|General Commands Manual
djpeg - decompress a JPEG file to an image file
djpeg [ options ] [ filename ]
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, or Targa output format can be selected.
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.
- 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.
- Force grayscale output even if JPEG file is color. Useful for viewing on monochrome displays; also, djpeg runs noticeably faster in this mode.
- 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.
- 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.
- Select GIF output format (LZW-compressed). Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors). If you specify -fast, the default number of colors is 216.
- Select GIF output format (uncompressed). Since GIF does not support more than 256 colors, -colors 256 is assumed (unless you specify a smaller number of colors). If you specify -fast, the default number of colors is 216.
- 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.
- 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.
- 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 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 algorithm 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 generally
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 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 [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 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.
- -icc file
- Extract ICC color management profile to the specified file.
- -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.
- Use a faster, lower-quality upsampling routine.
- 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, an error will occur.
- -maxscans N
- 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 images 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 name
- Send output image to the named file, not to standard output.
- 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 decompression progress.
- -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. Currently this option only works with the PBMPLUS (PPM/PGM), GIF, and Targa output formats.
- Treat all warnings as fatal. This feature also demonstrates a method by which applications can guard against attacks instigated by specially-crafted malformed JPEG images. Enabling this option will cause the decompressor to abort if the JPEG image contains incomplete or corrupt image data.
- Enable debug printout. More -v's give more output. Also, version information is printed at startup.
- Same as -verbose.
- Print version information and exit.
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
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 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.
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.
|4 November 2020