djpeg.1 - third_party/libjpeg-turbo - Git at Google

 .TH DJPEG 1 "21 November 2014"
 .SH NAME
 djpeg \- decompress a JPEG file to an image file
 .SH SYNOPSIS
 .B djpeg
 [
 .I options
 ]
 [
 .I filename
 ]
 .LP
 .SH DESCRIPTION
 .LP
 .B 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.)
 .SH OPTIONS
 All switch names may be abbreviated; for example,
 .B \-grayscale
 may be written
 .B \-gray
 or
 .BR \-gr .
 Most of the "basic" switches can be abbreviated to as little as one letter.
 Upper and lower case are equivalent (thus
 .B \-BMP
 is the same as
 .BR \-bmp ).
 British spellings are also accepted (e.g.,
 .BR \-greyscale ),
 though for brevity these are not mentioned below.
 .PP
 The basic switches are:
 .TP
 .BI \-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.
 .TP
 .BI \-quantize " N"
 Same as
 .BR \-colors .
 .B \-colors
 is the recommended name,
 .B \-quantize
 is provided only for backwards compatibility.
 .TP
 .B \-fast
 Select recommended processing options for fast, low quality output.  (The
 default options are chosen for highest quality output.)  Currently, this is
 equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
 .TP
 .B \-grayscale
 Force grayscale output even if JPEG file is color.  Useful for viewing on
 monochrome displays; also,
 .B djpeg
 runs noticeably faster in this mode.
 .TP
 .B \-rgb
 Force RGB output even if JPEG file is grayscale.
 .TP
 .BI \-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,
 .B djpeg
 runs much faster when scaling down the output.
 .TP
 .B \-bmp
 Select BMP output format (Windows flavor).  8-bit colormapped format is
 emitted if
 .B \-colors
 or
 .B \-grayscale
 is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
 format is emitted.
 .TP
 .B \-gif
 Select GIF output format.  Since GIF does not support more than 256 colors,
 .B \-colors 256
 is assumed (unless you specify a smaller number of colors).
 .TP
 .B \-os2
 Select BMP output format (OS/2 1.x flavor).  8-bit colormapped format is
 emitted if
 .B \-colors
 or
 .B \-grayscale
 is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
 format is emitted.
 .TP
 .B \-pnm
 Select PBMPLUS (PPM/PGM) output format (this is the default format).
 PGM is emitted if the JPEG file is grayscale or if
 .B \-grayscale
 is specified; otherwise PPM is emitted.
 .TP
 .B \-rle
 Select RLE output format.  (Requires URT library.)
 .TP
 .B \-targa
 Select Targa output format.  Grayscale format is emitted if the JPEG file is
 grayscale or if
 .B \-grayscale
 is specified; otherwise, colormapped format is emitted if
 .B \-colors
 is specified; otherwise, 24-bit full-color format is emitted.
 .PP
 Switches for advanced users:
 .TP
 .B \-dct int
 Use integer DCT method (default).
 .TP
 .B \-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.
 .TP
 .B \-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.
 .TP
 .B \-dither fs
 Use Floyd-Steinberg dithering in color quantization.
 .TP
 .B \-dither ordered
 Use ordered dithering in color quantization.
 .TP
 .B \-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
 .B \-onepass
 mode.
 .TP
 .BI \-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
 .I file
 must be a GIF or PPM file. This option overrides
 .B \-colors
 and
 .BR \-onepass .
 .TP
 .B \-nosmooth
 Use a faster, lower-quality upsampling routine.
 .TP
 .B \-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.
 .B \-onepass
 is ignored unless you also say
 .B \-colors
 .IR N .
 Also, the one-pass method is always used for grayscale output (the two-pass
 method is no improvement then).
 .TP
 .BI \-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,
 .B \-max 4m
 selects 4000000 bytes.  If more space is needed, temporary files will be used.
 .TP
 .BI \-outfile " name"
 Send output image to the named file, not to standard output.
 .TP
 .BI \-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().)
 .TP
 .B \-verbose
 Enable debug printout.  More
 .BR \-v 's
 give more output.  Also, version information is printed at startup.
 .TP
 .B \-debug
 Same as
 .BR \-verbose .
 .TP
 .B \-version
 Print version information and exit.
 .SH EXAMPLES
 .LP
 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:
 .IP
 .B djpeg \-colors 256 \-bmp
 .I foo.jpg
 .B >
 .I foo.bmp
 .SH HINTS
 To get a quick preview of an image, use the
 .B \-grayscale
 and/or
 .B \-scale
 switches.
 .B \-grayscale \-scale 1/8
 is the fastest case.
 .PP
 Several options are available that trade off image quality to gain speed.
 .B \-fast
 turns on the recommended settings.
 .PP
 .B \-dct fast
 and/or
 .B \-nosmooth
 gain speed at a small sacrifice in quality.
 When producing a color-quantized image,
 .B \-onepass \-dither ordered
 is fast but much lower quality than the default behavior.
 .B \-dither none
 may give acceptable results in two-pass mode, but is seldom tolerable in
 one-pass mode.
 .PP
 If you are fortunate enough to have very fast floating point hardware,
 \fB\-dct float\fR may be even faster than \fB\-dct fast\fR.  But on most
 machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
 not worth using, because its theoretical accuracy advantage is too small to be
 significant in practice.
 .SH ENVIRONMENT
 .TP
 .B JPEGMEM
 If this environment variable is set, its value is the default memory limit.
 The value is specified as described for the
 .B \-maxmemory
 switch.
 .B JPEGMEM
 overrides the default value specified when the program was compiled, and
 itself is overridden by an explicit
 .BR \-maxmemory .
 .SH SEE ALSO
 .BR cjpeg (1),
 .BR jpegtran (1),
 .BR rdjpgcom (1),
 .BR wrjpgcom (1)
 .br
 .BR ppm (5),
 .BR pgm (5)
 .br
 Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
 .SH AUTHOR
 Independent JPEG Group
 .PP
 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.
 .SH BUGS
 To avoid the Unisys LZW patent,
 .B djpeg
 produces uncompressed GIF files.  These are larger than they should be, but
 are readable by standard GIF decoders.
	.TH DJPEG 1 "21 November 2014"
	.SH NAME
	djpeg \- decompress a JPEG file to an image file
	.SH SYNOPSIS
	.B djpeg
	[
	.I options
	]
	[
	.I filename
	]
	.LP
	.SH DESCRIPTION
	.LP
	.B 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.)
	.SH OPTIONS
	All switch names may be abbreviated; for example,
	.B \-grayscale
	may be written
	.B \-gray
	or
	.BR \-gr .
	Most of the "basic" switches can be abbreviated to as little as one letter.
	Upper and lower case are equivalent (thus
	.B \-BMP
	is the same as
	.BR \-bmp ).
	British spellings are also accepted (e.g.,
	.BR \-greyscale ),
	though for brevity these are not mentioned below.
	.PP
	The basic switches are:
	.TP
	.BI \-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.
	.TP
	.BI \-quantize " N"
	Same as
	.BR \-colors .
	.B \-colors
	is the recommended name,
	.B \-quantize
	is provided only for backwards compatibility.
	.TP
	.B \-fast
	Select recommended processing options for fast, low quality output. (The
	default options are chosen for highest quality output.) Currently, this is
	equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
	.TP
	.B \-grayscale
	Force grayscale output even if JPEG file is color. Useful for viewing on
	monochrome displays; also,
	.B djpeg
	runs noticeably faster in this mode.
	.TP
	.B \-rgb
	Force RGB output even if JPEG file is grayscale.
	.TP
	.BI \-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,
	.B djpeg
	runs much faster when scaling down the output.
	.TP
	.B \-bmp
	Select BMP output format (Windows flavor). 8-bit colormapped format is
	emitted if
	.B \-colors
	or
	.B \-grayscale
	is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
	format is emitted.
	.TP
	.B \-gif
	Select GIF output format. Since GIF does not support more than 256 colors,
	.B \-colors 256
	is assumed (unless you specify a smaller number of colors).
	.TP
	.B \-os2
	Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
	emitted if
	.B \-colors
	or
	.B \-grayscale
	is specified, or if the JPEG file is grayscale; otherwise, 24-bit full-color
	format is emitted.
	.TP
	.B \-pnm
	Select PBMPLUS (PPM/PGM) output format (this is the default format).
	PGM is emitted if the JPEG file is grayscale or if
	.B \-grayscale
	is specified; otherwise PPM is emitted.
	.TP
	.B \-rle
	Select RLE output format. (Requires URT library.)
	.TP
	.B \-targa
	Select Targa output format. Grayscale format is emitted if the JPEG file is
	grayscale or if
	.B \-grayscale
	is specified; otherwise, colormapped format is emitted if
	.B \-colors
	is specified; otherwise, 24-bit full-color format is emitted.
	.PP
	Switches for advanced users:
	.TP
	.B \-dct int
	Use integer DCT method (default).
	.TP
	.B \-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.
	.TP
	.B \-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.
	.TP
	.B \-dither fs
	Use Floyd-Steinberg dithering in color quantization.
	.TP
	.B \-dither ordered
	Use ordered dithering in color quantization.
	.TP
	.B \-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
	.B \-onepass
	mode.
	.TP
	.BI \-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
	.I file
	must be a GIF or PPM file. This option overrides
	.B \-colors
	and
	.BR \-onepass .
	.TP
	.B \-nosmooth
	Use a faster, lower-quality upsampling routine.
	.TP
	.B \-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.
	.B \-onepass
	is ignored unless you also say
	.B \-colors
	.IR N .
	Also, the one-pass method is always used for grayscale output (the two-pass
	method is no improvement then).
	.TP
	.BI \-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,
	.B \-max 4m
	selects 4000000 bytes. If more space is needed, temporary files will be used.
	.TP
	.BI \-outfile " name"
	Send output image to the named file, not to standard output.
	.TP
	.BI \-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().)
	.TP
	.B \-verbose
	Enable debug printout. More
	.BR \-v 's
	give more output. Also, version information is printed at startup.
	.TP
	.B \-debug
	Same as
	.BR \-verbose .
	.TP
	.B \-version
	Print version information and exit.
	.SH EXAMPLES
	.LP
	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:
	.IP
	.B djpeg \-colors 256 \-bmp
	.I foo.jpg
	.B >
	.I foo.bmp
	.SH HINTS
	To get a quick preview of an image, use the
	.B \-grayscale
	and/or
	.B \-scale
	switches.
	.B \-grayscale \-scale 1/8
	is the fastest case.
	.PP
	Several options are available that trade off image quality to gain speed.
	.B \-fast
	turns on the recommended settings.
	.PP
	.B \-dct fast
	and/or
	.B \-nosmooth
	gain speed at a small sacrifice in quality.
	When producing a color-quantized image,
	.B \-onepass \-dither ordered
	is fast but much lower quality than the default behavior.
	.B \-dither none
	may give acceptable results in two-pass mode, but is seldom tolerable in
	one-pass mode.
	.PP
	If you are fortunate enough to have very fast floating point hardware,
	\fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
	machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
	not worth using, because its theoretical accuracy advantage is too small to be
	significant in practice.
	.SH ENVIRONMENT
	.TP
	.B JPEGMEM
	If this environment variable is set, its value is the default memory limit.
	The value is specified as described for the
	.B \-maxmemory
	switch.
	.B JPEGMEM
	overrides the default value specified when the program was compiled, and
	itself is overridden by an explicit
	.BR \-maxmemory .
	.SH SEE ALSO
	.BR cjpeg (1),
	.BR jpegtran (1),
	.BR rdjpgcom (1),
	.BR wrjpgcom (1)
	.br
	.BR ppm (5),
	.BR pgm (5)
	.br
	Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
	Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
	.SH AUTHOR
	Independent JPEG Group
	.PP
	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.
	.SH BUGS
	To avoid the Unisys LZW patent,
	.B djpeg
	produces uncompressed GIF files. These are larger than they should be, but
	are readable by standard GIF decoders.