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 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. 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 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 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 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 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 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 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.

If you're trying to control the quality/file size tradeoff, you might consider the JPEG2000 format instead. See pamtojpeg2k(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 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 , 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 message, "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 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.

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.