convert PNM image to a JFIF ("JPEG") image
see also :
cjpeg - djpeg - jpegtran - rdjpgcom - wrjpgcom - jpegtopnm
[ options ] [ filename ]
add an example, a script, a trick and tips
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
converts the named PBM, PGM, or PPM image file, or the
standard input if no file is named, to a JFIF file on the
uses the Independent JPEG Group’s JPEG library to
create the output file. See http://www.ijg.org for
information on the library.
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
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
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.
Scale quantization tables to
adjust image quality. n is 0 (worst) to 100 (best);
default is 75. (See below for more info.)
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.
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.
Create a progressive JPEG file
Include a comment marker in the
JFIF output, with comment text text. Without this
option, there are no comment markers in the output.
--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.)
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
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.)
--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.
Use integer DCT method
Use fast integer DCT (less
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.
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 the input image to
eliminate dithering noise. n, ranging from 1 to 100,
indicates the strength of smoothing. 0 (the default) means
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
Print to the Standard Error
file messages about the conversion process. This can be
helpful in debugging problems.
--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
--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.
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
Use the quantization tables
given in the specified text file.
Select which quantization table
to use for each color component.
Set JPEG sampling factors for
each color component.
Use the scan script given in
the specified text file. See below for information on scan
"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.
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.
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
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
Arithmetic coding is not supported for legal reasons.
The program could be much faster.
Use the -scan option to specify a scan script. Or use the
-progressive option to specify a particular built-in scan
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
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
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
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."
djpeg , jpegtran , rdjpgcom ,
ppm, pgm, jpegtopnm
Wallace, Gregory K. "The JPEG Still Picture Compression
Standard", Communications of the ACM, April 1991 (vol.
34, no. 4), pp. 30-44.
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.