# djpeg(1) - man - phpman [DJPEG(1)](https://www.chedong.com/phpMan.php/man/DJPEG/1/markdown) General Commands Manual [DJPEG(1)](https://www.chedong.com/phpMan.php/man/DJPEG/1/markdown) ## 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 pro‐ duces an image file on the standard output. PBMPLUS (PPM/PGM), BMP, GIF, or Targa output format can be selected. ## 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 Reduce image to at most N colors. This reduces the number of colors used in the out‐ put image, so that it can be displayed on a colormapped display or stored in a col‐ ormapped file format. For example, if you have an 8-bit display, you'd need to reduce to 256 or fewer colors. ### -quantize Same as **-colors**. **-colors** is the recommended name, **-quantize** is provided only for backwards compatibility. ### -fast tions 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 ### -scale 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 **-colors** or **-grayscale** is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color format is emitted. ### -gif colors, **-colors** **256** is assumed (unless you specify a smaller number of colors). If you specify **-fast,** the default number of colors is 216. ### -gif0 colors, **-colors** **256** is assumed (unless you specify a smaller number of colors). If you specify **-fast,** the default number of colors is 216. ### -os2 **-colors** or **-grayscale** is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color format is emitted. ### -pnm if the JPEG file is grayscale or if **-grayscale** is specified; otherwise PPM is emitted. ### -targa 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 accurate integer DCT method (default). ### -dct fast Use less accurate integer DCT method [legacy feature]. When the Independent JPEG Group's software was first released in 1991, the decompression time for a 1-megapixel JPEG image on a mainstream PC was measured in minutes. Thus, the **fast** integer DCT al‐ gorithm provided noticeable performance benefits. On modern CPUs running libjpeg- turbo, however, the decompression time for a 1-megapixel JPEG image is measured in milliseconds, and thus the performance benefits of the **fast** algorithm are much less noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, the **fast** and **int** methods have similar performance. On other types of CPUs, the **fast** method is gen‐ erally about 5-15% faster than the **int** method. If the JPEG image was compressed using a quality level of 85 or below, then there should be little or no perceptible quality 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 im‐ ages. 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 [legacy feature]. The **float** method 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 ma‐ chines. ### -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 quantiza‐ tion is being done. Ordered dither is only available in **-onepass** mode. ### -icc Extract ICC color management profile to the specified file. ### -map 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 un‐ less 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 Set limit for amount of memory to use in processing large images. Value is in thou‐ sands 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, an error will occur. ### -maxscans Abort if the JPEG image contains more than _N_ scans. This feature demonstrates a method by which applications can guard against denial-of-service attacks instigated by specially-crafted malformed JPEG images containing numerous scans with missing image data or image data consisting only of "EOB runs" (a feature of progressive JPEG images that allows potentially hundreds of thousands of adjoining zero-value pixels to be represented using only a few bytes.) Attempting to decompress such malformed JPEG im‐ ages can cause excessive CPU activity, since the decompressor must fully process each scan (even if the scan is corrupt) before it can proceed to the next scan. ### -outfile 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().) ### -report Report decompression progress. ### -skip 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 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. Cur‐ rently this option only works with the PBMPLUS (PPM/PGM), GIF, and Targa output for‐ mats. ### -strict Treat all warnings as fatal. This feature also demonstrates a method by which appli‐ cations can guard against attacks instigated by specially-crafted malformed JPEG im‐ ages. Enabling this option will cause the decompressor to abort if the JPEG image contains incomplete or corrupt image data. ### -verbose Enable debug printout. More **-v**'s give more output. Also, version information is printed at startup. ### -debug -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 Several options are available that trade off image quality to gain speed. **-fast** turns on the recommended settings. ### -dct -nosmooth color-quantized image, **-onepass** **-dither** **ordered** is fast but much lower quality than the de‐ fault behavior. **-dither** **none** may give acceptable results in two-pass mode, but is seldom tolerable in one-pass mode. ## 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**. ## SEE ALSO [**cjpeg**(1)](https://www.chedong.com/phpMan.php/man/cjpeg/1/markdown), [**jpegtran**(1)](https://www.chedong.com/phpMan.php/man/jpegtran/1/markdown), [**rdjpgcom**(1)](https://www.chedong.com/phpMan.php/man/rdjpgcom/1/markdown), [**wrjpgcom**(1)](https://www.chedong.com/phpMan.php/man/wrjpgcom/1/markdown) [**ppm**(5)](https://www.chedong.com/phpMan.php/man/ppm/5/markdown), [**pgm**(5)](https://www.chedong.com/phpMan.php/man/pgm/5/markdown) Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. ## 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 lib‐ jpeg. 4 November 2020 [DJPEG(1)](https://www.chedong.com/phpMan.php/man/DJPEG/1/markdown)