Error during parsing of djvu text layer to produce metadata leads to page offset in ProofreadPage extension
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	Mpaa
	Jan 25 2019, 9:15 PM

Description

Error during parsing of djvu text layer to produce metadata leads to page offset in ProofreadPage extension.

For this file,: https://commons.wikimedia.org/wiki/File:Philosophical_Transactions_-_Volume_053.djvu
parsing of some pages when text is loaded from file to metadata fails.

Metadata of djvu files contain the text layer of pages.
See https://commons.wikimedia.org/w/api.php?action=query&prop=imageinfo&iiprop=metadata&titles=File:Philosophical_Transactions_-_Volume_053.djvu

The format of the metadata is as follows:

<?xml version=\"1.0\" ?>
<!DOCTYPE DjVuXML PUBLIC \"-//W3C//DTD DjVuXML 1.1//EN\" \"pubtext/DjVuXML-s.dtd\">
<mw-djvu>
  <DjVuXML>
    <HEAD></HEAD>
    <BODY><OBJECT height=\"1500\" width=\"1201\">
        <PARAM name=\"DPI\" value=\"300\" />  <-- One per page, they are  actually as many as the pages
        <PARAM name=\"GAMMA\" value=\"2.2\" />
      </OBJECT>
      ...
      <OBJECT height=\"1500\" width=\"1026\">
        <PARAM name=\"DPI\" value=\"300\" />
        <PARAM name=\"GAMMA\" value=\"2.2\" />
      </OBJECT>
    </BODY>
  </DjVuXML>
  
  /DjVuXML>
  <DjVuTxt>
    <HEAD></HEAD>
    <BODY>
      <PAGE value=\"\" /> <-- One per page when OK, less if parsing fails.
      ...
      <PAGE value=\"\" />
    </BODY>
  </DjVuTxt>
</mw-djvu>

Evidence of parsing failure is the fact instead of a page, this is returned by the API.

<PAGE value=\"[ 30 ] ...... &#10;\" />
failed                                                                   <--- ERROR! this is suposed to be page 51
<PAGE value=\"1 &#10;\u2666 &#10;\" />

So one page is lost and the text goes out of sync in ProofreadPage extension.

In this file there are 7 of such failures.

I checked the XML of page 51 and I got no error regarding tag structure.
Maybe an encoding error?!

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Mentioned In: T219376: retrieveMetaData() in DjVuImage.php creates knock-on error when a page has invalid text layer

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Mpaa created this task.Jan 25 2019, 9:15 PM

Restricted Application added a subscriber: Aklapper. · View Herald TranscriptJan 25 2019, 9:15 PM

I guess the problem might lie somewhere here:
https://doc.wikimedia.org/mediawiki-core/master/php/DjVuImage_8php_source.html#l00308

It is the djvutxt that fails for this page 51:

$ djvutxt philosophicaltra5317roya.djvu --page=51 --detail=page
failed

And also djvused fails:

$ djvused philosophicaltra5317roya.djvu -e 'select 51; print-txt' 
*** [1-10100] Text layer hierarchy is corrupt"
*** (DjVuText.cpp:287)
*** 'void DJVU::DjVuTXT::Zone::decode(const DJVU::GP<DJVU::ByteStream>&, int, const DJVU::DjVuTXT::Zone*, const DJVU::DjVuTXT::Zone*)'

While there is nothing to be done in MW to fetch the correct text, a better solution would be to at least keep the page tagging number correct.
That is, instead of just writng

failed

something like this:

<PAGE value=\"failed" />

So text would not go out of sync.

Reedy updated the task description. (Show Details)Jan 25 2019, 10:53 PM

There does not seem to be anything to do with MediaWiki-Action-API here, the API is just returning the data given to it.

Anomie unsubscribed.Jan 25 2019, 10:58 PM

The improvement is to be done in DjVuImage.php: function retrieveMetaData()
https://doc.wikimedia.org/mediawiki-core/master/php/DjVuImage_8php_source.html#l00246

# Text layer
         if ( isset( $wgDjvuTxt ) ) {
             $cmd = wfEscapeShellArg( $wgDjvuTxt ) . ' --detail=page ' . wfEscapeShellArg( $this->mFilename );
             wfDebug( __METHOD__ . ": $cmd\n" );
             $retval = '';
             $txt = wfShellExec( $cmd, $retval, [], [ 'memory' => self::DJVUTXT_MEMORY_LIMIT ] );
             if ( $retval == 0 ) {
                 # Strip some control characters
                 $txt = preg_replace( "/[\013\035\037]/", "", $txt );
                 $reg = <<<EOR
                     /\(page\s[\d-]*\s[\d-]*\s[\d-]*\s[\d-]*\s*"
                     ((?>    # Text to match is composed of atoms of either:
                         \\\\. # - any escaped character
                         |     # - any character different from " and \
                         [^"\\\\]+
                     )*?)
                     "\s*\)
                     | # Or page can be empty ; in this case, djvutxt dumps ()
                     \(\s*()\)/sx
 EOR;
                 $txt = preg_replace_callback( $reg, [ $this, 'pageTextCallback' ], $txt );
                 $txt = "<DjVuTxt>\n<HEAD></HEAD>\n<BODY>\n" . $txt . "</BODY>\n</DjVuTxt>\n";
                 $xml = preg_replace( "/<DjVuXML>/", "<mw-djvu><DjVuXML>", $xml, 1 );
                 $xml = $xml . $txt . '</mw-djvu>';
             }
  }

retval is == 0 even if there are errors.

So the error, which is not processed by the callback as it does not match any regex, is just kept in $txt as is:

This can be seen running:

$djvutxt --detail=page philosophicaltra5317roya.djvu | grep failed

At least it should be checked if "failed" matches at the beginning of a line (a bit hackish).

Unfortunately my php ends here. Help would be appreciated.
Thanks

Something like this.

...
$txt = preg_replace_callback( $reg, [ $this, 'pageTextCallback' ], $txt );
$reg_failed = '/(?m)^failed$/'; 
$txt = preg_replace_callback( $reg_failed, [ $this, 'pageTextCallbackFailed' ], $txt );
txt = "<DjVuTxt>\n<HEAD></HEAD>\n<BODY>\n" . $txt . "</BODY>\n</DjVuTxt>\n";

...

function pageTextCallbackFailed( $matches ) {
	 $val = $matches[0];
         return '<PAGE value="' . $val . '" />';
  }
...

Note:
I fixed the file on Commons.
The buggy file is https://commons.wikimedia.org/w/index.php?title=File:Philosophical_Transactions_-_Volume_053.djvu&oldid=336270264

Mpaa mentioned this in T219376: retrieveMetaData() in DjVuImage.php creates knock-on error when a page has invalid text layer.Apr 9 2019, 7:46 PM

kaldari added a project: good first task.Oct 9 2019, 5:28 AM

Soda removed a project: good first task.Aug 10 2023, 4:10 PM

Error during parsing of djvu text layer to produce metadata leads to page offset in ProofreadPage extensionOpen, Needs TriagePublicActions

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Error during parsing of djvu text layer to produce metadata leads to page offset in ProofreadPage extension
Open, Needs TriagePublic
Actions