--- /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