Home | History | Annotate | Download | only in jpeg
      1 Advanced usage instructions for the Independent JPEG Group's JPEG software
      2 ==========================================================================
      3 
      4 This file describes cjpeg's "switches for wizards".
      5 
      6 The "wizard" switches are intended for experimentation with JPEG by persons
      7 who are reasonably knowledgeable about the JPEG standard.  If you don't know
      8 what you are doing, DON'T USE THESE SWITCHES.  You'll likely produce files
      9 with worse image quality and/or poorer compression than you'd get from the
     10 default settings.  Furthermore, these switches must be used with caution
     11 when making files intended for general use, because not all JPEG decoders
     12 will support unusual JPEG parameter settings.
     13 
     14 
     15 Quantization Table Adjustment
     16 -----------------------------
     17 
     18 Ordinarily, cjpeg starts with a default set of tables (the same ones given
     19 as examples in the JPEG standard) and scales them up or down according to
     20 the -quality setting.  The details of the scaling algorithm can be found in
     21 jcparam.c.  At very low quality settings, some quantization table entries
     22 can get scaled up to values exceeding 255.  Although 2-byte quantization
     23 values are supported by the IJG software, this feature is not in baseline
     24 JPEG and is not supported by all implementations.  If you need to ensure
     25 wide compatibility of low-quality files, you can constrain the scaled
     26 quantization values to no more than 255 by giving the -baseline switch.
     27 Note that use of -baseline will result in poorer quality for the same file
     28 size, since more bits than necessary are expended on higher AC coefficients.
     29 
     30 You can substitute a different set of quantization values by using the
     31 -qtables switch:
     32 
     33 	-qtables file	Use the quantization tables given in the named file.
     34 
     35 The specified file should be a text file containing decimal quantization
     36 values.  The file should contain one to four tables, each of 64 elements.
     37 The tables are implicitly numbered 0,1,etc. in order of appearance.  Table
     38 entries appear in normal array order (NOT in the zigzag order in which they
     39 will be stored in the JPEG file).
     40 
     41 Quantization table files are free format, in that arbitrary whitespace can
     42 appear between numbers.  Also, comments can be included: a comment starts
     43 with '#' and extends to the end of the line.  Here is an example file that
     44 duplicates the default quantization tables:
     45 
     46 	# Quantization tables given in JPEG spec, section K.1
     47 
     48 	# This is table 0 (the luminance table):
     49 	  16  11  10  16  24  40  51  61
     50 	  12  12  14  19  26  58  60  55
     51 	  14  13  16  24  40  57  69  56
     52 	  14  17  22  29  51  87  80  62
     53 	  18  22  37  56  68 109 103  77
     54 	  24  35  55  64  81 104 113  92
     55 	  49  64  78  87 103 121 120 101
     56 	  72  92  95  98 112 100 103  99
     57 
     58 	# This is table 1 (the chrominance table):
     59 	  17  18  24  47  99  99  99  99
     60 	  18  21  26  66  99  99  99  99
     61 	  24  26  56  99  99  99  99  99
     62 	  47  66  99  99  99  99  99  99
     63 	  99  99  99  99  99  99  99  99
     64 	  99  99  99  99  99  99  99  99
     65 	  99  99  99  99  99  99  99  99
     66 	  99  99  99  99  99  99  99  99
     67 
     68 If the -qtables switch is used without -quality, then the specified tables
     69 are used exactly as-is.  If both -qtables and -quality are used, then the
     70 tables taken from the file are scaled in the same fashion that the default
     71 tables would be scaled for that quality setting.  If -baseline appears, then
     72 the quantization values are constrained to the range 1-255.
     73 
     74 By default, cjpeg will use quantization table 0 for luminance components and
     75 table 1 for chrominance components.  To override this choice, use the -qslots
     76 switch:
     77 
     78 	-qslots N[,...]		Select which quantization table to use for
     79 				each color component.
     80 
     81 The -qslots switch specifies a quantization table number for each color
     82 component, in the order in which the components appear in the JPEG SOF marker.
     83 For example, to create a separate table for each of Y,Cb,Cr, you could
     84 provide a -qtables file that defines three quantization tables and say
     85 "-qslots 0,1,2".  If -qslots gives fewer table numbers than there are color
     86 components, then the last table number is repeated as necessary.
     87 
     88 
     89 Sampling Factor Adjustment
     90 --------------------------
     91 
     92 By default, cjpeg uses 2:1 horizontal and vertical downsampling when
     93 compressing YCbCr data, and no downsampling for all other color spaces.
     94 You can override this default with the -sample switch:
     95 
     96 	-sample HxV[,...]	Set JPEG sampling factors for each color
     97 				component.
     98 
     99 The -sample switch specifies the JPEG sampling factors for each color
    100 component, in the order in which they appear in the JPEG SOF marker.
    101 If you specify fewer HxV pairs than there are components, the remaining
    102 components are set to 1x1 sampling.  For example, the default YCbCr setting
    103 is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
    104 "-sample 2x2".
    105 
    106 There are still some JPEG decoders in existence that support only 2x1
    107 sampling (also called 4:2:2 sampling).  Compatibility with such decoders can
    108 be achieved by specifying "-sample 2x1".  This is not recommended unless
    109 really necessary, since it increases file size and encoding/decoding time
    110 with very little quality gain.
    111 
    112 
    113 Multiple Scan / Progression Control
    114 -----------------------------------
    115 
    116 By default, cjpeg emits a single-scan sequential JPEG file.  The
    117 -progressive switch generates a progressive JPEG file using a default series
    118 of progression parameters.  You can create multiple-scan sequential JPEG
    119 files or progressive JPEG files with custom progression parameters by using
    120 the -scans switch:
    121 
    122 	-scans file	Use the scan sequence given in the named file.
    123 
    124 The specified file should be a text file containing a "scan script".
    125 The script specifies the contents and ordering of the scans to be emitted.
    126 Each entry in the script defines one scan.  A scan definition specifies
    127 the components to be included in the scan, and for progressive JPEG it also
    128 specifies the progression parameters Ss,Se,Ah,Al for the scan.  Scan
    129 definitions are separated by semicolons (';').  A semicolon after the last
    130 scan definition is optional.
    131 
    132 Each scan definition contains one to four component indexes, optionally
    133 followed by a colon (':') and the four progressive-JPEG parameters.  The
    134 component indexes denote which color component(s) are to be transmitted in
    135 the scan.  Components are numbered in the order in which they appear in the
    136 JPEG SOF marker, with the first component being numbered 0.  (Note that these
    137 indexes are not the "component ID" codes assigned to the components, just
    138 positional indexes.)
    139 
    140 The progression parameters for each scan are:
    141 	Ss	Zigzag index of first coefficient included in scan
    142 	Se	Zigzag index of last coefficient included in scan
    143 	Ah	Zero for first scan of a coefficient, else Al of prior scan
    144 	Al	Successive approximation low bit position for scan
    145 If the progression parameters are omitted, the values 0,63,0,0 are used,
    146 producing a sequential JPEG file.  cjpeg automatically determines whether
    147 the script represents a progressive or sequential file, by observing whether
    148 Ss and Se values other than 0 and 63 appear.  (The -progressive switch is
    149 not needed to specify this; in fact, it is ignored when -scans appears.)
    150 The scan script must meet the JPEG restrictions on progression sequences.
    151 (cjpeg checks that the spec's requirements are obeyed.)
    152 
    153 Scan script files are free format, in that arbitrary whitespace can appear
    154 between numbers and around punctuation.  Also, comments can be included: a
    155 comment starts with '#' and extends to the end of the line.  For additional
    156 legibility, commas or dashes can be placed between values.  (Actually, any
    157 single punctuation character other than ':' or ';' can be inserted.)  For
    158 example, the following two scan definitions are equivalent:
    159 	0 1 2: 0 63 0 0;
    160 	0,1,2 : 0-63, 0,0 ;
    161 
    162 Here is an example of a scan script that generates a partially interleaved
    163 sequential JPEG file:
    164 
    165 	0;			# Y only in first scan
    166 	1 2;			# Cb and Cr in second scan
    167 
    168 Here is an example of a progressive scan script using only spectral selection
    169 (no successive approximation):
    170 
    171 	# Interleaved DC scan for Y,Cb,Cr:
    172 	0,1,2: 0-0,   0, 0 ;
    173 	# AC scans:
    174 	0:     1-2,   0, 0 ;	# First two Y AC coefficients
    175 	0:     3-5,   0, 0 ;	# Three more
    176 	1:     1-63,  0, 0 ;	# All AC coefficients for Cb
    177 	2:     1-63,  0, 0 ;	# All AC coefficients for Cr
    178 	0:     6-9,   0, 0 ;	# More Y coefficients
    179 	0:     10-63, 0, 0 ;	# Remaining Y coefficients
    180 
    181 Here is an example of a successive-approximation script.  This is equivalent
    182 to the default script used by "cjpeg -progressive" for YCbCr images:
    183 
    184 	# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
    185 	0,1,2: 0-0,   0, 1 ;
    186 	# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
    187 	0:     1-5,   0, 2 ;
    188 	# Send all Cr,Cb AC coefficients, minus lowest bit:
    189 	# (chroma data is usually too small to be worth subdividing further;
    190 	#  but note we send Cr first since eye is least sensitive to Cb)
    191 	2:     1-63,  0, 1 ;
    192 	1:     1-63,  0, 1 ;
    193 	# Send remaining Y AC coefficients, minus 2 lowest bits:
    194 	0:     6-63,  0, 2 ;
    195 	# Send next-to-lowest bit of all Y AC coefficients:
    196 	0:     1-63,  2, 1 ;
    197 	# At this point we've sent all but the lowest bit of all coefficients.
    198 	# Send lowest bit of DC coefficients
    199 	0,1,2: 0-0,   1, 0 ;
    200 	# Send lowest bit of AC coefficients
    201 	2:     1-63,  1, 0 ;
    202 	1:     1-63,  1, 0 ;
    203 	# Y AC lowest bit scan is last; it's usually the largest scan
    204 	0:     1-63,  1, 0 ;
    205 
    206 It may be worth pointing out that this script is tuned for quality settings
    207 of around 50 to 75.  For lower quality settings, you'd probably want to use
    208 a script with fewer stages of successive approximation (otherwise the
    209 initial scans will be really bad).  For higher quality settings, you might
    210 want to use more stages of successive approximation (so that the initial
    211 scans are not too large).
    212