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Pnmtojpeg User Manual(0) Pnmtojpeg User Manual(0)
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] [-qta-
bles=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 <http://www.ijg.org> for information on the library.
"JFIF" is the correct name for the image format commonly known as "JPEG." Strictly speak-
ing, 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 compres-
sion.
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, includ-
ing 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 pnmto-
jpeg, and you don't know whether or not it includes an EXIF header. jpegtopnm cre-
ates 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 program 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 val-
ues) 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 default, and in one experiment pnmtojpeg took 16% more
CPU time to convert it. The extra CPU time probably indicates 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 density. Each may be any inte-
ger 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 den-
sity 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 -den-
sity 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 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 recon-
structed 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 qual-
ity 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
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 quantiza-
tion 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-qual-
ity 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 pamtojpeg2k(1).
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.
-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
<http://www.faqs.org/faqs/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, arith-
metic 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 millions of bytes if you append M to the number. For exam-
ple, -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 com-
pressed 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 recom-
mend -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 quanti-
zation 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 wiz-
ard.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 partic-
ular program. So if you do the usual Netpbm thing and convert from JFIF to PNM, manipu-
late, 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 generate a pro-
gressive JPEG file).
You can do flipping and rotating transformations losslessly with the program jpegtran,
which is packaged with the Independent 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.
Another program, cjpeg, is similar. cjpeg is
maintained by the Independent JPEG Group and packaged with the JPEG library which pnmto-
jpeg uses for all its JPEG work. Because of that, you may expect it to exploit more cur-
rent 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 coefficient, 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 com-
ponent), 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 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.
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 otherwise 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(0)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
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