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1 | Advanced usage instructions for the Independent JPEG Group's JPEG software | ||
2 | ========================================================================== | ||
3 | |||
4 | This file describes cjpeg's "switches for wizards". | ||
5 | |||
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. | ||
13 | |||
14 | |||
15 | Quantization Table Adjustment | ||
16 | ----------------------------- | ||
17 | |||
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. | ||
29 | |||
30 | You can substitute a different set of quantization values by using the | ||
31 | -qtables switch: | ||
32 | |||
33 | -qtables file Use the quantization tables given in the named file. | ||
34 | |||
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). | ||
40 | |||
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: | ||
45 | |||
46 | # Quantization tables given in JPEG spec, section K.1 | ||
47 | |||
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 | ||
57 | |||
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 | ||
67 | |||
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. | ||
73 | |||
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 | ||
76 | switch: | ||
77 | |||
78 | -qslots N[,...] Select which quantization table to use for | ||
79 | each color component. | ||
80 | |||
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. | ||
87 | |||
88 | |||
89 | Sampling Factor Adjustment | ||
90 | -------------------------- | ||
91 | |||
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: | ||
95 | |||
96 | -sample HxV[,...] Set JPEG sampling factors for each color | ||
97 | component. | ||
98 | |||
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 | ||
104 | "-sample 2x2". | ||
105 | |||
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. | ||
111 | |||
112 | |||
113 | Multiple Scan / Progression Control | ||
114 | ----------------------------------- | ||
115 | |||
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 | ||
120 | the -scans switch: | ||
121 | |||
122 | -scans file Use the scan sequence given in the named file. | ||
123 | |||
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. | ||
131 | |||
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 | ||
138 | positional indexes.) | ||
139 | |||
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.) | ||
152 | |||
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: | ||
159 | 0 1 2: 0 63 0 0; | ||
160 | 0,1,2 : 0-63, 0,0 ; | ||
161 | |||
162 | Here is an example of a scan script that generates a partially interleaved | ||
163 | sequential JPEG file: | ||
164 | |||
165 | 0; # Y only in first scan | ||
166 | 1 2; # Cb and Cr in second scan | ||
167 | |||
168 | Here is an example of a progressive scan script using only spectral selection | ||
169 | (no successive approximation): | ||
170 | |||
171 | # Interleaved DC scan for Y,Cb,Cr: | ||
172 | 0,1,2: 0-0, 0, 0 ; | ||
173 | # AC scans: | ||
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 | ||
180 | |||
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: | ||
183 | |||
184 | # Initial DC scan for Y,Cb,Cr (lowest bit not sent) | ||
185 | 0,1,2: 0-0, 0, 1 ; | ||
186 | # First AC scan: send first 5 Y AC coefficients, minus 2 lowest bits: | ||
187 | 0: 1-5, 0, 2 ; | ||
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) | ||
191 | 2: 1-63, 0, 1 ; | ||
192 | 1: 1-63, 0, 1 ; | ||
193 | # Send remaining Y AC coefficients, minus 2 lowest bits: | ||
194 | 0: 6-63, 0, 2 ; | ||
195 | # Send next-to-lowest bit of all Y AC coefficients: | ||
196 | 0: 1-63, 2, 1 ; | ||
197 | # At this point we've sent all but the lowest bit of all coefficients. | ||
198 | # Send lowest bit of DC coefficients | ||
199 | 0,1,2: 0-0, 1, 0 ; | ||
200 | # Send lowest bit of AC coefficients | ||
201 | 2: 1-63, 1, 0 ; | ||
202 | 1: 1-63, 1, 0 ; | ||
203 | # Y AC lowest bit scan is last; it's usually the largest scan | ||
204 | 0: 1-63, 1, 0 ; | ||
205 | |||
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). | ||