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321 lines
11 KiB
321 lines
11 KiB
.TH DJPEG 1 "4 November 2020"
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.SH NAME
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djpeg \- decompress a JPEG file to an image file
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.SH SYNOPSIS
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.B djpeg
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[
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.I options
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B djpeg
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decompresses the named JPEG file, or the standard input if no file is named,
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and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
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GIF, or Targa output format can be selected.
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.SH OPTIONS
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All switch names may be abbreviated; for example,
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.B \-grayscale
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may be written
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.B \-gray
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or
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.BR \-gr .
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Most of the "basic" switches can be abbreviated to as little as one letter.
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Upper and lower case are equivalent (thus
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.B \-BMP
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is the same as
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.BR \-bmp ).
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British spellings are also accepted (e.g.,
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.BR \-greyscale ),
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though for brevity these are not mentioned below.
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.PP
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The basic switches are:
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.TP
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.BI \-colors " N"
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Reduce image to at most N colors. This reduces the number of colors used in
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the output image, so that it can be displayed on a colormapped display or
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stored in a colormapped file format. For example, if you have an 8-bit
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display, you'd need to reduce to 256 or fewer colors.
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.TP
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.BI \-quantize " N"
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Same as
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.BR \-colors .
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.B \-colors
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is the recommended name,
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.B \-quantize
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is provided only for backwards compatibility.
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.TP
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.B \-fast
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Select recommended processing options for fast, low quality output. (The
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default options are chosen for highest quality output.) Currently, this is
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equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
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.TP
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.B \-grayscale
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Force grayscale output even if JPEG file is color. Useful for viewing on
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monochrome displays; also,
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.B djpeg
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runs noticeably faster in this mode.
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.TP
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.B \-rgb
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Force RGB output even if JPEG file is grayscale.
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.TP
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.BI \-scale " M/N"
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Scale the output image by a factor M/N. Currently the scale factor must be
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M/8, where M is an integer between 1 and 16 inclusive, or any reduced fraction
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thereof (such as 1/2, 3/4, etc.) Scaling is handy if the image is larger than
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your screen; also,
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.B djpeg
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runs much faster when scaling down the output.
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.TP
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.B \-bmp
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Select BMP output format (Windows flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
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format is emitted.
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.TP
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.B \-gif
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Select GIF output format (LZW-compressed). Since GIF does not support more
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than 256 colors,
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.B \-colors 256
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is assumed (unless you specify a smaller number of colors). If you specify
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.BR \-fast,
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the default number of colors is 216.
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.TP
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.B \-gif0
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Select GIF output format (uncompressed). Since GIF does not support more than
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256 colors,
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.B \-colors 256
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is assumed (unless you specify a smaller number of colors). If you specify
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.BR \-fast,
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the default number of colors is 216.
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.TP
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.B \-os2
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Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
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emitted if
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.B \-colors
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or
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.B \-grayscale
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is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
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format is emitted.
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.TP
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.B \-pnm
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Select PBMPLUS (PPM/PGM) output format (this is the default format).
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PGM is emitted if the JPEG file is grayscale or if
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.B \-grayscale
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is specified; otherwise PPM is emitted.
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.TP
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.B \-targa
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Select Targa output format. Grayscale format is emitted if the JPEG file is
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grayscale or if
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.B \-grayscale
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is specified; otherwise, colormapped format is emitted if
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.B \-colors
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is specified; otherwise, 24-bit full-color format is emitted.
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.PP
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Switches for advanced users:
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.TP
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.B \-dct int
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Use accurate integer DCT method (default).
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.TP
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.B \-dct fast
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Use less accurate integer DCT method [legacy feature].
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When the Independent JPEG Group's software was first released in 1991, the
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decompression time for a 1-megapixel JPEG image on a mainstream PC was measured
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in minutes. Thus, the \fBfast\fR integer DCT algorithm provided noticeable
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performance benefits. On modern CPUs running libjpeg-turbo, however, the
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decompression time for a 1-megapixel JPEG image is measured in milliseconds,
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and thus the performance benefits of the \fBfast\fR algorithm are much less
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noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, the
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\fBfast\fR and \fBint\fR methods have similar performance. On other types of
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CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR
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method.
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If the JPEG image was compressed using a quality level of 85 or below, then
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there should be little or no perceptible quality difference between the two
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algorithms. When decompressing images that were compressed using quality
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levels above 85, however, the difference between the \fBfast\fR and \fBint\fR
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methods becomes more pronounced. With images compressed using quality=97, for
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instance, the \fBfast\fR method incurs generally about a 4-6 dB loss in PSNR
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relative to the \fBint\fR method, but this can be larger for some images. If
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you can avoid it, do not use the \fBfast\fR method when decompressing images
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that were compressed using quality levels above 97. The algorithm often
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degenerates for such images and can actually produce a more lossy output image
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than if the JPEG image had been compressed using lower quality levels.
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.TP
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.B \-dct float
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Use floating-point DCT method [legacy feature].
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The \fBfloat\fR method does not produce significantly more accurate results
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than the \fBint\fR method, and it is much slower. The \fBfloat\fR method may
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also give different results on different machines due to varying roundoff
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behavior, whereas the integer methods should give the same results on all
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machines.
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.TP
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.B \-dither fs
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Use Floyd-Steinberg dithering in color quantization.
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.TP
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.B \-dither ordered
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Use ordered dithering in color quantization.
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.TP
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.B \-dither none
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Do not use dithering in color quantization.
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By default, Floyd-Steinberg dithering is applied when quantizing colors; this
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is slow but usually produces the best results. Ordered dither is a compromise
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between speed and quality; no dithering is fast but usually looks awful. Note
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that these switches have no effect unless color quantization is being done.
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Ordered dither is only available in
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.B \-onepass
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mode.
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.TP
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.BI \-icc " file"
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Extract ICC color management profile to the specified file.
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.TP
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.BI \-map " file"
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Quantize to the colors used in the specified image file. This is useful for
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producing multiple files with identical color maps, or for forcing a
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predefined set of colors to be used. The
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.I file
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must be a GIF or PPM file. This option overrides
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.B \-colors
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and
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.BR \-onepass .
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.TP
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.B \-nosmooth
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Use a faster, lower-quality upsampling routine.
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.TP
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.B \-onepass
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Use one-pass instead of two-pass color quantization. The one-pass method is
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faster and needs less memory, but it produces a lower-quality image.
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.B \-onepass
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is ignored unless you also say
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.B \-colors
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.IR N .
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Also, the one-pass method is always used for grayscale output (the two-pass
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method is no improvement then).
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.TP
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.BI \-maxmemory " N"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-max 4m
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selects 4000000 bytes. If more space is needed, an error will occur.
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.TP
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.BI \-maxscans " N"
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Abort if the JPEG image contains more than
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.I N
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scans. This feature demonstrates a method by which applications can guard
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against denial-of-service attacks instigated by specially-crafted malformed
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JPEG images containing numerous scans with missing image data or image data
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consisting only of "EOB runs" (a feature of progressive JPEG images that allows
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potentially hundreds of thousands of adjoining zero-value pixels to be
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represented using only a few bytes.) Attempting to decompress such malformed
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JPEG images can cause excessive CPU activity, since the decompressor must fully
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process each scan (even if the scan is corrupt) before it can proceed to the
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next scan.
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.TP
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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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.TP
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.BI \-memsrc
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Load input file into memory before decompressing. This feature was implemented
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mainly as a way of testing the in-memory source manager (jpeg_mem_src().)
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.TP
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.BI \-report
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Report decompression progress.
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.TP
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.BI \-skip " Y0,Y1"
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Decompress all rows of the JPEG image except those between Y0 and Y1
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(inclusive.) Note that if decompression scaling is being used, then Y0 and Y1
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are relative to the scaled image dimensions.
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.TP
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.BI \-crop " WxH+X+Y"
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Decompress only a rectangular subregion of the image, starting at point X,Y
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with width W and height H. If necessary, X will be shifted left to the nearest
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iMCU boundary, and the width will be increased accordingly. Note that if
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decompression scaling is being used, then X, Y, W, and H are relative to the
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scaled image dimensions. Currently this option only works with the
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PBMPLUS (PPM/PGM), GIF, and Targa output formats.
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.TP
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.BI \-strict
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Treat all warnings as fatal. This feature also demonstrates a method by which
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applications can guard against attacks instigated by specially-crafted
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malformed JPEG images. Enabling this option will cause the decompressor to
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abort if the JPEG image contains incomplete or corrupt image data.
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.TP
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.B \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.TP
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.B \-version
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Print version information and exit.
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.SH EXAMPLES
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.LP
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This example decompresses the JPEG file foo.jpg, quantizes it to
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256 colors, and saves the output in 8-bit BMP format in foo.bmp:
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.IP
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.B djpeg \-colors 256 \-bmp
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.I foo.jpg
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.B >
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.I foo.bmp
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.SH HINTS
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To get a quick preview of an image, use the
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.B \-grayscale
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and/or
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.B \-scale
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switches.
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.B \-grayscale \-scale 1/8
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is the fastest case.
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.PP
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Several options are available that trade off image quality to gain speed.
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.B \-fast
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turns on the recommended settings.
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.PP
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.B \-dct fast
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and/or
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.B \-nosmooth
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gain speed at a small sacrifice in quality.
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When producing a color-quantized image,
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.B \-onepass \-dither ordered
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is fast but much lower quality than the default behavior.
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.B \-dither none
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may give acceptable results in two-pass mode, but is seldom tolerable in
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one-pass mode.
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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.SH SEE ALSO
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.BR cjpeg (1),
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.BR jpegtran (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (1)
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.br
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.BR ppm (5),
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.BR pgm (5)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.PP
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This file was modified by The libjpeg-turbo Project to include only information
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relevant to libjpeg-turbo, to wordsmith certain sections, and to describe
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features not present in libjpeg.
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