+++ /dev/null
-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).
projects / synfig.git / blobdiff