1
Advanced usage instructions for the Independent JPEG Group's JPEG software
2
==========================================================================
4
This file describes cjpeg's "switches for wizards".
6
The "wizard" switches are intended for experimentation with JPEG by persons
7
who are reasonably knowledgeable about the JPEG standard. If you don't know
8
what you are doing, DON'T USE THESE SWITCHES. You'll likely produce files
9
with worse image quality and/or poorer compression than you'd get from the
10
default settings. Furthermore, these switches must be used with caution
11
when making files intended for general use, because not all JPEG decoders
12
will support unusual JPEG parameter settings.
15
Quantization Table Adjustment
16
-----------------------------
18
Ordinarily, cjpeg starts with a default set of tables (the same ones given
19
as examples in the JPEG standard) and scales them up or down according to
20
the -quality setting. The details of the scaling algorithm can be found in
21
jcparam.c. At very low quality settings, some quantization table entries
22
can get scaled up to values exceeding 255. Although 2-byte quantization
23
values are supported by the IJG software, this feature is not in baseline
24
JPEG and is not supported by all implementations. If you need to ensure
25
wide compatibility of low-quality files, you can constrain the scaled
26
quantization values to no more than 255 by giving the -baseline switch.
27
Note that use of -baseline will result in poorer quality for the same file
28
size, since more bits than necessary are expended on higher AC coefficients.
30
You can substitute a different set of quantization values by using the
33
-qtables file Use the quantization tables given in the named file.
35
The specified file should be a text file containing decimal quantization
36
values. The file should contain one to four tables, each of 64 elements.
37
The tables are implicitly numbered 0,1,etc. in order of appearance. Table
38
entries appear in normal array order (NOT in the zigzag order in which they
39
will be stored in the JPEG file).
41
Quantization table files are free format, in that arbitrary whitespace can
42
appear between numbers. Also, comments can be included: a comment starts
43
with '#' and extends to the end of the line. Here is an example file that
44
duplicates the default quantization tables:
46
# Quantization tables given in JPEG spec, section K.1
48
# This is table 0 (the luminance table):
49
16 11 10 16 24 40 51 61
50
12 12 14 19 26 58 60 55
51
14 13 16 24 40 57 69 56
52
14 17 22 29 51 87 80 62
53
18 22 37 56 68 109 103 77
54
24 35 55 64 81 104 113 92
55
49 64 78 87 103 121 120 101
56
72 92 95 98 112 100 103 99
58
# This is table 1 (the chrominance table):
59
17 18 24 47 99 99 99 99
60
18 21 26 66 99 99 99 99
61
24 26 56 99 99 99 99 99
62
47 66 99 99 99 99 99 99
63
99 99 99 99 99 99 99 99
64
99 99 99 99 99 99 99 99
65
99 99 99 99 99 99 99 99
66
99 99 99 99 99 99 99 99
68
If the -qtables switch is used without -quality, then the specified tables
69
are used exactly as-is. If both -qtables and -quality are used, then the
70
tables taken from the file are scaled in the same fashion that the default
71
tables would be scaled for that quality setting. If -baseline appears, then
72
the quantization values are constrained to the range 1-255.
74
By default, cjpeg will use quantization table 0 for luminance components and
75
table 1 for chrominance components. To override this choice, use the -qslots
78
-qslots N[,...] Select which quantization table to use for
81
The -qslots switch specifies a quantization table number for each color
82
component, in the order in which the components appear in the JPEG SOF marker.
83
For example, to create a separate table for each of Y,Cb,Cr, you could
84
provide a -qtables file that defines three quantization tables and say
85
"-qslots 0,1,2". If -qslots gives fewer table numbers than there are color
86
components, then the last table number is repeated as necessary.
89
Sampling Factor Adjustment
90
--------------------------
92
By default, cjpeg uses 2:1 horizontal and vertical downsampling when
93
compressing YCbCr data, and no downsampling for all other color spaces.
94
You can override this default with the -sample switch:
96
-sample HxV[,...] Set JPEG sampling factors for each color
99
The -sample switch specifies the JPEG sampling factors for each color
100
component, in the order in which they appear in the JPEG SOF marker.
101
If you specify fewer HxV pairs than there are components, the remaining
102
components are set to 1x1 sampling. For example, the default YCbCr setting
103
is equivalent to "-sample 2x2,1x1,1x1", which can be abbreviated to
106
There are still some JPEG decoders in existence that support only 2x1
107
sampling (also called 4:2:2 sampling). Compatibility with such decoders can
108
be achieved by specifying "-sample 2x1". This is not recommended unless
109
really necessary, since it increases file size and encoding/decoding time
110
with very little quality gain.
113
Multiple Scan / Progression Control
114
-----------------------------------
116
By default, cjpeg emits a single-scan sequential JPEG file. The
117
-progressive switch generates a progressive JPEG file using a default series
118
of progression parameters. You can create multiple-scan sequential JPEG
119
files or progressive JPEG files with custom progression parameters by using
122
-scans file Use the scan sequence given in the named file.
124
The specified file should be a text file containing a "scan script".
125
The script specifies the contents and ordering of the scans to be emitted.
126
Each entry in the script defines one scan. A scan definition specifies
127
the components to be included in the scan, and for progressive JPEG it also
128
specifies the progression parameters Ss,Se,Ah,Al for the scan. Scan
129
definitions are separated by semicolons (';'). A semicolon after the last
130
scan definition is optional.
132
Each scan definition contains one to four component indexes, optionally
133
followed by a colon (':') and the four progressive-JPEG parameters. The
134
component indexes denote which color component(s) are to be transmitted in
135
the scan. Components are numbered in the order in which they appear in the
136
JPEG SOF marker, with the first component being numbered 0. (Note that these
137
indexes are not the "component ID" codes assigned to the components, just
140
The progression parameters for each scan are:
141
Ss Zigzag index of first coefficient included in scan
142
Se Zigzag index of last coefficient included in scan
143
Ah Zero for first scan of a coefficient, else Al of prior scan
144
Al Successive approximation low bit position for scan
145
If the progression parameters are omitted, the values 0,63,0,0 are used,
146
producing a sequential JPEG file. cjpeg automatically determines whether
147
the script represents a progressive or sequential file, by observing whether
148
Ss and Se values other than 0 and 63 appear. (The -progressive switch is
149
not needed to specify this; in fact, it is ignored when -scans appears.)
150
The scan script must meet the JPEG restrictions on progression sequences.
151
(cjpeg checks that the spec's requirements are obeyed.)
153
Scan script files are free format, in that arbitrary whitespace can appear
154
between numbers and around punctuation. Also, comments can be included: a
155
comment starts with '#' and extends to the end of the line. For additional
156
legibility, commas or dashes can be placed between values. (Actually, any
157
single punctuation character other than ':' or ';' can be inserted.) For
158
example, the following two scan definitions are equivalent:
162
Here is an example of a scan script that generates a partially interleaved
163
sequential JPEG file:
165
0; # Y only in first scan
166
1 2; # Cb and Cr in second scan
168
Here is an example of a progressive scan script using only spectral selection
169
(no successive approximation):
171
# Interleaved DC scan for Y,Cb,Cr:
174
0: 1-2, 0, 0 ; # First two Y AC coefficients
175
0: 3-5, 0, 0 ; # Three more
176
1: 1-63, 0, 0 ; # All AC coefficients for Cb
177
2: 1-63, 0, 0 ; # All AC coefficients for Cr
178
0: 6-9, 0, 0 ; # More Y coefficients
179
0: 10-63, 0, 0 ; # Remaining Y coefficients
181
Here is an example of a successive-approximation script. This is equivalent
182
to the default script used by "cjpeg -progressive" for YCbCr images:
184
# Initial DC scan for Y,Cb,Cr (lowest bit not sent)
186
# First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits:
188
# Send all Cr,Cb AC coefficients, minus lowest bit:
189
# (chroma data is usually too small to be worth subdividing further;
190
# but note we send Cr first since eye is least sensitive to Cb)
193
# Send remaining Y AC coefficients, minus 2 lowest bits:
195
# Send next-to-lowest bit of all Y AC coefficients:
197
# At this point we've sent all but the lowest bit of all coefficients.
198
# Send lowest bit of DC coefficients
200
# Send lowest bit of AC coefficients
203
# Y AC lowest bit scan is last; it's usually the largest scan
206
It may be worth pointing out that this script is tuned for quality settings
207
of around 50 to 75. For lower quality settings, you'd probably want to use
208
a script with fewer stages of successive approximation (otherwise the
209
initial scans will be really bad). For higher quality settings, you might
210
want to use more stages of successive approximation (so that the initial
211
scans are not too large).
added
removed
wizard.txt
