wizard.txt - third_party/libjpeg-turbo - Git at Google

 Advanced usage instructions for the Independent JPEG Group's JPEG software
 ==========================================================================

 This file describes cjpeg's "switches for wizards".

 The "wizard" switches are intended for experimentation with JPEG by persons
 who are reasonably knowledgeable about the JPEG standard.  If you don't know
 what you are doing, DON'T USE THESE SWITCHES.  You'll likely produce files
 with worse image quality and/or poorer compression than you'd get from the
 default settings.  Furthermore, these switches must be used with caution
 when making files intended for general use, because not all JPEG decoders
 will support unusual JPEG parameter settings.


 Quantization Table Adjustment
 -----------------------------

 Ordinarily, cjpeg starts with a default set of tables (the same ones given
 as examples in the JPEG standard) and scales them up or down according to
 the -quality setting.  The details of the scaling algorithm can be found in
 jcparam.c.  At very low quality settings, some quantization table entries
 can get scaled up to values exceeding 255.  Although 2-byte quantization
 values are supported by the IJG software, this feature is not in baseline
 JPEG and is not supported by all implementations.  If you need to ensure
 wide compatibility of low-quality files, you can constrain the scaled
 quantization values to no more than 255 by giving the -baseline switch.
 Note that use of -baseline will result in poorer quality for the same file
 size, since more bits than necessary are expended on higher AC coefficients.

 You can substitute a different set of quantization values by using the
 -qtables switch:

 	-qtables file	Use the quantization tables given in the named file.

 The specified file should be a text file containing decimal quantization
 values.  The file should contain one to four tables, each of 64 elements.
 The tables are implicitly numbered 0,1,etc. in order of appearance.  Table
 entries appear in normal array order (NOT in the zigzag order in which they
 will be stored in the JPEG file).

 Quantization table files are free format, in that arbitrary whitespace can
 appear between numbers.  Also, comments can be included: a comment starts
 with '#' and extends to the end of the line.  Here is an example file that
 duplicates the default quantization tables:

 	# Quantization tables given in JPEG spec, section K.1

 	# This is table 0 (the luminance table):
 	  16  11  10  16  24  40  51  61
 	  12  12  14  19  26  58  60  55
 	  14  13  16  24  40  57  69  56
 	  14  17  22  29  51  87  80  62
 	  18  22  37  56  68 109 103  77
 	  24  35  55  64  81 104 113  92
 	  49  64  78  87 103 121 120 101
 	  72  92  95  98 112 100 103  99

 	# This is table 1 (the chrominance table):
 	  17  18  24  47  99  99  99  99
 	  18  21  26  66  99  99  99  99
 	  24  26  56  99  99  99  99  99
 	  47  66  99  99  99  99  99  99
 	  99  99  99  99  99  99  99  99
 	  99  99  99  99  99  99  99  99
 	  99  99  99  99  99  99  99  99
 	  99  99  99  99  99  99  99  99

 If the -qtables switch is used without -quality, then the specified tables
 are used exactly as-is.  If both -qtables and -quality are used, then the
 tables taken from the file are scaled in the same fashion that the default
 tables would be scaled for that quality setting.  If -baseline appears, then
 the quantization values are constrained to the range 1-255.

 By default, cjpeg will use quantization table 0 for luminance components and
 table 1 for chrominance components.  To override this choice, use the -qslots
 switch:

 	-qslots N[,...]		Select which quantization table to use for
 				each color component.

 The -qslots switch specifies a quantization table number for each color
 component, in the order in which the components appear in the JPEG SOF marker.
 For example, to create a separate table for each of Y,Cb,Cr, you could
 provide a -qtables file that defines three quantization tables and say
 "-qslots 0,1,2".  If -qslots gives fewer table numbers than there are color
 components, then the last table number is repeated as necessary.


 Sampling Factor Adjustment
 --------------------------

 By default, cjpeg uses 2:1 horizontal and vertical downsampling when
 compressing YCbCr data, and no downsampling for all other color spaces.
 You can override this default with the -sample switch:

 	-sample HxV[,...]	Set JPEG sampling factors for each color
 				component.

 The -sample switch specifies the JPEG sampling factors for each color
 component, in the order in which they appear in the JPEG SOF marker.
 If you specify fewer HxV pairs than there are components, the remaining
 components are set to 1x1 sampling.  For example, the default YCbCr setting
 is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
 "-sample 2x2".

 There are still some JPEG decoders in existence that support only 2x1
 sampling (also called 4:2:2 sampling).  Compatibility with such decoders can
 be achieved by specifying "-sample 2x1".  This is not recommended unless
 really necessary, since it increases file size and encoding/decoding time
 with very little quality gain.


 Multiple Scan / Progression Control
 -----------------------------------

 By default, cjpeg emits a single-scan sequential JPEG file.  The
 -progressive switch generates a progressive JPEG file using a default series
 of progression parameters.  You can create multiple-scan sequential JPEG
 files or progressive JPEG files with custom progression parameters by using
 the -scans switch:

 	-scans file	Use the scan sequence given in the named file.

 The specified file should be a text file containing a "scan script".
 The script specifies the contents and ordering of the scans to be emitted.
 Each entry in the script defines one scan.  A scan definition specifies
 the components to be included in the scan, and for progressive JPEG it also
 specifies the progression parameters Ss,Se,Ah,Al for the scan.  Scan
 definitions are separated by semicolons (';').  A semicolon after the last
 scan definition is optional.

 Each scan definition contains one to four component indexes, optionally
 followed by a colon (':') and the four progressive-JPEG parameters.  The
 component indexes denote which color component(s) are to be transmitted in
 the scan.  Components are numbered in the order in which they appear in the
 JPEG SOF marker, with the first component being numbered 0.  (Note that these
 indexes are not the "component ID" codes assigned to the components, just
 positional indexes.)

 The progression parameters for each scan are:
 	Ss	Zigzag index of first coefficient included in scan
 	Se	Zigzag index of last coefficient included in scan
 	Ah	Zero for first scan of a coefficient, else Al of prior scan
 	Al	Successive approximation low bit position for scan
 If the progression parameters are omitted, the values 0,63,0,0 are used,
 producing a sequential JPEG file.  cjpeg automatically determines whether
 the script represents a progressive or sequential file, by observing whether
 Ss and Se values other than 0 and 63 appear.  (The -progressive switch is
 not needed to specify this; in fact, it is ignored when -scans appears.)
 The scan script must meet the JPEG restrictions on progression sequences.
 (cjpeg checks that the spec's requirements are obeyed.)

 Scan script files are free format, in that arbitrary whitespace can appear
 between numbers and around punctuation.  Also, comments can be included: a
 comment starts with '#' and extends to the end of the line.  For additional
 legibility, commas or dashes can be placed between values.  (Actually, any
 single punctuation character other than ':' or ';' can be inserted.)  For
 example, the following two scan definitions are equivalent:
 	0 1 2: 0 63 0 0;
 	0,1,2 : 0-63, 0,0 ;

 Here is an example of a scan script that generates a partially interleaved
 sequential JPEG file:

 	0;			# Y only in first scan
 	1 2;			# Cb and Cr in second scan

 Here is an example of a progressive scan script using only spectral selection
 (no successive approximation):

 	# Interleaved DC scan for Y,Cb,Cr:
 	0,1,2: 0-0,   0, 0 ;
 	# AC scans:
 	0:     1-2,   0, 0 ;	# First two Y AC coefficients
 	0:     3-5,   0, 0 ;	# Three more
 	1:     1-63,  0, 0 ;	# All AC coefficients for Cb
 	2:     1-63,  0, 0 ;	# All AC coefficients for Cr
 	0:     6-9,   0, 0 ;	# More Y coefficients
 	0:     10-63, 0, 0 ;	# Remaining Y coefficients

 Here is an example of a successive-approximation script.  This is equivalent
 to the default script used by "cjpeg -progressive" for YCbCr images:

 	# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
 	0,1,2: 0-0,   0, 1 ;
 	# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
 	0:     1-5,   0, 2 ;
 	# Send all Cr,Cb AC coefficients, minus lowest bit:
 	# (chroma data is usually too small to be worth subdividing further;
 	#  but note we send Cr first since eye is least sensitive to Cb)
 	2:     1-63,  0, 1 ;
 	1:     1-63,  0, 1 ;
 	# Send remaining Y AC coefficients, minus 2 lowest bits:
 	0:     6-63,  0, 2 ;
 	# Send next-to-lowest bit of all Y AC coefficients:
 	0:     1-63,  2, 1 ;
 	# At this point we've sent all but the lowest bit of all coefficients.
 	# Send lowest bit of DC coefficients
 	0,1,2: 0-0,   1, 0 ;
 	# Send lowest bit of AC coefficients
 	2:     1-63,  1, 0 ;
 	1:     1-63,  1, 0 ;
 	# Y AC lowest bit scan is last; it's usually the largest scan
 	0:     1-63,  1, 0 ;

 It may be worth pointing out that this script is tuned for quality settings
 of around 50 to 75.  For lower quality settings, you'd probably want to use
 a script with fewer stages of successive approximation (otherwise the
 initial scans will be really bad).  For higher quality settings, you might
 want to use more stages of successive approximation (so that the initial
 scans are not too large).
	Advanced usage instructions for the Independent JPEG Group's JPEG software
	==========================================================================

	This file describes cjpeg's "switches for wizards".

	The "wizard" switches are intended for experimentation with JPEG by persons
	who are reasonably knowledgeable about the JPEG standard. If you don't know
	what you are doing, DON'T USE THESE SWITCHES. You'll likely produce files
	with worse image quality and/or poorer compression than you'd get from the
	default settings. Furthermore, these switches must be used with caution
	when making files intended for general use, because not all JPEG decoders
	will support unusual JPEG parameter settings.


	Quantization Table Adjustment
	-----------------------------

	Ordinarily, cjpeg starts with a default set of tables (the same ones given
	as examples in the JPEG standard) and scales them up or down according to
	the -quality setting. The details of the scaling algorithm can be found in
	jcparam.c. At very low quality settings, some quantization table entries
	can get scaled up to values exceeding 255. Although 2-byte quantization
	values are supported by the IJG software, this feature is not in baseline
	JPEG and is not supported by all implementations. If you need to ensure
	wide compatibility of low-quality files, you can constrain the scaled
	quantization values to no more than 255 by giving the -baseline switch.
	Note that use of -baseline will result in poorer quality for the same file
	size, since more bits than necessary are expended on higher AC coefficients.

	You can substitute a different set of quantization values by using the
	-qtables switch:

	-qtables file Use the quantization tables given in the named file.

	The specified file should be a text file containing decimal quantization
	values. The file should contain one to four tables, each of 64 elements.
	The tables are implicitly numbered 0,1,etc. in order of appearance. Table
	entries appear in normal array order (NOT in the zigzag order in which they
	will be stored in the JPEG file).

	Quantization table files are free format, in that arbitrary whitespace can
	appear between numbers. Also, comments can be included: a comment starts
	with '#' and extends to the end of the line. Here is an example file that
	duplicates the default quantization tables:

	# Quantization tables given in JPEG spec, section K.1

	# This is table 0 (the luminance table):
	16 11 10 16 24 40 51 61
	12 12 14 19 26 58 60 55
	14 13 16 24 40 57 69 56
	14 17 22 29 51 87 80 62
	18 22 37 56 68 109 103 77
	24 35 55 64 81 104 113 92
	49 64 78 87 103 121 120 101
	72 92 95 98 112 100 103 99

	# This is table 1 (the chrominance table):
	17 18 24 47 99 99 99 99
	18 21 26 66 99 99 99 99
	24 26 56 99 99 99 99 99
	47 66 99 99 99 99 99 99
	99 99 99 99 99 99 99 99
	99 99 99 99 99 99 99 99
	99 99 99 99 99 99 99 99
	99 99 99 99 99 99 99 99

	If the -qtables switch is used without -quality, then the specified tables
	are used exactly as-is. If both -qtables and -quality are used, then the
	tables taken from the file are scaled in the same fashion that the default
	tables would be scaled for that quality setting. If -baseline appears, then
	the quantization values are constrained to the range 1-255.

	By default, cjpeg will use quantization table 0 for luminance components and
	table 1 for chrominance components. To override this choice, use the -qslots
	switch:

	-qslots N[,...] Select which quantization table to use for
	each color component.

	The -qslots switch specifies a quantization table number for each color
	component, in the order in which the components appear in the JPEG SOF marker.
	For example, to create a separate table for each of Y,Cb,Cr, you could
	provide a -qtables file that defines three quantization tables and say
	"-qslots 0,1,2". If -qslots gives fewer table numbers than there are color
	components, then the last table number is repeated as necessary.


	Sampling Factor Adjustment
	--------------------------

	By default, cjpeg uses 2:1 horizontal and vertical downsampling when
	compressing YCbCr data, and no downsampling for all other color spaces.
	You can override this default with the -sample switch:

	-sample HxV[,...] Set JPEG sampling factors for each color
	component.

	The -sample switch specifies the JPEG sampling factors for each color
	component, in the order in which they appear in the JPEG SOF marker.
	If you specify fewer HxV pairs than there are components, the remaining
	components are set to 1x1 sampling. For example, the default YCbCr setting
	is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
	"-sample 2x2".

	There are still some JPEG decoders in existence that support only 2x1
	sampling (also called 4:2:2 sampling). Compatibility with such decoders can
	be achieved by specifying "-sample 2x1". This is not recommended unless
	really necessary, since it increases file size and encoding/decoding time
	with very little quality gain.


	Multiple Scan / Progression Control
	-----------------------------------

	By default, cjpeg emits a single-scan sequential JPEG file. The
	-progressive switch generates a progressive JPEG file using a default series
	of progression parameters. You can create multiple-scan sequential JPEG
	files or progressive JPEG files with custom progression parameters by using
	the -scans switch:

	-scans file Use the scan sequence given in the named file.

	The specified file should be a text file containing a "scan script".
	The script specifies the contents and ordering of the scans to be emitted.
	Each entry in the script defines one scan. A scan definition specifies
	the components to be included in the scan, and for progressive JPEG it also
	specifies the progression parameters Ss,Se,Ah,Al for the scan. Scan
	definitions are separated by semicolons (';'). A semicolon after the last
	scan definition is optional.

	Each scan definition contains one to four component indexes, optionally
	followed by a colon (':') and the four progressive-JPEG parameters. The
	component indexes denote which color component(s) are to be transmitted in
	the scan. Components are numbered in the order in which they appear in the
	JPEG SOF marker, with the first component being numbered 0. (Note that these
	indexes are not the "component ID" codes assigned to the components, just
	positional indexes.)

	The progression parameters for each scan are:
	Ss Zigzag index of first coefficient included in scan
	Se Zigzag index of last coefficient included in scan
	Ah Zero for first scan of a coefficient, else Al of prior scan
	Al Successive approximation low bit position for scan
	If the progression parameters are omitted, the values 0,63,0,0 are used,
	producing a sequential JPEG file. cjpeg automatically determines whether
	the script represents a progressive or sequential file, by observing whether
	Ss and Se values other than 0 and 63 appear. (The -progressive switch is
	not needed to specify this; in fact, it is ignored when -scans appears.)
	The scan script must meet the JPEG restrictions on progression sequences.
	(cjpeg checks that the spec's requirements are obeyed.)

	Scan script files are free format, in that arbitrary whitespace can appear
	between numbers and around punctuation. Also, comments can be included: a
	comment starts with '#' and extends to the end of the line. For additional
	legibility, commas or dashes can be placed between values. (Actually, any
	single punctuation character other than ':' or ';' can be inserted.) For
	example, the following two scan definitions are equivalent:
	0 1 2: 0 63 0 0;
	0,1,2 : 0-63, 0,0 ;

	Here is an example of a scan script that generates a partially interleaved
	sequential JPEG file:

	0; # Y only in first scan
	1 2; # Cb and Cr in second scan

	Here is an example of a progressive scan script using only spectral selection
	(no successive approximation):

	# Interleaved DC scan for Y,Cb,Cr:
	0,1,2: 0-0, 0, 0 ;
	# AC scans:
	0: 1-2, 0, 0 ; # First two Y AC coefficients
	0: 3-5, 0, 0 ; # Three more
	1: 1-63, 0, 0 ; # All AC coefficients for Cb
	2: 1-63, 0, 0 ; # All AC coefficients for Cr
	0: 6-9, 0, 0 ; # More Y coefficients
	0: 10-63, 0, 0 ; # Remaining Y coefficients

	Here is an example of a successive-approximation script. This is equivalent
	to the default script used by "cjpeg -progressive" for YCbCr images:

	# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
	0,1,2: 0-0, 0, 1 ;
	# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
	0: 1-5, 0, 2 ;
	# Send all Cr,Cb AC coefficients, minus lowest bit:
	# (chroma data is usually too small to be worth subdividing further;
	# but note we send Cr first since eye is least sensitive to Cb)
	2: 1-63, 0, 1 ;
	1: 1-63, 0, 1 ;
	# Send remaining Y AC coefficients, minus 2 lowest bits:
	0: 6-63, 0, 2 ;
	# Send next-to-lowest bit of all Y AC coefficients:
	0: 1-63, 2, 1 ;
	# At this point we've sent all but the lowest bit of all coefficients.
	# Send lowest bit of DC coefficients
	0,1,2: 0-0, 1, 0 ;
	# Send lowest bit of AC coefficients
	2: 1-63, 1, 0 ;
	1: 1-63, 1, 0 ;
	# Y AC lowest bit scan is last; it's usually the largest scan
	0: 1-63, 1, 0 ;

	It may be worth pointing out that this script is tuned for quality settings
	of around 50 to 75. For lower quality settings, you'd probably want to use
	a script with fewer stages of successive approximation (otherwise the
	initial scans will be really bad). For higher quality settings, you might
	want to use more stages of successive approximation (so that the initial
	scans are not too large).