Ubuntu Manpage: pnmtojpeg - convert PNM image to a JFIF ("JPEG") image

NAME

       pnmtojpeg - convert PNM image to a JFIF ("JPEG") image

SYNOPSIS

       pnmtojpeg [ options ] [ filename ]

DESCRIPTION

       pnmtojpeg converts the named PBM, PGM, or PPM image file, or the standard input if no file is named, to a
       JFIF file on the standard output.

       pnmtojpeg   uses   the   Independent   JPEG  Group's  JPEG  library  to  create  the  output  file.   See
       http://www.ijg.org for information on the library.

       "JFIF" is the correct name for the image format commonly known as "JPEG."  Strictly speaking, JPEG  is  a
       method  of  compression.  The image format using JPEG compression that is by far the most common is JFIF.
       There is also a subformat of TIFF that uses JPEG compression.

       EXIF is an image format that is a subformat of JFIF (to wit, a JFIF file that contains an EXIF header  as
       an APP1 marker).  pnmtojpeg creates an EXIF image when you specify the -exif option.

OPTIONS

The basic options are:

--exif=filespec
This option specifies that the output image is to be EXIF (a subformat of JFIF), i.e. it will have
an EXIF header as a JFIF APP1 marker. The contents of that marker are the contents of the
specified file. The special value - means to read the EXIF header contents from standard input.
It is invalid to specify standard input for both the EXIF header and the input image.

The EXIF file starts with a two byte field which is the length of the file, including the length
field, in pure binary, most significant byte first. The special value of zero for the length
field means there is to be no EXIF header, i.e. the same as no -exif option. This is useful for
when you convert a file from JFIF to PNM using jpegtopnm, then transform it, then convert it back
to JFIF with pnmtojpeg, and you don't know whether or not it includes an EXIF header. jpegtopnm
creates an EXIF file containing nothing but two bytes of zero when the input JFIF file has no EXIF
header. Thus, you can transfer any EXIF header from the input JFIF to the output JFIF without
worrying about whether an EXIF header actually exists.

The contents of the EXIF file after the length field are the exact byte for byte contents of the
APP1 marker, not counting the length field, that constitutes the EXIF header.

--quality=n
Scale quantization tables to adjust image quality. n is 0 (worst) to 100 (best); default is 75.
(See below for more info.)

--grayscale

--greyscale
Create gray scale JFIF file. With this option, pnmtojpeg converts color input to gray scale. If
you don't specify this option, The output file is in color format if the input is PPM, and
grayscale format if the input is PBM or PGM.

In the PPM input case, even if all the colors in the image are gray, the output is in color
format. Of course, the colors in it are still gray. The difference is that color format takes up
a lot more space and takes longer to create and process.

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

--progressive
Create a progressive JPEG file (see below).

--comment=text
Include a comment marker in the JFIF output, with comment text text. Without this option, there
are no comment markers in the output.

The --quality option lets you trade off compressed file size against quality of the reconstructed image:
the higher the quality setting, the larger the JFIF 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 generates 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 JFIF standard. pnmtojpeg emits a
warning message when you give such a quality value, because some other JFIF programs may be unable to
decode the resulting file. Use --baseline if you need to ensure compatibility at low quality values.)

The --progressive option creates a "progressive JPEG" file. In this type of JFIF 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 JFIF file of the same quality setting, and the total file size is about the same -- often a
little smaller. Caution: progressive JPEG is not yet widely implemented, so many decoders will be unable
to view a progressive JPEG file at all.

Options 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.

--restart=n
Emit a JPEG restart marker every n MCU rows, or every n MCU blocks if you append B 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 a limit for amount of memory to use in processing large images. Value is in thousands of
bytes, or millions of bytes if you append M to the number. For example, --max=4m selects
4,000,000 bytes. If pnmtojpeg needs more space, it will use temporary files.

--verbose
Print to the Standard Error file messages about the conversion process. This can be helpful in
debugging problems.

The --restart option tells pnmtojpeg to insert 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 JFIF: a moderate smoothing factor of 10 to 50 gets rid of dithering
patterns in the input file, resulting in a smaller JFIF file and a better-looking image. Too large a
smoothing factor will visibly blur the image, however.

Options 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=filespec
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=filespec
Use the scan script given in the specified text file. See below for information on scan scripts.

The "wizard" options 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.doc that comes with the
Independent JPEG Group's JPEG library.

EXAMPLES

       This example compresses the PPM file foo.ppm with a quality factor of 60 and saves the output as foo.jpg:

              pnmtojpeg --quality=60 foo.ppm > foo.jpg

              cat foo.bmp | bmptoppm | pnmtojpeg > foo.jpg

HINTS

JFIF is not ideal for cartoons, line drawings, and other images that have only a few distinct colors.
For those, try instead pnmtopng or ppmtobmp. If you need to convert such an image to JFIF, though, you
should experiment with pnmtojpeg's --quality and --smooth options to get a satisfactory conversion.
--smooth 10 or so is often helpful.

JPEG compression is notable for being a "lossy." This means that, unlike with most graphics conversions,
you lose information, which means image quality, when you convert to JFIF. If you convert from PPM to
JFIF and back repeatedly, image quality loss will accumulate. After ten or so cycles the image may be
noticeably worse than it was after one cycle.

Because of this, you should do all the manipulation you have to do on the image in some other format and
convert to JFIF as the last step. And if you can keep a copy in the original format, so much the better.
PNG is a good choice for a format that is lossless, yet fairly compact. GIF is another way to go, but
chances are you can't create a GIF image without owing a lot of money to Unisys and IBM, holders of
patents on the LZW compression used in the GIF format.

The --optimize option to pnmtojpeg 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 JFIF files; the
percentage improvement is often a lot more than it is on larger files. (At present, --optimize mode is
automatically in effect when you generate a progressive JPEG file).

Another program, cjpeg, is similar. cjpeg is maintained by the Independent JPEG Group and packaged with
the JPEG library which pnmtojpeg uses for all its JPEG work. Because of that, you may expect it to
exploit more current JPEG features. Also, since you have to have the library to run pnmtojpeg, but not
vice versa, cjpeg may be more commonly available.

On the other hand, cjpeg does not use the NetPBM libraries to process its input, as all the NetPBM tools
such as pnmtojpeg do. This means it is less likely to be consistent with all the other programs that
deal with the NetPBM formats. Also, the command syntax of pnmtojpeg is consistent with that of the other
Netpbm tools, unlike cjpeg.

SCAN SCRIPTS

Use the -scan option to specify a scan script. Or use the -progressive option to specify a particular
built-in scan script.

Just what a scan script is, and the basic format of the scan script file, is covered in the wizard.doc
file that comes with the Independent JPEG Group's JPEG library. Scan scripts are same for pnmtojpeg as
the are for cjpeg.

This section contains additional information that isn't, but probably should be, in that document.

First, there are many restrictions on what is a valid scan script. The JPEG library, and thus pnmtojpeg,
checks thoroughly for any lack of compliance with these restrictions, but does little to tell you how the
script fails to comply. The messages are very general and sometimes untrue.

To start with, the entries for the DC coefficient must come before any entries for the AC coefficients.
The DC coefficient is Coefficient 0; all the other coefficients are AC coefficients. So in an entry for
the DC coefficient, the two numbers after the colon must be 0 and 0. In an entry for AC coefficients,
the first number after the colon must not be 0.

In a DC entry, the color components must be in increasing order. E.g. "0,2,1" before the colon is wrong.
So is "0,0,0".

In an entry for an AC coeffient, you must specify only one color component. I.e. there can be only one
number before the colon.

In the first entry for a particular coefficient for a particular color component, the "Ah" value must be
zero, but the Al value can be any valid bit number. In subsequent entries, Ah must be the Al value from
the previous entry (for that coefficient for that color component), and the Al value must be one less
than the Ah value.

The script must ultimately specify at least some of the DC coefficent for every color component.
Otherwise, you get the error message "Script does not transmit all the data." You need not specify all
of the bits of the DC coefficient, or any of the AC coefficients.

There is a standard option in building the JPEG library to omit scan script capability. If for some
reason your library was built with this option, you get the message "Requested feature was omitted at
compile time."

ENVIRONMENT

       JPEGMEM
              If  this  environment  variable  is  set,  its  value  is  the default memory limit.  The value is
              specified as described for the --maxmemory option.  An explicit --maxmemory option  overrides  any
              JPEGMEM.

LIMITATIONS

       Arithmetic coding is not supported for legal reasons.

       The program could be much faster.

AUTHOR

       pnmtojpeg  and  this  man page were derived in large part from cjpeg, by the Independent JPEG Group.  The
       program is otherwise by Bryan Henderson on March 07, 2000.

                                                  07 March 2000                                     PNMTOJPEG(1)