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

 .TH CJPEG 1 "15 June 1995"
 .SH NAME
 cjpeg \- compress an image file to a JPEG file
 .SH SYNOPSIS
 .B cjpeg
 [
 .I options
 ]
 [
 .I filename
 ]
 .LP
 .SH DESCRIPTION
 .LP
 .B cjpeg
 compresses the named image file, or the standard input if no file is
 named, and produces a JPEG/JFIF file on the standard output.
 The currently supported input file formats are: PPM (PBMPLUS color
 format), PGM (PBMPLUS gray-scale format), BMP, GIF, Targa, and RLE (Utah Raster
 Toolkit format).  (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 \-GIF
 is the same as
 .BR \-gif ).
 British spellings are also accepted (e.g.,
 .BR \-greyscale ),
 though for brevity these are not mentioned below.
 .PP
 The basic switches are:
 .TP
 .BI \-quality " N"
 Scale quantization tables to adjust image quality.  Quality is 0 (worst) to
 100 (best); default is 75.  (See below for more info.)
 .TP
 .B \-grayscale
 Create monochrome JPEG file from color input.  Be sure to use this switch when
 compressing a grayscale GIF file, because
 .B cjpeg
 isn't bright enough to notice whether a GIF file uses only shades of gray.
 By saying
 .BR \-grayscale ,
 you'll get a smaller JPEG file that takes less time to process.
 .TP
 .B \-optimize
 Perform optimization of entropy encoding parameters.  Without this, default
 encoding parameters are used.
 .B \-optimize
 usually makes the JPEG file a little smaller, but
 .B cjpeg
 runs somewhat slower and needs much more memory.  Image quality and speed of
 decompression are unaffected by
 .BR \-optimize .
 .TP
 .B \-progressive
 Create progressive JPEG file (see below).
 .TP
 .B \-targa
 Input file is Targa format.  Targa files that contain an "identification"
 field will not be automatically recognized by
 .BR cjpeg ;
 for such files you must specify
 .B \-targa
 to make
 .B cjpeg
 treat the input as Targa format.
 For most Targa files, you won't need this switch.
 .PP
 The
 .B \-quality
 switch lets you trade off compressed file size against quality of the
 reconstructed image: the higher the quality setting, the larger the JPEG file,
 and the closer the output image will be to the original input.  Normally you
 want to use the lowest quality setting (smallest file) that decompresses into
 something visually indistinguishable from the original image.  For this
 purpose the quality setting should be between 50 and 95; the default of 75 is
 often about right.  If you see defects at
 .B \-quality
 75, then go up 5 or 10 counts at a time until you are happy with the output
 image.  (The optimal setting will vary from one image to another.)
 .PP
 .B \-quality
 100 will generate a quantization table of all 1's, minimizing loss in the
 quantization step (but there is still information loss in subsampling, as well
 as roundoff error).  This setting is mainly of interest for experimental
 purposes.  Quality values above about 95 are
 .B not
 recommended for normal use; the compressed file size goes up dramatically for
 hardly any gain in output image quality.
 .PP
 In the other direction, quality values below 50 will produce very small files
 of low image quality.  Settings around 5 to 10 might be useful in preparing an
 index of a large image library, for example.  Try
 .B \-quality
 2 (or so) for some amusing Cubist effects.  (Note: quality
 values below about 25 generate 2-byte quantization tables, which are
 considered optional in the JPEG standard.
 .B cjpeg
 emits a warning message when you give such a quality value, because some
 other JPEG programs may be unable to decode the resulting file.  Use
 .B \-baseline
 if you need to ensure compatibility at low quality values.)
 .PP
 The
 .B \-progressive
 switch creates a "progressive JPEG" file.  In this type of JPEG file, the data
 is stored in multiple scans of increasing quality.  If the file is being
 transmitted over a slow communications link, the decoder can use the first
 scan to display a low-quality image very quickly, and can then improve the
 display with each subsequent scan.  The final image is exactly equivalent to a
 standard JPEG file of the same quality setting, and the total file size is
 about the same --- often a little smaller.
 .B Caution:
 progressive JPEG is not yet widely implemented, so many decoders will be
 unable to view a progressive JPEG file at all.
 .PP
 Switches for advanced users:
 .TP
 .B \-dct int
 Use integer DCT method (default).
 .TP
 .B \-dct fast
 Use fast integer DCT (less accurate).
 .TP
 .B \-dct float
 Use floating-point DCT method.
 The float method is very slightly more accurate than the int method, but is
 much slower unless your machine has very fast floating-point hardware.  Also
 note that results of the floating-point method may vary slightly across
 machines, while the integer methods should give the same results everywhere.
 The fast integer method is much less accurate than the other two.
 .TP
 .BI \-restart " N"
 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
 attached to the number.
 .B \-restart 0
 (the default) means no restart markers.
 .TP
 .BI \-smooth " N"
 Smooth the input image to eliminate dithering noise.  N, ranging from 1 to
 100, indicates the strength of smoothing.  0 (the default) means no smoothing.
 .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
 .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 .
 .PP
 The
 .B \-restart
 option inserts extra markers that allow a JPEG decoder to resynchronize after
 a transmission error.  Without restart markers, any damage to a compressed
 file will usually ruin the image from the point of the error to the end of the
 image; with restart markers, the damage is usually confined to the portion of
 the image up to the next restart marker.  Of course, the restart markers
 occupy extra space.  We recommend
 .B \-restart 1
 for images that will be transmitted across unreliable networks such as Usenet.
 .PP
 The
 .B \-smooth
 option filters the input to eliminate fine-scale noise.  This is often useful
 when converting GIF files to JPEG: a moderate smoothing factor of 10 to 50
 gets rid of dithering patterns in the input file, resulting in a smaller JPEG
 file and a better-looking image.  Too large a smoothing factor will visibly
 blur the image, however.
 .PP
 Switches for wizards:
 .TP
 .B \-baseline
 Force a baseline JPEG file to be generated.  This clamps quantization values
 to 8 bits even at low quality settings.
 .TP
 .BI \-qtables " file"
 Use the quantization tables given in the specified text file.
 .TP
 .BI \-qslots " N[,...]"
 Select which quantization table to use for each color component.
 .TP
 .BI \-sample " HxV[,...]"
 Set JPEG sampling factors for each color component.
 .TP
 .BI \-scans " file"
 Use the scan script given in the specified text file.
 .PP
 The "wizard" switches are intended for experimentation with JPEG.  If you
 don't know what you are doing, \fBdon't use them\fR.  These switches are
 documented further in the file wizard.doc.
 .SH EXAMPLES
 .LP
 This example compresses the PPM file foo.ppm with a quality factor of
 60 and saves the output as foo.jpg:
 .IP
 .B cjpeg \-quality
 .I 60 foo.ppm
 .B >
 .I foo.jpg
 .SH HINTS
 Color GIF files are not the ideal input for JPEG; JPEG is really intended for
 compressing full-color (24-bit) images.  In particular, don't try to convert
 cartoons, line drawings, and other images that have only a few distinct
 colors.  GIF works great on these, JPEG does not.  If you want to convert a
 GIF to JPEG, you should experiment with
 .BR cjpeg 's
 .B \-quality
 and
 .B \-smooth
 options to get a satisfactory conversion.
 .B \-smooth 10
 or so is often helpful.
 .PP
 Avoid running an image through a series of JPEG compression/decompression
 cycles.  Image quality loss will accumulate; after ten or so cycles the image
 may be noticeably worse than it was after one cycle.  It's best to use a
 lossless format while manipulating an image, then convert to JPEG format when
 you are ready to file the image away.
 .PP
 The
 .B \-optimize
 option to
 .B cjpeg
 is worth using when you are making a "final" version for posting or archiving.
 It's also a win when you are using low quality settings to make very small
 JPEG files; the percentage improvement is often a lot more than it is on
 larger files.  (At present,
 .B \-optimize
 mode is always selected when generating progressive JPEG files.)
 .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 djpeg (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
 .SH BUGS
 Arithmetic coding is not supported for legal reasons.
 .PP
 Not all variants of BMP and Targa file formats are supported.
 .PP
 The
 .B \-targa
 switch is not a bug, it's a feature.  (It would be a bug if the Targa format
 designers had not been clueless.)
 .PP
 Still not as fast as we'd like.
	.TH CJPEG 1 "15 June 1995"
	.SH NAME
	cjpeg \- compress an image file to a JPEG file
	.SH SYNOPSIS
	.B cjpeg
	[
	.I options
	]
	[
	.I filename
	]
	.LP
	.SH DESCRIPTION
	.LP
	.B cjpeg
	compresses the named image file, or the standard input if no file is
	named, and produces a JPEG/JFIF file on the standard output.
	The currently supported input file formats are: PPM (PBMPLUS color
	format), PGM (PBMPLUS gray-scale format), BMP, GIF, Targa, and RLE (Utah Raster
	Toolkit format). (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 \-GIF
	is the same as
	.BR \-gif ).
	British spellings are also accepted (e.g.,
	.BR \-greyscale ),
	though for brevity these are not mentioned below.
	.PP
	The basic switches are:
	.TP
	.BI \-quality " N"
	Scale quantization tables to adjust image quality. Quality is 0 (worst) to
	100 (best); default is 75. (See below for more info.)
	.TP
	.B \-grayscale
	Create monochrome JPEG file from color input. Be sure to use this switch when
	compressing a grayscale GIF file, because
	.B cjpeg
	isn't bright enough to notice whether a GIF file uses only shades of gray.
	By saying
	.BR \-grayscale ,
	you'll get a smaller JPEG file that takes less time to process.
	.TP
	.B \-optimize
	Perform optimization of entropy encoding parameters. Without this, default
	encoding parameters are used.
	.B \-optimize
	usually makes the JPEG file a little smaller, but
	.B cjpeg
	runs somewhat slower and needs much more memory. Image quality and speed of
	decompression are unaffected by
	.BR \-optimize .
	.TP
	.B \-progressive
	Create progressive JPEG file (see below).
	.TP
	.B \-targa
	Input file is Targa format. Targa files that contain an "identification"
	field will not be automatically recognized by
	.BR cjpeg ;
	for such files you must specify
	.B \-targa
	to make
	.B cjpeg
	treat the input as Targa format.
	For most Targa files, you won't need this switch.
	.PP
	The
	.B \-quality
	switch lets you trade off compressed file size against quality of the
	reconstructed image: the higher the quality setting, the larger the JPEG file,
	and the closer the output image will be to the original input. Normally you
	want to use the lowest quality setting (smallest file) that decompresses into
	something visually indistinguishable from the original image. For this
	purpose the quality setting should be between 50 and 95; the default of 75 is
	often about right. If you see defects at
	.B \-quality
	75, then go up 5 or 10 counts at a time until you are happy with the output
	image. (The optimal setting will vary from one image to another.)
	.PP
	.B \-quality
	100 will generate a quantization table of all 1's, minimizing loss in the
	quantization step (but there is still information loss in subsampling, as well
	as roundoff error). This setting is mainly of interest for experimental
	purposes. Quality values above about 95 are
	.B not
	recommended for normal use; the compressed file size goes up dramatically for
	hardly any gain in output image quality.
	.PP
	In the other direction, quality values below 50 will produce very small files
	of low image quality. Settings around 5 to 10 might be useful in preparing an
	index of a large image library, for example. Try
	.B \-quality
	2 (or so) for some amusing Cubist effects. (Note: quality
	values below about 25 generate 2-byte quantization tables, which are
	considered optional in the JPEG standard.
	.B cjpeg
	emits a warning message when you give such a quality value, because some
	other JPEG programs may be unable to decode the resulting file. Use
	.B \-baseline
	if you need to ensure compatibility at low quality values.)
	.PP
	The
	.B \-progressive
	switch creates a "progressive JPEG" file. In this type of JPEG file, the data
	is stored in multiple scans of increasing quality. If the file is being
	transmitted over a slow communications link, the decoder can use the first
	scan to display a low-quality image very quickly, and can then improve the
	display with each subsequent scan. The final image is exactly equivalent to a
	standard JPEG file of the same quality setting, and the total file size is
	about the same --- often a little smaller.
	.B Caution:
	progressive JPEG is not yet widely implemented, so many decoders will be
	unable to view a progressive JPEG file at all.
	.PP
	Switches for advanced users:
	.TP
	.B \-dct int
	Use integer DCT method (default).
	.TP
	.B \-dct fast
	Use fast integer DCT (less accurate).
	.TP
	.B \-dct float
	Use floating-point DCT method.
	The float method is very slightly more accurate than the int method, but is
	much slower unless your machine has very fast floating-point hardware. Also
	note that results of the floating-point method may vary slightly across
	machines, while the integer methods should give the same results everywhere.
	The fast integer method is much less accurate than the other two.
	.TP
	.BI \-restart " N"
	Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
	attached to the number.
	.B \-restart 0
	(the default) means no restart markers.
	.TP
	.BI \-smooth " N"
	Smooth the input image to eliminate dithering noise. N, ranging from 1 to
	100, indicates the strength of smoothing. 0 (the default) means no smoothing.
	.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
	.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 .
	.PP
	The
	.B \-restart
	option inserts extra markers that allow a JPEG decoder to resynchronize after
	a transmission error. Without restart markers, any damage to a compressed
	file will usually ruin the image from the point of the error to the end of the
	image; with restart markers, the damage is usually confined to the portion of
	the image up to the next restart marker. Of course, the restart markers
	occupy extra space. We recommend
	.B \-restart 1
	for images that will be transmitted across unreliable networks such as Usenet.
	.PP
	The
	.B \-smooth
	option filters the input to eliminate fine-scale noise. This is often useful
	when converting GIF files to JPEG: a moderate smoothing factor of 10 to 50
	gets rid of dithering patterns in the input file, resulting in a smaller JPEG
	file and a better-looking image. Too large a smoothing factor will visibly
	blur the image, however.
	.PP
	Switches for wizards:
	.TP
	.B \-baseline
	Force a baseline JPEG file to be generated. This clamps quantization values
	to 8 bits even at low quality settings.
	.TP
	.BI \-qtables " file"
	Use the quantization tables given in the specified text file.
	.TP
	.BI \-qslots " N[,...]"
	Select which quantization table to use for each color component.
	.TP
	.BI \-sample " HxV[,...]"
	Set JPEG sampling factors for each color component.
	.TP
	.BI \-scans " file"
	Use the scan script given in the specified text file.
	.PP
	The "wizard" switches are intended for experimentation with JPEG. If you
	don't know what you are doing, \fBdon't use them\fR. These switches are
	documented further in the file wizard.doc.
	.SH EXAMPLES
	.LP
	This example compresses the PPM file foo.ppm with a quality factor of
	60 and saves the output as foo.jpg:
	.IP
	.B cjpeg \-quality
	.I 60 foo.ppm
	.B >
	.I foo.jpg
	.SH HINTS
	Color GIF files are not the ideal input for JPEG; JPEG is really intended for
	compressing full-color (24-bit) images. In particular, don't try to convert
	cartoons, line drawings, and other images that have only a few distinct
	colors. GIF works great on these, JPEG does not. If you want to convert a
	GIF to JPEG, you should experiment with
	.BR cjpeg 's
	.B \-quality
	and
	.B \-smooth
	options to get a satisfactory conversion.
	.B \-smooth 10
	or so is often helpful.
	.PP
	Avoid running an image through a series of JPEG compression/decompression
	cycles. Image quality loss will accumulate; after ten or so cycles the image
	may be noticeably worse than it was after one cycle. It's best to use a
	lossless format while manipulating an image, then convert to JPEG format when
	you are ready to file the image away.
	.PP
	The
	.B \-optimize
	option to
	.B cjpeg
	is worth using when you are making a "final" version for posting or archiving.
	It's also a win when you are using low quality settings to make very small
	JPEG files; the percentage improvement is often a lot more than it is on
	larger files. (At present,
	.B \-optimize
	mode is always selected when generating progressive JPEG files.)
	.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 djpeg (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
	.SH BUGS
	Arithmetic coding is not supported for legal reasons.
	.PP
	Not all variants of BMP and Targa file formats are supported.
	.PP
	The
	.B \-targa
	switch is not a bug, it's a feature. (It would be a bug if the Targa format
	designers had not been clueless.)
	.PP
	Still not as fast as we'd like.