# man > pnmtojpeg(1) > **TLDR:** Convert a PNM image file to the JPEG/JFIF/EXIF image format. > - Read a PNM image as input and produce a JPEG/JFIF/EXIF image as output: `pnmtojpeg {{path/to/file.pnm}} > {{path/to/file.jpg}}` - Display version: `pnmtojpeg -version` *Source: tldr-pages* --- _Pnmtojpeg_ _User_ [_Manual_(1)](https://www.chedong.com/phpMan.php/man/Manual/1/markdown) General Commands Manual _Pnmtojpeg_ _User_ [_Manual_(1)](https://www.chedong.com/phpMan.php/man/Manual/1/markdown) ## NAME pnmtojpeg - convert PNM image to a JFIF ("JPEG") image ## SYNOPSIS **pnmtojpeg **[**-exif=_**filespec_] [**-quality=_**n_] [{**-grayscale**|**-greyscale**}] [**-density=_**n_**x_**n_[**dpi**,**dpcm**]] [**-optimize**|**-optimise**] [**-rgb**] [**-progressive**] [**-comment=_**text_] [**-dct=**{**int**|**fast**|**float**}] [**-arith‐** **metic**] [**-restart=_**n_] [**-smooth=_**n_] [**-maxmemory=_**n_] [**-verbose**] [**-baseline**] [**-qtables=_**filespec_] [**-qslots=n[,...]**] [**-sample=_**H_**x_**V_[,...]] [**-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 sep‐ arate an option name from its value. ## DESCRIPTION This program is part of [**Netpbm**(1)](https://www.chedong.com/phpMan.php/man/Netpbm/1/markdown). **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 ]8;;\]8;;\ 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 In addition to the options common to all programs based on libnetpbm (most notably **-quiet**, see ]8;;index.html#commonoptions\ Common Options]8;;\ ), **pnmtojpeg **recognizes the following command line options: ### Basic Options ### -exif=_ 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 **jpeg‐** **topnm**, 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 con‐ taining 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 con‐ tents of the APP1 marker, not counting the length field, that constitutes the EXIF header. ### -quality=_ Scale quantization tables to adjust image quality. _n_ is 0 (worst) to 100 (best); de‐ fault is 75. Below about 25 can produce images some interpreters won't be able to in‐ terpret. See below for more info. ### -grayscale ### -greyscale ### -rgb -grayscale **-greyscale**) means to create a gray scale JFIF, converting from color PPM input if nec‐ essary. **-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 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 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 convert the RGB in‐ puts 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=_ 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 _x_**x_**y_ followed by an optional unit specifier of **dpi **or **dpcm**. Examples: **1x1**, **3x2**, **300x300dpi**, **100x200dpcm**. The first number is the horizon‐ tal density; the 2nd number is the vertical density. Each may be any integer from 1 to 65535. The unit specifier is **dpi **for dots per inch or **dpcm **for dots per centime‐ ter. If you don't specify the units, the density information goes into the JFIF ex‐ plicitly 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 **con‐ verts 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 re‐ sulting 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 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=_ 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 quality set‐ ting (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 reason‐ able 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 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 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 li‐ brary, 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, be‐ cause 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 to‐ tal 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)](https://www.chedong.com/phpMan.php/man/pamtojpeg2k/1/markdown). ### 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 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 ]8;;\the JPEG FAQ]8;;\ , 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 (ei‐ ther 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 message, "Sorry, there are legal restrictions on arithmetic coding" or "Sorry, arithmetic coding not supported," this is the problem. ### -restart=_ 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=_ Smooth the input image to eliminate dithering noise. _n_, ranging from 1 to 100, indi‐ cates the strength of smoothing. 0 (the default) means no smoothing. ### -maxmemory=_ Set a limit for amount of memory to use in processing large images. Value is in thou‐ sands 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 re‐ synchronize 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. ### Wizard Options ### -baseline Force baseline-compatible quantization tables to be generated. This clamps quantiza‐ tion 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=_ Use the quantization tables given in the specified text file. ### -qslots=n[,...] Select which quantization table to use for each color component. ### -sample=_ Set JPEG sampling factors for each color component. ### -scans=_ Use the scan script given in the specified text file. See below for information on scan scripts. ### -tracelevel=_ 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. Draw‐ ings (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 -smooth 10 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 origi‐ nal 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 progressive 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. ## OTHER PROGRAMS 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 doc‐ ument. 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 co‐ efficients. The DC coefficient is Coefficient 0; all the other coefficients are AC coeffi‐ cients. 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 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 compo‐ nent. 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 fea‐ ture 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)](https://www.chedong.com/phpMan.php/man/jpegtopnm/1/markdown), [**pnm**(1)](https://www.chedong.com/phpMan.php/man/pnm/1/markdown), **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 **** netpbm documentation 23 April 2007 _Pnmtojpeg_ _User_ [_Manual_(1)](https://www.chedong.com/phpMan.php/man/Manual/1/markdown)