| 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). |
