say:
	cjpeg [switches] -outfile jpegfile  imagefile
or
	djpeg [switches] -outfile imagefile  jpegfile
This syntax works on all systems, so it is useful for scripts.

The currently supported image file formats are: PPM (PBMPLUS color format),
PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster Toolkit
format).  (RLE is supported only if the URT library is available.)
cjpeg recognizes the input image format automatically, with the exception
of some Targa-format files.  You have to tell djpeg which format to generate.

JPEG files are in the defacto standard JFIF file format.  There are other,
less widely used JPEG-based file formats, but we don't support them.

All switch names may be abbreviated; for example, -grayscale may be written
-gray or -gr.  Most of the "basic" switches can be abbreviated to as little as
one letter.  Upper and lower case are equivalent (-BMP is the same as -bmp).
British spellings are also accepted (e.g., -greyscale), though for brevity
these are not mentioned below.


CJPEG DETAILS

The basic command line switches for cjpeg are:

	-quality N[,...]  Scale quantization tables to adjust image quality.
			Quality is 0 (worst) to 100 (best); default is 75.
			(See below for more info.)

	-grayscale	Create monochrome JPEG file from color input.
			Be sure to use this switch when compressing a grayscale
			BMP file, because cjpeg isn't bright enough to notice
			whether a BMP file uses only shades of gray.  By
			saying -grayscale, you'll get a smaller JPEG file that
			takes less time to process.

	-rgb		Create RGB JPEG file.
			Using this switch suppresses the conversion from RGB
			colorspace input to the default YCbCr JPEG colorspace.

	-optimize	Perform optimization of entropy encoding parameters.
			Without this, default encoding parameters are used.
			-optimize usually makes the JPEG file a little smaller,
			but cjpeg runs somewhat slower and needs much more
			memory.  Image quality and speed of decompression are
			unaffected by -optimize.

	-progressive	Create progressive JPEG file (see below).

	-targa		Input file is Targa format.  Targa files that contain
			an "identification" field will not be automatically
			recognized by cjpeg; for such files you must specify
			-targa to make cjpeg treat the input as Targa format.
			For most Targa files, you won't need this switch.

The -quality switch lets you trade off compressed file size against quality of
the reconstructed image: the higher the quality setting, the larger the JPEG
file, and the closer the output image will be to the original input.  Normally
you want to use the lowest quality setting (smallest file) that decompresses
into something visually indistinguishable from the original image.  For this
purpose the quality setting should be between 50 and 95; the default of 75 is
often about right.  If you see defects at -quality 75, then go up 5 or 10
counts at a time until you are happy with the output image.  (The optimal
setting will vary from one image to another.)

-quality 100 will generate a quantization table of all 1's, minimizing loss
in the quantization step (but there is still information loss in subsampling,
as well as roundoff error).  This setting is mainly of interest for
experimental purposes.  Quality values above about 95 are NOT recommended for
normal use; the compressed file size goes up dramatically for hardly any gain
in output image quality.

In the other direction, quality values below 50 will produce very small files
of low image quality.  Settings around 5 to 10 might be useful in preparing an
index of a large image library, for example.  Try -quality 2 (or so) for some
amusing Cubist effects.  (Note: quality values below about 25 generate 2-byte
quantization tables, which are considered optional in the JPEG standard.
cjpeg emits a warning message when you give such a quality value, because some
other JPEG programs may be unable to decode the resulting file.  Use -baseline
if you need to ensure compatibility at low quality values.)

The -quality option has been extended in this version of cjpeg to support
separate quality settings for luminance and chrominance (or, in general,
separate settings for every quantization table slot.)  The principle is the
same as chrominance subsampling:  since the human eye is more sensitive to
spatial changes in brightness than spatial changes in color, the chrominance
components can be quantized more than the luminance components without
incurring any visible image quality loss.  However, unlike subsampling, this
feature reduces data in the frequency domain instead of the spatial domain,
which allows for more fine-grained control.  This option is useful in
quality-sensitive applications, for which the artifacts generated by
subsampling may be unacceptable.

The -quality option accepts a comma-separated list of parameters, which
respectively refer to the quality levels that should be assigned to the
quantization table slots.  If there are more q-table slots than parameters,
then the last parameter is replicated.  Thus, if only one quality parameter is
given, this is used for both luminance and chrominance (slots 0 and 1,
respectively), preserving the legacy behavior of cjpeg v6b and prior.  More (or
customized) quantization tables can be set with the -qtables option and
assigned to components with the -qslots option (see the "wizard" switches
below.)

JPEG  files  generated  with separate luminance and chrominance quality are
fully compliant with standard JPEG decoders.

CAUTION: For this setting to be useful, be sure to pass an argument of
-sample 1x1 to cjpeg to disable chrominance subsampling.  Otherwise, the
default subsampling level (2x2, AKA "4:2:0") will be used.

The -progressive switch creates a "progressive JPEG" file.  In this type of
JPEG file, the data is stored in multiple scans of increasing quality.  If the
file is being transmitted over a slow communications link, the decoder can use
the first scan to display a low-quality image very quickly, and can then
improve the display with each subsequent scan.  The final image is exactly
equivalent to a standard JPEG file of the same quality setting, and the total
file size is about the same --- often a little smaller.

Switches for advanced users:

	-arithmetic	Use arithmetic coding.  CAUTION: arithmetic coded JPEG
			is not yet widely implemented, so many decoders will
			be unable to view an arithmetic coded JPEG file at
			all.

	-dct int	Use integer DCT method (default).
	-dct fast	Use fast integer DCT (less accurate).
	-dct float	Use floating-point DCT method.
			The float method is very slightly more accurate than
			the int method, but is much slower unless your machine
			has very fast floating-point hardware.  Also note that
			results of the floating-point method may vary slightly
			across machines, while the integer methods should give
			the same results everywhere.  The fast integer method
			is much less accurate than the other two.

	-restart N	Emit a JPEG restart marker every N MCU rows, or every
			N MCU blocks if "B" is attached to the number.
			-restart 0 (the default) means no restart markers.

	-smooth N	Smooth the input image to eliminate dithering noise.
			N, ranging from 1 to 100, indicates the strength of
			smoothing.  0 (the default) means no smoothing.

	-maxmemory N	Set limit for amount of memory to use in processing
			large images.  Value is in thousands of bytes, or
			millions of bytes if "M" is attached to the number.
			For example, -max 4m selects 4000000 bytes.  If more
			space is needed, temporary files will be used.

	-verbose	Enable debug printout.  More -v's give more printout.
	or  -debug	Also, version information is printed at startup.

The -restart option inserts extra markers that allow a JPEG decoder to
resynchronize after a transmission error.  Without restart markers, any damage
to a compressed file will usually ruin the image from the point of the error
to the end of the image; with restart markers, the damage is usually confined
to the portion of the image up to the next restart marker.  Of course, the
restart markers occupy extra space.  We recommend -restart 1 for images that
will be transmitted across unreliable networks such as Usenet.

The -smooth option filters the input to eliminate fine-scale noise.  This is
often useful when converting dithered images to JPEG: a moderate smoothing
factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
in a smaller JPEG file and a better-looking image.  Too large a smoothing
factor will visibly blur the image, however.

Switches for wizards:

	-baseline	Force baseline-compatible quantization tables to be
			generated.  This clamps quantization values to 8 bits
			even at low quality settings.  (This switch is poorly
			named, since it does not ensure that the output is
			actually baseline JPEG.  For example, you can use
			-baseline and -progressive together.)

	-qtables file	Use the quantization tables given in the specified
			text file.

	-qslots N[,...] Select which quantization table to use for each color
			component.

	-sample HxV[,...]  Set JPEG sampling factors for each color component.

	-scans file	Use the scan script given in the specified text file.

The "wizard" switches are intended for experimentation with JPEG.  If you
don't know what you are doing, DON'T USE THEM.  These switches are documented
further in the file wizard.txt.


DJPEG DETAILS

The basic command line switches for djpeg are:

	-colors N	Reduce image to at most N colors.  This reduces the
	or -quantize N	number of colors used in the output image, so that it
			can be displayed on a colormapped display or stored in
			a colormapped file format.  For example, if you have
			an 8-bit display, you'd need to reduce to 256 or fewer
			colors.  (-colors is the recommended name, -quantize
			is provided only for backwards compatibility.)

	-fast		Select recommended processing options for fast, low
			quality output.  (The default options are chosen for
			highest quality output.)  Currently, this is equivalent
			to "-dct fast -nosmooth -onepass -dither ordered".

	-grayscale	Force gray-scale output even if JPEG file is color.
			Useful for viewing on monochrome displays; also,
			djpeg runs noticeably faster in this mode.

	-scale M/N	Scale the output image by a factor M/N.  Currently
			the scale factor must be M/8, where M is an integer
			between 1 and 16 inclusive, or any reduced fraction
			thereof (such as 1/2, 3/4, etc.  Scaling is handy if
			the image is larger than your screen; also, djpeg runs
			much faster when scaling down the output.

	-bmp		Select BMP output format (Windows flavor).  8-bit
			colormapped format is emitted if -colors or -grayscale
			is specified, or if the JPEG file is gray-scale;
			otherwise, 24-bit full-color format is emitted.

	-gif		Select GIF output format.  Since GIF does not support
			more than 256 colors, -colors 256 is assumed (unless
			you specify a smaller number of colors).  If you
			specify -fast, the default number of colors is 216.

	-os2		Select BMP output format (OS/2 1.x flavor).  8-bit
			colormapped format is emitted if -colors or -grayscale
			is specified, or if the JPEG file is gray-scale;
			otherwise, 24-bit full-color format is emitted.

	-pnm		Select PBMPLUS (PPM/PGM) output format (this is the
			default format).  PGM is emitted if the JPEG file is
			gray-scale or if -grayscale is specified; otherwise
			PPM is emitted.

	-rle		Select RLE output format.  (Requires URT library.)

	-targa		Select Targa output format.  Gray-scale format is
			emitted if the JPEG file is gray-scale or if
			-grayscale is specified; otherwise, colormapped format
			is emitted if -colors is specified; otherwise, 24-bit
			full-color format is emitted.

Switches for advanced users:

	-dct int	Use integer DCT method (default).
	-dct fast	Use fast integer DCT (less accurate).
	-dct float	Use floating-point DCT method.
			The float method is very slightly more accurate than
			the int method, but is much slower unless your machine
			has very fast floating-point hardware.  Also note that
			results of the floating-point method may vary slightly
			across machines, while the integer methods should give
			the same results everywhere.  The fast integer method
			is much less accurate than the other two.

	-dither fs	Use Floyd-Steinberg dithering in color quantization.
	-dither ordered	Use ordered dithering in color quantization.
	-dither none	Do not use dithering in color quantization.
			By default, Floyd-Steinberg dithering is applied when
			quantizing colors; this is slow but usually produces
			the best results.  Ordered dither is a compromise
			between speed and quality; no dithering is fast but
			usually looks awful.  Note that these switches have
			no effect unless color quantization is being done.
			Ordered dither is only available in -onepass mode.

	-map FILE	Quantize to the colors used in the specified image
			file.  This is useful for producing multiple files
			with identical color maps, or for forcing a predefined
			set of colors to be used.  The FILE must be a GIF
			or PPM file.  This option overrides -colors and
			-onepass.

	-nosmooth	Use a faster, lower-quality upsampling routine.

	-onepass	Use one-pass instead of two-pass color quantization.
			The one-pass method is faster and needs less memory,
			but it produces a lower-quality image.  -onepass is
			ignored unless you also say -colors N.  Also,
			the one-pass method is always used for gray-scale
			output (the two-pass method is no improvement then).

	-maxmemory N	Set limit for amount of memory to use in processing
			large images.  Value is in thousands of bytes, or
			millions of bytes if "M" is attached to the number.
			For example, -max 4m selects 4000000 bytes.  If more
			space is needed, temporary files will be used.

	-verbose	Enable debug printout.  More -v's give more printout.
	or  -debug	Also, version information is printed at startup.


HINTS FOR CJPEG

Color GIF files are not the ideal input for JPEG; JPEG is really intended for
compressing full-color (24-bit) images.  In particular, don't try to convert
cartoons, line drawings, and other images that have only a few distinct
colors.  GIF works great on these, JPEG does not.  If you want to convert a
GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
to get a satisfactory conversion.  -smooth 10 or so is often helpful.

Avoid running an image through a series of JPEG compression/decompression
cycles.  Image quality loss will accumulate; after ten or so cycles the image
may be noticeably worse than it was after one cycle.  It's best to use a
lossless format while manipulating an image, then convert to JPEG format when
you are ready to file the image away.

The -optimize option to cjpeg is worth using when you are making a "final"
version for posting or archiving.  It's also a win when you are using low
quality settings to make very small JPEG files; the percentage improvement
is often a lot more than it is on larger files.  (At present, -optimize
mode is always selected when generating progressive JPEG files.)

Support for GIF input files was removed in cjpeg v6b due to concerns over
the Unisys LZW patent.  Although this patent expired in 2006, cjpeg still
lacks GIF support, for these historical reasons.  (Conversion of GIF files to
JPEG is usually a bad idea anyway.)


HINTS FOR DJPEG

To get a quick preview of an image, use the -grayscale and/or -scale switches.
"-grayscale -scale 1/8" is the fastest case.

Several options are available that trade off image quality to gain speed.
"-fast" turns on the recommended settings.

"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
When producing a color-quantized image, "-onepass -dither ordered" is fast but
much lower quality than the default behavior.  "-dither none" may give
acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.

If you are fortunate enough to have very fast floating point hardware,
"-dct float" may be even faster than "-dct fast".  But on most machines
"-dct float" is slower than "-dct int"; in this case it is not worth using,
because its theoretical accuracy advantage is too small to be significant
in practice.

Two-pass color quantization requires a good deal of memory; on MS-DOS machines
it may run out of memory even with -maxmemory 0.  In that case you can still
decompress, with some loss of image quality, by specifying -onepass for
one-pass quantization.

To avoid the Unisys LZW patent, djpeg produces uncompressed GIF files.  These
are larger than they should be, but are readable by standard GIF decoders.


HINTS FOR BOTH PROGRAMS

If more space is needed than will fit in the available main memory (as
determined by -maxmemory), temporary files will be used.  (MS-DOS versions
will try to get extended or expanded memory first.)  The temporary files are
often rather large: in typical cases they occupy three bytes per pixel, for
example 3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enough
free disk space, leave out -progressive and -optimize (for cjpeg) or specify
-onepass (for djpeg).

On MS-DOS, the temporary files are created in the directory named by the TMP
or TEMP environment variable, or in the current directory if neither of those
exist.  Amiga implementations put the temp files in the directory named by
JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
space.

The default memory usage limit (-maxmemory) is set when the software is
compiled.  If you get an "insufficient memory" error, try specifying a smaller
-maxmemory value, even -maxmemory 0 to use the absolute minimum space.  You
may want to recompile with a smaller default value if this happens often.

On machines that have "environment" variables, you can define the environment
variable JPEGMEM to set the default memory limit.  The value is specified as
described for the -maxmemory switch.  JPEGMEM overrides the default value
specified when the program was compiled, and itself is overridden by an
explicit -maxmemory switch.

On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
use.  (Extended or expanded memory is also used if available.)  Most
DOS-specific versions of this software do their own memory space estimation
and do not need you to specify -maxmemory.


JPEGTRAN

jpegtran performs various useful transformations of JPEG files.
It can translate the coded representation from one variant of JPEG to another,
for example from baseline JPEG to progressive JPEG or vice versa.  It can also
perform some rearrangements of the image data, for example turning an image
from landscape to portrait format by rotation.

jpegtran works by rearranging the compressed data (DCT coefficients), without
ever fully decoding the image.  Therefore, its transformations are lossless:
there is no image degradation at all, which would not be true if you used
djpeg followed by cjpeg to accomplish the same conversion.  But by the same
token, jpegtran cannot perform lossy operations such as changing the image
quality.

jpegtran uses a command line syntax similar to cjpeg or djpeg.
On Unix-like systems, you say:
	jpegtran [switches] [inputfile] >outputfile
On most non-Unix systems, you say:
	jpegtran [switches] inputfile outputfile
where both the input and output files are JPEG files.

To specify the coded JPEG representation used in the output file,
jpegtran accepts a subset of the switches recognized by cjpeg:
	-optimize	Perform optimization of entropy encoding parameters.
	-progressive	Create progressive JPEG file.
	-arithmetic	Use arithmetic coding.
	-restart N	Emit a JPEG restart marker every N MCU rows, or every
			N MCU blocks if "B" is attached to the number.
	-scans file	Use the scan script given in the specified text file.
See the previous discussion of cjpeg for more details about these switches.
If you specify none of these switches, you get a plain baseline-JPEG output
file.  The quality setting and so forth are determined by the input file.

The image can be losslessly transformed by giving one of these switches:
	-flip horizontal	Mirror image horizontally (left-right).
	-flip vertical		Mirror image vertically (top-bottom).
	-rotate 90		Rotate image 90 degrees clockwise.
	-rotate 180		Rotate image 180 degrees.
	-rotate 270		Rotate image 270 degrees clockwise (or 90 ccw).
	-transpose		Transpose image (across UL-to-LR axis).
	-transverse		Transverse transpose (across UR-to-LL axis).

The transpose transformation has no restrictions regarding image dimensions.
The other transformations operate rather oddly if the image dimensions are not
a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
transform complete blocks of DCT coefficient data in the desired way.

jpegtran's default behavior when transforming an odd-size image is designed
to preserve exact reversibility and mathematical consistency of the
transformation set.  As stated, transpose is able to flip the entire image
area.  Horizontal mirroring leaves any partial iMCU column at the right edge
untouched, but is able to flip all rows of the image.  Similarly, vertical
mirroring leaves any partial iMCU row at the bottom edge untouched, but is
able to flip all columns.  The other transforms can be built up as sequences
of transpose and flip operations; for consistency, their actions on edge
pixels are defined to be the same as the end result of the corresponding
transpose-and-flip sequence.

For practical use, you may prefer to discard any untransformable edge pixels
rather than having a strange-looking strip along the right and/or bottom edges
of a transformed image.  To do this, add the -trim switch:
	-trim		Drop non-transformable edge blocks.
Obviously, a transformation with -trim is not reversible, so strictly speaking
jpegtran with this switch is not lossless.  Also, the expected mathematical
equivalences between the transformations no longer hold.  For example,
"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
"-rot 180 -trim" trims both edges.

If you are only interested in perfect transformations, add the -perfect switch:
	-perfect	Fail with an error if the transformation is not
			perfect.
For example, you may want to do
  jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
to do a perfect rotation, if available, or an approximated one if not.

This version of jpegtran also offers a lossless crop option, which discards
data outside of a given image region but losslessly preserves what is inside.
Like the rotate and flip transforms, lossless crop is restricted by the current
JPEG format; the upper left corner of the selected region must fall on an iMCU
boundary.  If it doesn't, then it is silently moved up and/or left to the
nearest iMCU boundary (the lower right corner is unchanged.)

The image can be losslessly cropped by giving the switch:
	-crop WxH+X+Y	Crop to a rectangular region of width W and height H,
			starting at point X,Y.

Other not-strictly-lossless transformation switches are:

	-grayscale	Force grayscale output.
This option discards the chrominance channels if the input image is YCbCr
(ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
luminance channel is preserved exactly, so this is a better method of reducing
to grayscale than decompression, conversion, and recompression.  This switch
is particularly handy for fixing a monochrome picture that was mistakenly
encoded as a color JPEG.  (In such a case, the space savings from getting rid
of the near-empty chroma channels won't be large; but the decoding time for
a grayscale JPEG is substantially less than that for a color JPEG.)

jpegtran also recognizes these switches that control what to do with "extra"
markers, such as comment blocks:
	-copy none	Copy no extra markers from source file.  This setting
			suppresses all comments and other excess baggage
			present in the source file.
	-copy comments	Copy only comment markers.  This setting copies
			comments from the source file but discards
			any other data that is inessential for image display.
	-copy all	Copy all extra markers.  This setting preserves
			miscellaneous markers found in the source file, such
			as JFIF thumbnails, Exif data, and Photoshop settings.
			In some files, these extra markers can be sizable.
The default behavior is -copy comments.  (Note: in IJG releases v6 and v6a,
jpegtran always did the equivalent of -copy none.)

Additional switches recognized by jpegtran are:
	-outfile filename
	-maxmemory N
	-verbose
	-debug
These work the same as in cjpeg or djpeg.


THE COMMENT UTILITIES

The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
Although the standard doesn't actually define what COM blocks are for, they
are widely used to hold user-supplied text strings.  This lets you add
annotations, titles, index terms, etc to your JPEG files, and later retrieve
them as text.  COM blocks do not interfere with the image stored in the JPEG
file.  The maximum size of a COM block is 64K, but you can have as many of
them as you like in one JPEG file.

We provide two utility programs to display COM block contents and add COM
blocks to a JPEG file.

rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
standard output.  The command line syntax is
	rdjpgcom [-raw] [-verbose] [inputfilename]
The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable
characters in JPEG comments.  These characters are normally escaped for
security reasons.
The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
image dimensions.  If you omit the input file name from the command line,
the JPEG file is read from standard input.  (This may not work on some
operating systems, if binary data can't be read from stdin.)

wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
Ordinarily, the COM block is added after any existing COM blocks, but you
can delete the old COM blocks if you wish.  wrjpgcom produces a new JPEG
file; it does not modify the input file.  DO NOT try to overwrite the input
file by directing wrjpgcom's output back into it; on most systems this will
just destroy your file.

The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-like
systems, it is
	wrjpgcom [switches] [inputfilename]
The output file is written to standard output.  The input file comes from
the named file, or from standard input if no input file is named.

On most non-Unix systems, the syntax is
	wrjpgcom [switches] inputfilename outputfilename
where both input and output file names must be given explicitly.

wrjpgcom understands three switches:
	-replace		 Delete any existing COM blocks from the file.
	-comment "Comment text"	 Supply new COM text on command line.
	-cfile name		 Read text for new COM block from named file.
(Switch names can be abbreviated.)  If you have only one line of comment text
to add, you can provide it on the command line with -comment.  The comment
text must be surrounded with quotes so that it is treated as a single
argument.  Longer comments can be read from a text file.

If you give neither -comment nor -cfile, then wrjpgcom will read the comment
text from standard input.  (In this case an input image file name MUST be
supplied, so that the source JPEG file comes from somewhere else.)  You can
enter multiple lines, up to 64KB worth.  Type an end-of-file indicator
(usually control-D or control-Z) to terminate the comment text entry.

wrjpgcom will not add a COM block if the provided comment string is empty.
Therefore -replace -comment "" can be used to delete all COM blocks from a
file.

These utility programs do not depend on the IJG JPEG library.  In
particular, the source code for rdjpgcom is intended as an illustration of
the minimum amount of code required to parse a JPEG file header correctly.