jpegtopnm
JPEGTOPNM(1) JPEGTOPNM(1)NAME
jpegtopnm - convert JPEG/JFIF file to portable pixmap or graymapSYNOPSIS
jpegtopnm [-dct {int|fast|float}] [-nosmooth] [-maxmemory N] [{-adobe|-notadobe}] [-comments] [-verbose] [-tracelevel N] [ filename ] All options may be abbreviated to their shortest unique prefix.DESCRIPTION
jpegtopnm converts the named JPEG/JFIF file, or the stan dard input if no file is named to a PPM or PGM image file on the standard output. If the JPEG file is of the grayscale variety, jpegtopnm generates a PGM (Portable Graymap) file. Otherwise, it generates a PPM (Portable Pixmap) file. jpegtopnm uses the Independent JPEG Group's JPEG library to interpret the input file. See http://www.ijg.org for information on the library. JPEG files can have either 8 bits per sample or 12 bits per sample. The 8 bit variety is by far the most common. There are two versions of the IJG JPEG library. One reads only 8 bit files and the other reads only 12 bit files. You must link the appropriate one of these libraries with jpegtopnm. Ordinarily, this means the library is in your shared library search path when you run jpegtopnm. jpegtopnm generates output with either one byte or two bytes per sample depending on whether the JPEG input has either 8 bits or 12 bits per sample. You can use pnmdepth to reduce a two-byte-per-sample file to a one-byte-per- sample file if you need to. If the JPEG file uses the CMYK or YCCK color space, the input does not actually contain enough information to know what color each pixel is. To know what color a pixel is, one would have to know the properties of the inks to which the color space refers. jpegtopnm interprets the colors using the common transformation which assumes all the inks are simply subtractive and linear.OPTIONS
The options are only 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. -nosmooth Use a faster, lower-quality upsampling routine. -maxmemory N Set limit on the amount of memory jpegtopnm uses in processing large images. Value is in thousands of bytes, or millions of bytes if "M" is suffixed to the number. For example, -maxmemory 4m selects 4000000 bytes. If jpegtopnm needs more space, it uses temporary files. -adobe -notadobe There are two variations on the CMYK (and likewise YCCK) color space that may be used in the JPEG input. In the normal one, a zero value for a color components indicates absence of ink. In the other, a zero value means the maximum ink coverage. The latter is used by Adobe Photoshop when it creates a bare JPEG output file (but not when it creates JPEG output as part of Encapsulated Postscript output). These options tell jpegtopnm which version of the CMYK or YCCK color space the image uses. If you specify neither, jpegtopnm tries to figure it out on its own. In the present version, it doesn't try very hard at all: It just assumes the Photoshop version, since Photoshop and its emulators seem to be the main source of CMYK and YCCK images. But with experience of use, future versions might be more sophisticated. If the JPEG image does not indicate that it is CMYK or YCCK, these options have no effect. If you don't use the right one of these options, the symptom is output that looks like a negative. -comments Print any comments in the input file to the Stan dard Error file. -verbose Print details about the conversion to the Standard Error file. -tracelevel n Turn on the JPEG library's trace messages to the Standard Error file. A higher value of n gets more trace information. -verbose implies a trace level of at least 1. EXAMPLES This example converts the color JPEG/JFIF file foo.jpg to a PPM file named foo.ppm: jpegtopnm foo.jpg >foo.ppm HINTS You can use ppmquant to color quantize the result, i.e. to reduce the number of distinct colors in the image. In fact, you may have to if you want to convert the PPM file to certain other formats. ppmdither Does a more sophisti cated quantization. Use pnmscale to change the dimensions of the resulting image. Use ppmtopgm to convert a color JPEG file to a grayscale PGM file. You can use these converters together. E.g.: jpegtopnm foo.jpg | ppmtopgm | pnmscale .25 >foo.pgm -dct fast and/or -nosmooth gain speed at a small sacrifice in quality. If you are fortunate enough to have very fast floating point hardware, -dct float may be even faster than -dct fast. But on most machines -dct float is slower than -dct int; in this case it is not worth using, because its theo retical accuracy advantage is too small to be significant in practice. Another program, djpeg, is similar. djpeg is maintained by the Independent JPEG Group and packaged with the JPEG library which jpegtopnm 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 jpegtopnm, but not vice versa, cjpeg may be more commonly available. On the other hand, djpeg does not use the NetPBM libraries to generate its output, as all the NetPBM tools such as jpegtopnm do. This means it is less likely to be consis tent with all the other programs that deal with the NetPBM formats. Also, the command syntax of jpegtopnm is consis tent with that of the other Netpbm tools, unlike djpeg. 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
ppm(5), pgm(5), ppmtojpeg(1), ppmquant(1), pnmscale(1), ppmtopgm(1), ppmdither(1), pnmdepth(1), djpeg(1), cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1) Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. LIMITATIONS Arithmetic coding is not supported for legal reasons. The program could be much faster.AUTHOR
jpegtopnm and this man page were derived in large part from djpeg, by the Independent JPEG Group. The program is otherwise by Bryan Henderson on March 19, 2000. 19 March 2000 JPEGTOPNM(1)