212 lines
9.7 KiB
Plaintext
Executable File
212 lines
9.7 KiB
Plaintext
Executable File
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).
|