Mercurial > hgrepos > Python2 > PyMuPDF

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>>>>> tag v1.26.5+1 for changeset b74429b0f5c4
author Franz Glasner <fzglas.hg@dom66.de>
date Sat, 11 Oct 2025 17:17:30 +0200
parents b50eed0cc0ef
children
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1 LCMS 2.10.MT
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2 ============
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4 Why does this fork exist?
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5 ~~~~~~~~~~~~~~~~~~~~~~~~~
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7 We, Artifex Software, use LCMS 2 in both our Ghostscript and MuPDF
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8 projects. It's a great library that performs well, and does almost
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9 exactly what we want.
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11 Almost.
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13 In the course of pulling LCMS 2 into MuPDF, we hit some problems
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14 with the library (described in more detail below). We've changed
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15 the code to fix these problems, and therefore released this
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16 version of the library.
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19 Why don't you just pass the changes back to mainline LCMS?
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20 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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22 Sadly, the changes we've made require changes to some of the API
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23 to LCMS. We consider these changes to be pretty minor, and to
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24 improve the overall API, but they are unavoidable.
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26 You can't just drop this version of the library into an existing
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27 project that uses vanilla LCMS and expect it to work. You will
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28 need to make some changes in your code. Small ones, but changes
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29 nonetheless.
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31 We have offered these changes upstream, with a view to getting
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32 them adopted into mainstream LCMS - perhaps as LCMS 3. Marti
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33 Maria, the original author of LCMS is considering them at the
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34 moment, but no decision has been made.
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36 Marti has plans of his own for LCMS, so until such time as we
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37 figure out a mutually satisfactory way of working, we're doing
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38 this separate release.
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41 So what problem was this intended to solve?
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42 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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44 Put simply, we ran into problems with using LCMS in a multi-threaded
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45 environment.
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47 A few years ago, Marti kindly made some changes within LittleCMS
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48 to enable us to safely use LCMS with Ghostscript when run with
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49 multiple threads.
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51 Specifically, Ghostscript needed to use different allocators from
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52 each thread. In order to account for this, Marti added the idea
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53 of the 'cmsContext'. These would carry a 'void *UserData' value
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54 that could be retrieved in the allocators. By using a different
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55 cmsContext in each thread, we could therefore pass thread specific
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56 information through LCMS and safely have it reach the allocators.
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58 In order to make this change without breaking the existing API
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59 Marti added new functions, suffixed with THR, that took these
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60 cmsContext's.
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62 So where in old lcms we had:
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64 CMSAPI cmsBool CMSEXPORT cmsPlugin(void* Plugin);
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66 we now also had:
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68 CMSAPI cmsBool CMSEXPORT cmsPluginTHR(cmsContext ContextID, void* Plugin);
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70 Internally within LCMS, the cmsContext values were often stored
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71 within things like profiles and links. For Ghostscript this was
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72 absolutely fine, because we didn't share profiles or links between
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73 different threads.
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74
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75 For MuPDF, however, this was a significant restriction. MuPDF is
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76 designed as a C level library from which people can build applications.
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77 Data objects are reference counted, and are designed to be able to
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78 be passed around the system as required. It would be almost impossible
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79 to enforce the idea that profiles and links can only be used within
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80 the original thread (cmsContext) within which they were created.
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81 Also new versions of Ghostscript will also share profiles and links
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82 among threads to enhance performance with multi-threaded rendering.
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83
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84 Lastly, Ghostscript made use of cmsChangeBuffersFormat to switch the
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85 input or output data format (planar, bytes per component, etc.) to
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86 allow a link profile to be re-used without the computation needed to
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87 create a new link profile. Since the input and output format are
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88 stored within the link profile, one thread changing the format while
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89 another thread was using it for color transform would cause problems.
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90
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91 So what changes have been made?
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92 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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93
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94 The changes are very simple and systematic.
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96 1) Every API function (or at least those that might allocate) now takes
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97 a cmsContext pointer.
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99 2) cmsContexts are now passed explicitly throughout the system. They
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100 are never stored in any structures.
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101
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102 3) Accordingly, we have removed the 'THR' API functions (as they no
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103 longer serve any purpose).
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104
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105 4) We have removed the cmsChangeBuffersFormat function (use of which
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106 could lead to a link profile being changed by one thread while it
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107 was in use by another) and replaced it with a thread safe alternative,
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108 cmsCloneTransformChangingFormats. This creates a new transform that
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109 shares the underlying tables of the original transform, but with
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110 new buffer format handlers. Since the underlying tables for the link
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111 are shared, the cost of cloning the profile is very low.
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112
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113 In addition, we've made 1 other change that breaks the ABI, but not
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114 the API:
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115
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116 5) The organisation of the flags word used to describe the format of
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117 input/output pixels has been altered to accommodate more 'extra'
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118 channels (now a maximum of 63 rather than 7).
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119
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120 Finally:
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121
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122 6) We have renamed lcms2.h to be lcms2mt.h.
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123
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124 7) We have tweaked the LCMS_VERSION value (and some of the code
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125 that reads it), to ensure that people don't link binary plugins
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126 expecting other versions against us without an error being given.
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127
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128
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129 So what's the plan from here?
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130 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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131
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132 Well, at the very least we hope to keep pulling fixes from mainline
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133 LCMS into this version. We (Artifex Software) need to keep it up to
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134 date and secure for our use of the library.
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135
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136 In the fullness of time we'd love to see these fixes folded back
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137 into a new LCMS release, but this will have to wait for this to be
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138 a convenient time for Marti.
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139
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140
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141 So where should we report bugs?
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142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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143
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144 Report bugs to us, by all means, but please also report them to Marti.
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145 He's likely to be in a far better position to evaluate potential
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146 problems and fixes than we are - this is his library after all.
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147 Ideally every problem that gets fixed in mainline LCMS should get
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148 pulled into our version.
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149
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150 Indeed, to keep this simple, we don't really want to diverge our
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151 version from mainline more than we have to.