212 lines
9.5 KiB
Plaintext
212 lines
9.5 KiB
Plaintext
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Advanced usage instructions for the Independent JPEG Group's JPEG software
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==========================================================================
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This file describes cjpeg's "switches for wizards".
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The "wizard" switches are intended for experimentation with JPEG by persons
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who are reasonably knowledgeable about the JPEG standard. If you don't know
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what you are doing, DON'T USE THESE SWITCHES. You'll likely produce files
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with worse image quality and/or poorer compression than you'd get from the
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default settings. Furthermore, these switches must be used with caution
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when making files intended for general use, because not all JPEG decoders
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will support unusual JPEG parameter settings.
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Quantization Table Adjustment
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-----------------------------
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Ordinarily, cjpeg starts with a default set of tables (the same ones given
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as examples in the JPEG standard) and scales them up or down according to
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the -quality setting. The details of the scaling algorithm can be found in
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jcparam.c. At very low quality settings, some quantization table entries
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can get scaled up to values exceeding 255. Although 2-byte quantization
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values are supported by the IJG software, this feature is not in baseline
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JPEG and is not supported by all implementations. If you need to ensure
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wide compatibility of low-quality files, you can constrain the scaled
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quantization values to no more than 255 by giving the -baseline switch.
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Note that use of -baseline will result in poorer quality for the same file
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size, since more bits than necessary are expended on higher AC coefficients.
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You can substitute a different set of quantization values by using the
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-qtables switch:
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-qtables file Use the quantization tables given in the named file.
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The specified file should be a text file containing decimal quantization
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values. The file should contain one to four tables, each of 64 elements.
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The tables are implicitly numbered 0,1,etc. in order of appearance. Table
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entries appear in normal array order (NOT in the zigzag order in which they
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will be stored in the JPEG file).
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Quantization table files are free format, in that arbitrary whitespace can
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appear between numbers. Also, comments can be included: a comment starts
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with '#' and extends to the end of the line. Here is an example file that
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duplicates the default quantization tables:
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# Quantization tables given in JPEG spec, section K.1
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# This is table 0 (the luminance table):
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16 11 10 16 24 40 51 61
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12 12 14 19 26 58 60 55
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14 13 16 24 40 57 69 56
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14 17 22 29 51 87 80 62
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18 22 37 56 68 109 103 77
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24 35 55 64 81 104 113 92
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49 64 78 87 103 121 120 101
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72 92 95 98 112 100 103 99
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# This is table 1 (the chrominance table):
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17 18 24 47 99 99 99 99
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18 21 26 66 99 99 99 99
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24 26 56 99 99 99 99 99
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47 66 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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99 99 99 99 99 99 99 99
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If the -qtables switch is used without -quality, then the specified tables
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are used exactly as-is. If both -qtables and -quality are used, then the
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tables taken from the file are scaled in the same fashion that the default
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tables would be scaled for that quality setting. If -baseline appears, then
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the quantization values are constrained to the range 1-255.
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By default, cjpeg will use quantization table 0 for luminance components and
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table 1 for chrominance components. To override this choice, use the -qslots
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switch:
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-qslots N[,...] Select which quantization table to use for
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each color component.
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The -qslots switch specifies a quantization table number for each color
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component, in the order in which the components appear in the JPEG SOF marker.
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For example, to create a separate table for each of Y,Cb,Cr, you could
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provide a -qtables file that defines three quantization tables and say
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"-qslots 0,1,2". If -qslots gives fewer table numbers than there are color
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components, then the last table number is repeated as necessary.
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Sampling Factor Adjustment
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--------------------------
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By default, cjpeg uses 2:1 horizontal and vertical downsampling when
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compressing YCbCr data, and no downsampling for all other color spaces.
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You can override this default with the -sample switch:
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-sample HxV[,...] Set JPEG sampling factors for each color
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component.
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The -sample switch specifies the JPEG sampling factors for each color
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component, in the order in which they appear in the JPEG SOF marker.
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If you specify fewer HxV pairs than there are components, the remaining
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components are set to 1x1 sampling. For example, the default YCbCr setting
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is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
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"-sample 2x2".
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There are still some JPEG decoders in existence that support only 2x1
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sampling (also called 4:2:2 sampling). Compatibility with such decoders can
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be achieved by specifying "-sample 2x1". This is not recommended unless
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really necessary, since it increases file size and encoding/decoding time
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with very little quality gain.
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Multiple Scan / Progression Control
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-----------------------------------
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By default, cjpeg emits a single-scan sequential JPEG file. The
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-progressive switch generates a progressive JPEG file using a default series
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of progression parameters. You can create multiple-scan sequential JPEG
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files or progressive JPEG files with custom progression parameters by using
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the -scans switch:
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-scans file Use the scan sequence given in the named file.
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The specified file should be a text file containing a "scan script".
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The script specifies the contents and ordering of the scans to be emitted.
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Each entry in the script defines one scan. A scan definition specifies
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the components to be included in the scan, and for progressive JPEG it also
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specifies the progression parameters Ss,Se,Ah,Al for the scan. Scan
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definitions are separated by semicolons (';'). A semicolon after the last
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scan definition is optional.
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Each scan definition contains one to four component indexes, optionally
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followed by a colon (':') and the four progressive-JPEG parameters. The
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component indexes denote which color component(s) are to be transmitted in
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the scan. Components are numbered in the order in which they appear in the
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JPEG SOF marker, with the first component being numbered 0. (Note that these
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indexes are not the "component ID" codes assigned to the components, just
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positional indexes.)
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The progression parameters for each scan are:
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Ss Zigzag index of first coefficient included in scan
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Se Zigzag index of last coefficient included in scan
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Ah Zero for first scan of a coefficient, else Al of prior scan
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Al Successive approximation low bit position for scan
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If the progression parameters are omitted, the values 0,63,0,0 are used,
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producing a sequential JPEG file. cjpeg automatically determines whether
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the script represents a progressive or sequential file, by observing whether
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Ss and Se values other than 0 and 63 appear. (The -progressive switch is
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not needed to specify this; in fact, it is ignored when -scans appears.)
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The scan script must meet the JPEG restrictions on progression sequences.
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(cjpeg checks that the spec's requirements are obeyed.)
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Scan script files are free format, in that arbitrary whitespace can appear
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between numbers and around punctuation. Also, comments can be included: a
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comment starts with '#' and extends to the end of the line. For additional
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legibility, commas or dashes can be placed between values. (Actually, any
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single punctuation character other than ':' or ';' can be inserted.) For
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example, the following two scan definitions are equivalent:
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0 1 2: 0 63 0 0;
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0,1,2 : 0-63, 0,0 ;
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Here is an example of a scan script that generates a partially interleaved
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sequential JPEG file:
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0; # Y only in first scan
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1 2; # Cb and Cr in second scan
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Here is an example of a progressive scan script using only spectral selection
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(no successive approximation):
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# Interleaved DC scan for Y,Cb,Cr:
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0,1,2: 0-0, 0, 0 ;
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# AC scans:
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0: 1-2, 0, 0 ; # First two Y AC coefficients
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0: 3-5, 0, 0 ; # Three more
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1: 1-63, 0, 0 ; # All AC coefficients for Cb
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2: 1-63, 0, 0 ; # All AC coefficients for Cr
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0: 6-9, 0, 0 ; # More Y coefficients
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0: 10-63, 0, 0 ; # Remaining Y coefficients
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Here is an example of a successive-approximation script. This is equivalent
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to the default script used by "cjpeg -progressive" for YCbCr images:
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# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
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0,1,2: 0-0, 0, 1 ;
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# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
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0: 1-5, 0, 2 ;
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# Send all Cr,Cb AC coefficients, minus lowest bit:
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# (chroma data is usually too small to be worth subdividing further;
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# but note we send Cr first since eye is least sensitive to Cb)
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2: 1-63, 0, 1 ;
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1: 1-63, 0, 1 ;
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# Send remaining Y AC coefficients, minus 2 lowest bits:
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0: 6-63, 0, 2 ;
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# Send next-to-lowest bit of all Y AC coefficients:
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0: 1-63, 2, 1 ;
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# At this point we've sent all but the lowest bit of all coefficients.
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# Send lowest bit of DC coefficients
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0,1,2: 0-0, 1, 0 ;
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# Send lowest bit of AC coefficients
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2: 1-63, 1, 0 ;
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1: 1-63, 1, 0 ;
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# Y AC lowest bit scan is last; it's usually the largest scan
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0: 1-63, 1, 0 ;
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It may be worth pointing out that this script is tuned for quality settings
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of around 50 to 75. For lower quality settings, you'd probably want to use
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a script with fewer stages of successive approximation (otherwise the
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initial scans will be really bad). For higher quality settings, you might
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want to use more stages of successive approximation (so that the initial
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scans are not too large).
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