Manual: pnmtojpeg
Pnmtojpeg User Manual(1) General Commands Manual Pnmtojpeg User Manual(1)
NAME
pnmtojpeg - convert PNM image to a JFIF ("JPEG") image
SYNOPSIS
pnmtojpeg [-exif=filespec] [-quality=n] [{-grayscale|-greyscale}] [-density=nxn[dpi,dpcm]] [-optimize|-optimise]
[-rgb] [-progressive] [-comment=text] [-dct={int|fast|float}] [-arithmetic] [-restart=n] [-smooth=n] [-maxmemory=n]
[-verbose] [-baseline] [-qtables=filespec] [-qslots=n[,...]] [-sample=HxV[,...]] [-scans=filespec] [-tracelevel=N]
filename
Minimum unique abbreviation of option is acceptable. You may use double hyphens instead of single hyphen to denote
options. You may use white space in place of the equals sign to separate an option name from its value.
DESCRIPTION
This program is part of Netpbm(1).
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 in‐
formation on the library.
"JFIF" is the correct name for the image format commonly known as "JPEG." Strictly speaking, JPEG is a method of com‐
pression. 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
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options ), pn‐
mtojpeg recognizes the following command line options:
Basic Options
-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 spe‐
cial 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. Below about
25 can produce images some interpreters won't be able to interpret. See below for more info.
-grayscale
-greyscale
-rgb These options determine the color space used in the JFIF output. -grayscale (or -greyscale) means to create a
gray scale JFIF, converting from color PPM input if necessary. -rgb means to create an RGB JFIF, and the pro‐
gram fails if the input is not PPM.
If you specify neither, The output file is in YCbCr format if the input is PPM, and grayscale format if the
input is PBM or PGM.
YCbCr format (a color is represented by an intensity value and two chrominance values) usually compresses much
better than RGB (a color is represented by one red, one green, and one blue value). RGB is rare. But you may
be able to convert between JFIF and PPM faster with RGB, since it's the same color space PPM uses.
The testimg.ppm file that comes with Netpbm is 2.3 times larger with the -rgb option than with the YCbCr de‐
fault, and in one experiment pnmtojpeg took 16% more CPU time to convert it. The extra CPU time probably in‐
dicates that processing of all the extra compressed data consumed all the CPU time saved by not having to con‐
vert the RGB inputs to YCbCr.
Grayscale format takes up a lot less space and takes less time to create and process than the color formats,
even if the image contains nothing but black, white, and gray.
The -rgb option was added in Netpbm 10.11 in October 2002.
-density=density
This option determines the density (aka resolution) information recorded in the JFIF output image. It does
not affect the raster in any way; it just tells whoever reads the JFIF how to interpret the raster.
The density value takes the form xxy followed by an optional unit specifier of dpi or dpcm. Examples: 1x1,
3x2, 300x300dpi, 100x200dpcm. The first number is the horizontal density; the 2nd number is the vertical den‐
sity. Each may be any integer from 1 to 65535. The unit specifier is dpi for dots per inch or dpcm for dots
per centimeter. If you don't specify the units, the density information goes into the JFIF explicitly stating
"density unspecified" (also interpreted as "unknown"). This may seem pointless, but note that even without
specifying the units, the density numbers tell the aspect ratio of the pixels. E.g. 1x1 tells you the pixels
are square. 3x2 tells you the pixels are vertical rectangles.
Note that if you specify different horizontal and vertical densities, the resulting JFIF image is not a true
representation of the input PNM image, because pnmtojpeg converts the raster pixel-for-pixel and the pixels of
a PNM image are defined to be square. Thus, if you start with a square PNM image and specify -density=3x2,
the resulting JFIF image is a horizontally squashed version of the original. However, it is common to use an
input image which is a slight variation on PNM rather than true PNM such that the pixels are not square. In
that case, the appropriate -density option yields a faithful reproduction of the input pseudo-PNM image.
The default is 1x1 in unspecified units.
Before Netpbm 10.15 (April 2003), this option did not exist and the pnmtojpeg always created a JFIF with a
density of 1x1 in unspecified units.
-optimize
Perform optimization of entropy encoding parameters. Without this, pnmtojpeg uses default encoding parame‐
ters. -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. Nor‐
mally you want to use the lowest quality setting (smallest file) that decompresses into something visually indistin‐
guishable from the original image. For this purpose the quality setting should be between 50 and 95 for reasonable
results; 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 of interest mainly for experimental
purposes. Quality values above about 95 are not recommended for normal use; the compressed file size goes up dramat‐
ically 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 consid‐
ered 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 compatibil‐
ity 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.
If you're trying to control the quality/file size tradeoff, you might consider the JPEG2000 format instead. See pam‐
tojpeg2k(1).
Advanced options
-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 float‐
ing-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.
-arithmetic
Use arithmetic coding. Default is Huffman encoding. Arithmetic coding tends to get you a smaller result.
You may need patent licenses to use this option. According to the JPEG FAQ , This method is covered by
patents owned by IBM, AT&T, and Mitsubishi.
The author of the FAQ recommends against using arithmetic coding (and therefore this option) because the space
savings is not great enough to justify the legal hassles.
Most JPEG libraries, including any distributed by the Independent JPEG Group since about 1998 are not capable
of arithmetic encoding. pnmtojpeg uses a JPEG library (either bound to it when the pnmtojpeg executable was
built or accessed on your system at run time) to do the JPEG encoding. If pnmtojpeg terminates with the mes‐
sage, "Sorry, there are legal restrictions on arithmetic coding" or "Sorry, arithmetic coding not supported,"
this is the problem.
-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 mil‐
lions 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, how‐
ever.
Wizard Options
-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.
-tracelevel=N
This sets the level of debug tracing the program outputs as it runs. 0 means none, and is the default. This
level primarily controls tracing of the JPEG library, and you can get some pretty interesting information
about the compression process.
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
Here's a more typical example. It converts from BMP to JFIF:
cat foo.bmp | bmptoppm | pnmtojpeg > foo.jpg
JPEG LOSS
When you compress with JPEG, you lose information -- i.e. the resulting image has somewhat lower quality than the
original. This is a characteristic of JPEG itself, not any particular program. So if you do the usual Netpbm thing
and convert from JFIF to PNM, manipulate, then convert back to JFIF, you will lose quality. The more you do it, the
more you lose. Drawings (charts, cartoons, line drawings, and such with few colors and sharp edges) suffer the most.
To avoid this, you can use a compressed image format other than JPEG. PNG and JPEG2000 are good choices, and Netpbm
contains converters for those.
If you need to use JFIF on a drawing, you should experiment with pnmtojpeg's -quality and -smooth options to get a
satisfactory conversion. -smooth 10 or so is often helpful.
Because of the loss, 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.
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 gener‐
ate a progressive JPEG file).
You can do flipping and rotating transformations losslessly with the program jpegtran, which is packaged with the In‐
dependent Jpeg Group's JPEG library. jpegtran exercises its intimate knowledge of the way JPEG works to do the
transformation without ever actually decompressing the image.
OTHER PROGRAMS
Another program, cjpeg, is similar. cjpeg is maintained by the Independent JPEG Group and packaged with the JPEG li‐
brary 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 pnm‐
tojpeg do. This means it is less likely to be consistent with all the other programs that deal with the NetPBM for‐
mats. 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 coef‐
ficient 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 coefficient, you must specify only one color component. I.e. there can be only one number be‐
fore 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 coefficient 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 coeffi‐
cient, 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 li‐
brary 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 de‐
scribed for the -maxmemory option. An explicit -maxmemory option overrides any JPEGMEM.
SEE ALSO
jpegtopnm(1), pnm(1), cjpeg man page, djpeg man page, jpegtran man page, rdjpgcom man page, wrjpgcom man page
Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34,
no. 4), pp. 30-44.
AUTHOR
pnmtojpeg and this manual were derived in large part from cjpeg, by the Independent JPEG Group. The program is oth‐
erwise by Bryan Henderson on March 07, 2000.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at
http://netpbm.sourceforge.net/doc/pnmtojpeg.html
netpbm documentation 23 April 2007 Pnmtojpeg User Manual(1)