diff options
Diffstat (limited to 'jpeg/wizard.txt')
| -rw-r--r-- | jpeg/wizard.txt | 211 |
1 files changed, 0 insertions, 211 deletions
diff --git a/jpeg/wizard.txt b/jpeg/wizard.txt deleted file mode 100644 index 54170b2..0000000 --- a/jpeg/wizard.txt +++ /dev/null @@ -1,211 +0,0 @@ -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). |