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1 The Independent JPEG Group's JPEG software |
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2 ========================================== |
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3 |
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4 README for release 6b of 27-Mar-1998 |
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5 ==================================== |
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6 |
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7 This distribution contains the sixth public release of the Independent JPEG |
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8 Group's free JPEG software. You are welcome to redistribute this software and |
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9 to use it for any purpose, subject to the conditions under LEGAL ISSUES, below. |
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10 |
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11 Serious users of this software (particularly those incorporating it into |
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12 larger programs) should contact IJG at jpeg-info@uunet.uu.net to be added to |
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13 our electronic mailing list. Mailing list members are notified of updates |
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14 and have a chance to participate in technical discussions, etc. |
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15 |
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16 This software is the work of Tom Lane, Philip Gladstone, Jim Boucher, |
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17 Lee Crocker, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, |
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18 Guido Vollbeding, Ge' Weijers, and other members of the Independent JPEG |
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19 Group. |
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20 |
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21 IJG is not affiliated with the official ISO JPEG standards committee. |
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22 |
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23 |
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24 DOCUMENTATION ROADMAP |
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25 ===================== |
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26 |
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27 This file contains the following sections: |
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28 |
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29 OVERVIEW General description of JPEG and the IJG software. |
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30 LEGAL ISSUES Copyright, lack of warranty, terms of distribution. |
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31 REFERENCES Where to learn more about JPEG. |
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32 ARCHIVE LOCATIONS Where to find newer versions of this software. |
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33 RELATED SOFTWARE Other stuff you should get. |
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34 FILE FORMAT WARS Software *not* to get. |
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35 TO DO Plans for future IJG releases. |
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36 |
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37 Other documentation files in the distribution are: |
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38 |
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39 User documentation: |
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40 install.doc How to configure and install the IJG software. |
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41 usage.doc Usage instructions for cjpeg, djpeg, jpegtran, |
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42 rdjpgcom, and wrjpgcom. |
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43 *.1 Unix-style man pages for programs (same info as usage.doc). |
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44 wizard.doc Advanced usage instructions for JPEG wizards only. |
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45 change.log Version-to-version change highlights. |
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46 Programmer and internal documentation: |
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47 libjpeg.doc How to use the JPEG library in your own programs. |
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48 example.c Sample code for calling the JPEG library. |
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49 structure.doc Overview of the JPEG library's internal structure. |
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50 filelist.doc Road map of IJG files. |
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51 coderules.doc Coding style rules --- please read if you contribute code. |
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52 |
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53 Please read at least the files install.doc and usage.doc. Useful information |
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54 can also be found in the JPEG FAQ (Frequently Asked Questions) article. See |
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55 ARCHIVE LOCATIONS below to find out where to obtain the FAQ article. |
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56 |
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57 If you want to understand how the JPEG code works, we suggest reading one or |
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58 more of the REFERENCES, then looking at the documentation files (in roughly |
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59 the order listed) before diving into the code. |
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60 |
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61 |
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62 OVERVIEW |
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63 ======== |
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64 |
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65 This package contains C software to implement JPEG image compression and |
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66 decompression. JPEG (pronounced "jay-peg") is a standardized compression |
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67 method for full-color and gray-scale images. JPEG is intended for compressing |
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68 "real-world" scenes; line drawings, cartoons and other non-realistic images |
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69 are not its strong suit. JPEG is lossy, meaning that the output image is not |
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70 exactly identical to the input image. Hence you must not use JPEG if you |
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71 have to have identical output bits. However, on typical photographic images, |
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72 very good compression levels can be obtained with no visible change, and |
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73 remarkably high compression levels are possible if you can tolerate a |
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74 low-quality image. For more details, see the references, or just experiment |
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75 with various compression settings. |
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76 |
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77 This software implements JPEG baseline, extended-sequential, and progressive |
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78 compression processes. Provision is made for supporting all variants of these |
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79 processes, although some uncommon parameter settings aren't implemented yet. |
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80 For legal reasons, we are not distributing code for the arithmetic-coding |
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81 variants of JPEG; see LEGAL ISSUES. We have made no provision for supporting |
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82 the hierarchical or lossless processes defined in the standard. |
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83 |
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84 We provide a set of library routines for reading and writing JPEG image files, |
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85 plus two sample applications "cjpeg" and "djpeg", which use the library to |
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86 perform conversion between JPEG and some other popular image file formats. |
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87 The library is intended to be reused in other applications. |
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88 |
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89 In order to support file conversion and viewing software, we have included |
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90 considerable functionality beyond the bare JPEG coding/decoding capability; |
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91 for example, the color quantization modules are not strictly part of JPEG |
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92 decoding, but they are essential for output to colormapped file formats or |
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93 colormapped displays. These extra functions can be compiled out of the |
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94 library if not required for a particular application. We have also included |
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95 "jpegtran", a utility for lossless transcoding between different JPEG |
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96 processes, and "rdjpgcom" and "wrjpgcom", two simple applications for |
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97 inserting and extracting textual comments in JFIF files. |
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98 |
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99 The emphasis in designing this software has been on achieving portability and |
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100 flexibility, while also making it fast enough to be useful. In particular, |
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101 the software is not intended to be read as a tutorial on JPEG. (See the |
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102 REFERENCES section for introductory material.) Rather, it is intended to |
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103 be reliable, portable, industrial-strength code. We do not claim to have |
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104 achieved that goal in every aspect of the software, but we strive for it. |
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105 |
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106 We welcome the use of this software as a component of commercial products. |
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107 No royalty is required, but we do ask for an acknowledgement in product |
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108 documentation, as described under LEGAL ISSUES. |
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109 |
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110 |
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111 LEGAL ISSUES |
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112 ============ |
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113 |
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114 In plain English: |
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115 |
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116 1. We don't promise that this software works. (But if you find any bugs, |
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117 please let us know!) |
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118 2. You can use this software for whatever you want. You don't have to pay us. |
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119 3. You may not pretend that you wrote this software. If you use it in a |
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120 program, you must acknowledge somewhere in your documentation that |
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121 you've used the IJG code. |
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122 |
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123 In legalese: |
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124 |
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125 The authors make NO WARRANTY or representation, either express or implied, |
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126 with respect to this software, its quality, accuracy, merchantability, or |
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127 fitness for a particular purpose. This software is provided "AS IS", and you, |
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128 its user, assume the entire risk as to its quality and accuracy. |
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129 |
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130 This software is copyright (C) 1991-1998, Thomas G. Lane. |
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131 All Rights Reserved except as specified below. |
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132 |
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133 Permission is hereby granted to use, copy, modify, and distribute this |
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134 software (or portions thereof) for any purpose, without fee, subject to these |
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135 conditions: |
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136 (1) If any part of the source code for this software is distributed, then this |
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137 README file must be included, with this copyright and no-warranty notice |
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138 unaltered; and any additions, deletions, or changes to the original files |
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139 must be clearly indicated in accompanying documentation. |
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140 (2) If only executable code is distributed, then the accompanying |
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141 documentation must state that "this software is based in part on the work of |
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142 the Independent JPEG Group". |
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143 (3) Permission for use of this software is granted only if the user accepts |
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144 full responsibility for any undesirable consequences; the authors accept |
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145 NO LIABILITY for damages of any kind. |
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146 |
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147 These conditions apply to any software derived from or based on the IJG code, |
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148 not just to the unmodified library. If you use our work, you ought to |
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149 acknowledge us. |
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150 |
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151 Permission is NOT granted for the use of any IJG author's name or company name |
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152 in advertising or publicity relating to this software or products derived from |
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153 it. This software may be referred to only as "the Independent JPEG Group's |
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154 software". |
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155 |
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156 We specifically permit and encourage the use of this software as the basis of |
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157 commercial products, provided that all warranty or liability claims are |
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158 assumed by the product vendor. |
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159 |
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160 |
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161 ansi2knr.c is included in this distribution by permission of L. Peter Deutsch, |
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162 sole proprietor of its copyright holder, Aladdin Enterprises of Menlo Park, CA. |
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163 ansi2knr.c is NOT covered by the above copyright and conditions, but instead |
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164 by the usual distribution terms of the Free Software Foundation; principally, |
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165 that you must include source code if you redistribute it. (See the file |
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166 ansi2knr.c for full details.) However, since ansi2knr.c is not needed as part |
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167 of any program generated from the IJG code, this does not limit you more than |
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168 the foregoing paragraphs do. |
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169 |
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170 The Unix configuration script "configure" was produced with GNU Autoconf. |
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171 It is copyright by the Free Software Foundation but is freely distributable. |
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172 The same holds for its supporting scripts (config.guess, config.sub, |
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173 ltconfig, ltmain.sh). Another support script, install-sh, is copyright |
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174 by M.I.T. but is also freely distributable. |
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175 |
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176 It appears that the arithmetic coding option of the JPEG spec is covered by |
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177 patents owned by IBM, AT&T, and Mitsubishi. Hence arithmetic coding cannot |
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178 legally be used without obtaining one or more licenses. For this reason, |
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179 support for arithmetic coding has been removed from the free JPEG software. |
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180 (Since arithmetic coding provides only a marginal gain over the unpatented |
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181 Huffman mode, it is unlikely that very many implementations will support it.) |
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182 So far as we are aware, there are no patent restrictions on the remaining |
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183 code. |
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184 |
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185 The IJG distribution formerly included code to read and write GIF files. |
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186 To avoid entanglement with the Unisys LZW patent, GIF reading support has |
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187 been removed altogether, and the GIF writer has been simplified to produce |
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188 "uncompressed GIFs". This technique does not use the LZW algorithm; the |
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189 resulting GIF files are larger than usual, but are readable by all standard |
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190 GIF decoders. |
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191 |
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192 We are required to state that |
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193 "The Graphics Interchange Format(c) is the Copyright property of |
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194 CompuServe Incorporated. GIF(sm) is a Service Mark property of |
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195 CompuServe Incorporated." |
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196 |
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197 |
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198 REFERENCES |
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199 ========== |
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200 |
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201 We highly recommend reading one or more of these references before trying to |
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202 understand the innards of the JPEG software. |
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203 |
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204 The best short technical introduction to the JPEG compression algorithm is |
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205 Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
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206 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. |
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207 (Adjacent articles in that issue discuss MPEG motion picture compression, |
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208 applications of JPEG, and related topics.) If you don't have the CACM issue |
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209 handy, a PostScript file containing a revised version of Wallace's article is |
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210 available at ftp://ftp.uu.net/graphics/jpeg/wallace.ps.gz. The file (actually |
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211 a preprint for an article that appeared in IEEE Trans. Consumer Electronics) |
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212 omits the sample images that appeared in CACM, but it includes corrections |
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213 and some added material. Note: the Wallace article is copyright ACM and IEEE, |
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214 and it may not be used for commercial purposes. |
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215 |
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216 A somewhat less technical, more leisurely introduction to JPEG can be found in |
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217 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by |
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218 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides |
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219 good explanations and example C code for a multitude of compression methods |
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220 including JPEG. It is an excellent source if you are comfortable reading C |
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221 code but don't know much about data compression in general. The book's JPEG |
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222 sample code is far from industrial-strength, but when you are ready to look |
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223 at a full implementation, you've got one here... |
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224 |
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225 The best full description of JPEG is the textbook "JPEG Still Image Data |
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226 Compression Standard" by William B. Pennebaker and Joan L. Mitchell, published |
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227 by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp. |
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228 The book includes the complete text of the ISO JPEG standards (DIS 10918-1 |
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229 and draft DIS 10918-2). This is by far the most complete exposition of JPEG |
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230 in existence, and we highly recommend it. |
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231 |
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232 The JPEG standard itself is not available electronically; you must order a |
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233 paper copy through ISO or ITU. (Unless you feel a need to own a certified |
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234 official copy, we recommend buying the Pennebaker and Mitchell book instead; |
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235 it's much cheaper and includes a great deal of useful explanatory material.) |
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236 In the USA, copies of the standard may be ordered from ANSI Sales at (212) |
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237 642-4900, or from Global Engineering Documents at (800) 854-7179. (ANSI |
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238 doesn't take credit card orders, but Global does.) It's not cheap: as of |
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239 1992, ANSI was charging $95 for Part 1 and $47 for Part 2, plus 7% |
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240 shipping/handling. The standard is divided into two parts, Part 1 being the |
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241 actual specification, while Part 2 covers compliance testing methods. Part 1 |
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242 is titled "Digital Compression and Coding of Continuous-tone Still Images, |
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243 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS |
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244 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of |
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245 Continuous-tone Still Images, Part 2: Compliance testing" and has document |
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246 numbers ISO/IEC IS 10918-2, ITU-T T.83. |
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247 |
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248 Some extensions to the original JPEG standard are defined in JPEG Part 3, |
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249 a newer ISO standard numbered ISO/IEC IS 10918-3 and ITU-T T.84. IJG |
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250 currently does not support any Part 3 extensions. |
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251 |
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252 The JPEG standard does not specify all details of an interchangeable file |
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253 format. For the omitted details we follow the "JFIF" conventions, revision |
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254 1.02. A copy of the JFIF spec is available from: |
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255 Literature Department |
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256 C-Cube Microsystems, Inc. |
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257 1778 McCarthy Blvd. |
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258 Milpitas, CA 95035 |
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259 phone (408) 944-6300, fax (408) 944-6314 |
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260 A PostScript version of this document is available by FTP at |
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261 ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz. There is also a plain text |
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262 version at ftp://ftp.uu.net/graphics/jpeg/jfif.txt.gz, but it is missing |
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263 the figures. |
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264 |
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265 The TIFF 6.0 file format specification can be obtained by FTP from |
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266 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme |
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267 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems. |
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268 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6). |
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269 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2 |
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270 (Compression tag 7). Copies of this Note can be obtained from ftp.sgi.com or |
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271 from ftp://ftp.uu.net/graphics/jpeg/. It is expected that the next revision |
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272 of the TIFF spec will replace the 6.0 JPEG design with the Note's design. |
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273 Although IJG's own code does not support TIFF/JPEG, the free libtiff library |
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274 uses our library to implement TIFF/JPEG per the Note. libtiff is available |
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275 from ftp://ftp.sgi.com/graphics/tiff/. |
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276 |
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277 |
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278 ARCHIVE LOCATIONS |
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279 ================= |
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280 |
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281 The "official" archive site for this software is ftp.uu.net (Internet |
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282 address 192.48.96.9). The most recent released version can always be found |
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283 there in directory graphics/jpeg. This particular version will be archived |
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284 as ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz. If you don't have |
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285 direct Internet access, UUNET's archives are also available via UUCP; contact |
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286 help@uunet.uu.net for information on retrieving files that way. |
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287 |
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288 Numerous Internet sites maintain copies of the UUNET files. However, only |
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289 ftp.uu.net is guaranteed to have the latest official version. |
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290 |
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291 You can also obtain this software in DOS-compatible "zip" archive format from |
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292 the SimTel archives (ftp://ftp.simtel.net/pub/simtelnet/msdos/graphics/), or |
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293 on CompuServe in the Graphics Support forum (GO CIS:GRAPHSUP), library 12 |
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294 "JPEG Tools". Again, these versions may sometimes lag behind the ftp.uu.net |
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295 release. |
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296 |
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297 The JPEG FAQ (Frequently Asked Questions) article is a useful source of |
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298 general information about JPEG. It is updated constantly and therefore is |
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299 not included in this distribution. The FAQ is posted every two weeks to |
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300 Usenet newsgroups comp.graphics.misc, news.answers, and other groups. |
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301 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/ |
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302 and other news.answers archive sites, including the official news.answers |
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303 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/. |
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304 If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu |
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305 with body |
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306 send usenet/news.answers/jpeg-faq/part1 |
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307 send usenet/news.answers/jpeg-faq/part2 |
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308 |
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309 |
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310 RELATED SOFTWARE |
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311 ================ |
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312 |
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313 Numerous viewing and image manipulation programs now support JPEG. (Quite a |
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314 few of them use this library to do so.) The JPEG FAQ described above lists |
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315 some of the more popular free and shareware viewers, and tells where to |
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316 obtain them on Internet. |
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317 |
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318 If you are on a Unix machine, we highly recommend Jef Poskanzer's free |
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319 PBMPLUS software, which provides many useful operations on PPM-format image |
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320 files. In particular, it can convert PPM images to and from a wide range of |
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321 other formats, thus making cjpeg/djpeg considerably more useful. The latest |
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322 version is distributed by the NetPBM group, and is available from numerous |
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323 sites, notably ftp://wuarchive.wustl.edu/graphics/graphics/packages/NetPBM/. |
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324 Unfortunately PBMPLUS/NETPBM is not nearly as portable as the IJG software is; |
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325 you are likely to have difficulty making it work on any non-Unix machine. |
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326 |
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327 A different free JPEG implementation, written by the PVRG group at Stanford, |
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328 is available from ftp://havefun.stanford.edu/pub/jpeg/. This program |
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329 is designed for research and experimentation rather than production use; |
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330 it is slower, harder to use, and less portable than the IJG code, but it |
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331 is easier to read and modify. Also, the PVRG code supports lossless JPEG, |
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332 which we do not. (On the other hand, it doesn't do progressive JPEG.) |
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333 |
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334 |
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335 FILE FORMAT WARS |
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336 ================ |
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337 |
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338 Some JPEG programs produce files that are not compatible with our library. |
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339 The root of the problem is that the ISO JPEG committee failed to specify a |
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340 concrete file format. Some vendors "filled in the blanks" on their own, |
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341 creating proprietary formats that no one else could read. (For example, none |
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342 of the early commercial JPEG implementations for the Macintosh were able to |
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343 exchange compressed files.) |
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344 |
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345 The file format we have adopted is called JFIF (see REFERENCES). This format |
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346 has been agreed to by a number of major commercial JPEG vendors, and it has |
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347 become the de facto standard. JFIF is a minimal or "low end" representation. |
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348 We recommend the use of TIFF/JPEG (TIFF revision 6.0 as modified by TIFF |
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349 Technical Note #2) for "high end" applications that need to record a lot of |
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350 additional data about an image. TIFF/JPEG is fairly new and not yet widely |
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351 supported, unfortunately. |
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352 |
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353 The upcoming JPEG Part 3 standard defines a file format called SPIFF. |
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354 SPIFF is interoperable with JFIF, in the sense that most JFIF decoders should |
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355 be able to read the most common variant of SPIFF. SPIFF has some technical |
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356 advantages over JFIF, but its major claim to fame is simply that it is an |
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357 official standard rather than an informal one. At this point it is unclear |
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358 whether SPIFF will supersede JFIF or whether JFIF will remain the de-facto |
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359 standard. IJG intends to support SPIFF once the standard is frozen, but we |
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360 have not decided whether it should become our default output format or not. |
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361 (In any case, our decoder will remain capable of reading JFIF indefinitely.) |
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362 |
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363 Various proprietary file formats incorporating JPEG compression also exist. |
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364 We have little or no sympathy for the existence of these formats. Indeed, |
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365 one of the original reasons for developing this free software was to help |
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366 force convergence on common, open format standards for JPEG files. Don't |
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367 use a proprietary file format! |
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368 |
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369 |
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370 TO DO |
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371 ===== |
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372 |
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373 The major thrust for v7 will probably be improvement of visual quality. |
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374 The current method for scaling the quantization tables is known not to be |
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375 very good at low Q values. We also intend to investigate block boundary |
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376 smoothing, "poor man's variable quantization", and other means of improving |
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377 quality-vs-file-size performance without sacrificing compatibility. |
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378 |
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379 In future versions, we are considering supporting some of the upcoming JPEG |
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380 Part 3 extensions --- principally, variable quantization and the SPIFF file |
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381 format. |
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382 |
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383 As always, speeding things up is of great interest. |
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384 |
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385 Please send bug reports, offers of help, etc. to jpeg-info@uunet.uu.net. |