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diff --git a/jpeg/wizard.txt b/jpeg/wizard.txt new file mode 100644 index 0000000..54170b2 --- /dev/null +++ b/jpeg/wizard.txt @@ -0,0 +1,211 @@ +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). |