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DJPEG

Section: User Commands (1)
Updated: 18 February 2016
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NAME

djpeg - decompress a JPEG file to an image file

SYNOPSIS

djpeg [ options ] [ filename ]

DESCRIPTION

djpeg decompresses the named JPEG file, or the standard input if no file is named, and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP, GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected. (RLE is supported only if the URT library is available.)

OPTIONS

All switch names may be abbreviated; for example, -grayscale may be written -gray or -gr. Most of the "basic" switches can be abbreviated to as little as one letter. Upper and lower case are equivalent (thus -BMP is the same as -bmp). British spellings are also accepted (e.g., -greyscale), though for brevity these are not mentioned below.

The basic switches are:

-colors N: Reduce image to at most N colors. This reduces the number of colors used in the output image, so that it can be displayed on a colormapped display or stored in a colormapped file format. For example, if you have an 8-bit display, you'd need to reduce to 256 or fewer colors.
-quantize N: Same as -colors. -colors is the recommended name, -quantize is provided only for backwards compatibility.
-fast: Select recommended processing options for fast, low quality output. (The default options are chosen for highest quality output.) Currently, this is equivalent to -dct fast -nosmooth -onepass -dither ordered.
-grayscale: Force grayscale output even if JPEG file is color. Useful for viewing on monochrome displays; also, djpeg runs noticeably faster in this mode.
-rgb: Force RGB output even if JPEG file is grayscale.
-scale M/N: Scale the output image by a factor M/N. Currently the scale factor must be M/8, where M is an integer between 1 and 16 inclusive, or any reduced fraction thereof (such as 1/2, 3/4, etc.) Scaling is handy if the image is larger than your screen; also, djpeg runs much faster when scaling down the output.
-bmp: Select BMP output format (Windows flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color format is emitted.
-gif: Select GIF output format. Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors).
-os2: Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is emitted if -colors or -grayscale is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color format is emitted.
-pnm: Select PBMPLUS (PPM/PGM) output format (this is the default format). PGM is emitted if the JPEG file is grayscale or if -grayscale is specified; otherwise PPM is emitted.
-rle: Select RLE output format. (Requires URT library.)
-targa: Select Targa output format. Grayscale format is emitted if the JPEG file is grayscale or if -grayscale is specified; otherwise, colormapped format is emitted if -colors is specified; otherwise, 24-bit full-color format is emitted.

Switches for advanced users:

-dct int: Use integer DCT method (default).
-dct fast: Use fast integer DCT (less accurate). In libjpeg-turbo, the fast method is generally about 5-15% faster than the int method when using the x86/x86-64 SIMD extensions (results may vary with other SIMD implementations, or when using libjpeg-turbo without SIMD extensions.) If the JPEG image was compressed using a quality level of 85 or below, then there should be little or no perceptible difference between the two algorithms. When decompressing images that were compressed using quality levels above 85, however, the difference between the fast and int methods becomes more pronounced. With images compressed using quality=97, for instance, the fast method incurs generally about a 4-6 dB loss (in PSNR) relative to the int method, but this can be larger for some images. If you can avoid it, do not use the fast method when decompressing images that were compressed using quality levels above 97. The algorithm often degenerates for such images and can actually produce a more lossy output image than if the JPEG image had been compressed using lower quality levels.
-dct float: Use floating-point DCT method. The float method is mainly a legacy feature. It does not produce significantly more accurate results than the int method, and it is much slower. The float method may also give different results on different machines due to varying roundoff behavior, whereas the integer methods should give the same results on all machines.
-dither fs: Use Floyd-Steinberg dithering in color quantization.
-dither ordered: Use ordered dithering in color quantization.
-dither none: Do not use dithering in color quantization. By default, Floyd-Steinberg dithering is applied when quantizing colors; this is slow but usually produces the best results. Ordered dither is a compromise between speed and quality; no dithering is fast but usually looks awful. Note that these switches have no effect unless color quantization is being done. Ordered dither is only available in -onepass mode.
-map file: Quantize to the colors used in the specified image file. This is useful for producing multiple files with identical color maps, or for forcing a predefined set of colors to be used. The file must be a GIF or PPM file. This option overrides -colors and -onepass.
-nosmooth: Use a faster, lower-quality upsampling routine.
-onepass: Use one-pass instead of two-pass color quantization. The one-pass method is faster and needs less memory, but it produces a lower-quality image. -onepass is ignored unless you also say -colors N. Also, the one-pass method is always used for grayscale output (the two-pass method is no improvement then).
-maxmemory N: Set limit for amount of memory to use in processing large images. Value is in thousands of bytes, or millions of bytes if "M" is attached to the number. For example, -max 4m selects 4000000 bytes. If more space is needed, temporary files will be used.
-outfile name: Send output image to the named file, not to standard output.
-memsrc: Load input file into memory before decompressing. This feature was implemented mainly as a way of testing the in-memory source manager (jpeg_mem_src().)
-skip Y0,Y1: Decompress all rows of the JPEG image except those between Y0 and Y1 (inclusive.) Note that if decompression scaling is being used, then Y0 and Y1 are relative to the scaled image dimensions.
-crop WxH+X+Y: Decompress only a rectangular subregion of the image, starting at point X,Y with width W and height H. If necessary, X will be shifted left to the nearest iMCU boundary, and the width will be increased accordingly. Note that if decompression scaling is being used, then X, Y, W, and H are relative to the scaled image dimensions.
-verbose: Enable debug printout. More -v's give more output. Also, version information is printed at startup.
-debug: Same as -verbose.
-version: Print version information and exit.

EXAMPLES

This example decompresses the JPEG file foo.jpg, quantizes it to 256 colors, and saves the output in 8-bit BMP format in foo.bmp:

: djpeg -colors 256 -bmp foo.jpg > foo.bmp

HINTS

To get a quick preview of an image, use the -grayscale and/or -scale switches. -grayscale -scale 1/8 is the fastest case.

Several options are available that trade off image quality to gain speed. -fast turns on the recommended settings.

-dct fast and/or -nosmooth gain speed at a small sacrifice in quality. When producing a color-quantized image, -onepass -dither ordered is fast but much lower quality than the default behavior. -dither none may give acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.

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 theoretical accuracy advantage is too small to be significant in practice.

ENVIRONMENT

JPEGMEM: If this environment variable is set, its value is the default memory limit. The value is specified as described for the -maxmemory switch. JPEGMEM overrides the default value specified when the program was compiled, and itself is overridden by an explicit -maxmemory.

AUTHOR

Independent JPEG Group

This file was modified by The libjpeg-turbo Project to include only information relevant to libjpeg-turbo, to wordsmith certain sections, and to describe features not present in libjpeg.

ISSUES

Support for compressed GIF output files was removed in djpeg v6b due to concerns over the Unisys LZW patent. Although this patent expired in 2006, djpeg still lacks compressed GIF support, for these historical reasons. (Conversion of JPEG files to GIF is usually a bad idea anyway, since GIF is a 256-color format.) The uncompressed GIF files that djpeg generates are larger than they should be, but they are readable by standard GIF decoders.