Managing a JPEG structure object¶

JPEG::new

[arguments: "($input, $regex, $options)"] The first thing you need in order to interact with a JPEG picture is to create an Image::MetaData::JPEG structure object. This is done with a call to the new method, whose first argument is an input source, either a scalar, interpreted as a file name to be opened and read, or a scalar reference, interpreted as a pointer to an in-memory buffer containing a JPEG stream. This interface is similar to that of Image::Info, but no open file handle is (currently) accepted. The constructor then parses the picture content and stores its segments internally. The memory footprint is close to the size of the disk file plus a few tens of kilobytes.

    my $file = new Image::MetaData::JPEG('a_file_name.jpg');
    my $file = new Image::MetaData::JPEG(\ $a_JPEG_stream);
    

The constructor method accepts two optional arguments, a regular expression and an option string. If the regular expression is present, it is matched against segment names, and only those segments with a positive match are parsed (they are nonetheless stored); this allows for some speed-up if you just need partial information, but be sure not to miss something necessary; e.g., SOF segments are needed for reading the picture dimensions. For instance, if you just want to manipulate the comments, you could set the string to 'COM'.

    my $file = new Image::MetaData::JPEG('a_file_name.jpg', 'COM');
    

The third optional argument is an option string. If it matches the string 'FASTREADONLY', only the segments matching the regular expression are actually stored; also, everything which is found after a Start Of Scan is completely neglected. This allows for very large speed-ups, but, obviously, you cannot rebuild the file afterwards, so this is only for getting information fast, e.g., when doing a directory scan.

    my $file = new Image::MetaData::JPEG('a_file.jpg', 'COM', 'FASTREADONLY');
    

Nota bene: an old version of "Arles Image Web Page Creator" had a bug which caused the application to generate JPEG's with illegal comment segments, reportedly due to a bug in the Intel JPEG library the developers used at that time (these segments had to 0x00 bytes appended). It is true that a JPEG file with garbage between segments is to be considered invalid, but some libraries like IJG's try to forgive, so this module tries to forgive too, if the amount of garbage isn't too large (only a warning is printed).

JPEG::Error

[arguments: none] If the file reference remains undefined after a call to new, the file is to be considered not parseable by this module, and one should issue some error message and go to another file. An error message explaining the reason of the failure can be retrieved with the Error method:

    die 'Error: ' . Image::MetaData::JPEG::Error() unless $file;
    

JPEG::get_segments

[arguments: "($regex, $do_indexes)"] If the new call is successful, the returned reference points to an Image::MetaData::JPEG structure object containing a list of references to Image::MetaData::JPEG::Segment objects, which can be retrieved with the get_segments method. This method returns a list containing the references (or their indexes in the Segment references' list, if the second argument is the string INDEXES) to those Segments whose name matches the $regex regular expression. For instance, if $regex is 'APP', all application Segments will be returned. If you want only APP1 Segments you need to specify '^APP1$'. The output can become invalid after adding/removing any Segment. If $regex is undefined, all references are returned.

    my @segments = $file->get_segments($regex, $do_indexes);
    

JPEG::drop_segments

[arguments: "($regex)"] Similarly, if you are only interested in eliminating some segments, you can use the drop_segments method, which erases from the internal segment list all segments matching a given regular expression. If the regular expression is undefined or evaluates to the empty string, this method throws an exception, because I don't want the user to erase the whole file just because he/she did not understand what he was doing. One should also remember that it is not wise to drop non-meta-data segments, because this in general invalidates the file. As a special case, if $regex == 'METADATA', all APP* and COM segments are erased.

    $file->drop_segments('^APP1$');
    

JPEG::insert_segments

[arguments: "($segref, $pos, $overwrite)"] Inserting a Segment into the picture's segment list is done with the insert_segments method. This method inserts the segments referenced by $segref into the current list of segments at position $pos. If $segref is undefined, the method fails silently. If $pos is undefined, the position is chosen automatically (using find_new_app_segment_position ); if $pos is out of bound, an exception is thrown; this happens also if $pos points to the first segment, and it is an SOI. $segref may be a reference to a single segment or a reference to a list of segment references; everything else throws an exception. If $overwrite is defined, it must be the number of segments to overwrite during the splice.

    $file->insert_segments([$my_comment_1, $my_comment_2], 3, 1);
    

JPEG::get_description

JPEG::get_dimensions

[arguments: none] Getting a string describing the findings of the parsing stage is as easy as calling the get_description method. Those Segments whose parsing failed have the first line of their description stating the stopping error condition. Non-printable characters are replaced, in the string returned by get_description, by a slash followed by the two digit hexadecimal code of the character. The (x,y) dimensions of the JPEG picture are returned by get_dimensions from the Start of Frame (SOF*) Segment:

    print $file->get_description();
    my ($dim_x, $dim_y) = $file->get_dimensions();
    

JPEG::find_new_app_segment_position

[arguments: "($name)"] If a new comment or application Segment is to be added to the file, the module provides a standard algorithm for deciding the location of the new Segment, in the find_new_app_segment_position method. The argument is the name of the Segment to be inserted (it defaults to 'COM', producing a warning). The position is chosen immediately before the first (or after the last) element of some list, provided that the list is not empty, otherwise the next list is taken into account: 1) [for COM segments only] after 'COM' segments; otherwise after APP segments; 2) [for APPx segments only] after APPy's (trying y = x..0, in sequence); otherwise before APPy's (trying y = x+1..15, in sequence); 3) before DHP segments; 4) before SOF segments. If all these tentatives fail, the position immediately after the SOI segment is returned (i.e., 1).

    my $new_position = $file->find_new_app_segment_position('APP2');
    

JPEG::save

[arguments: "($filename)"] The data areas of each Segment in the in-memory JPEG structure object can be rewritten to a disk file or to an in-memory scalar, thus recreating the (possibly modified) JPEG picture. This is accomplished by the save method, accepting a filename or a scalar reference as argument; if the file name is undefined, it defaults to the file originally used to create the JPEG structure object. This method returns "true" (1) if it works, "false" (undefined) otherwise. Remember that if the file had initially been opened with the 'FASTREADONLY' option, it is not possible to save it, and this call fails immediately.

    print "Creation of $newJPEG failed!" unless $file->save($newJPEG);
    

An example of how to proficiently use the in-memory feature to read the content of a JPEG thumbnail is the following (see later for get_Exif_data, and also do some error checking!):

    my $thumbnail = $file->get_Exif_data('THUMBNAIL');
    print Image::MetaData::JPEG->new($thumbnail)->get_description();
    

Managing a JPEG Segment object¶

JPEG::Segment::name

JPEG::Segment::error

An Image::MetaData::JPEG::Segment object is created for each Segment found in the JPEG image during the creation of a JPEG object (see JPEG::new), and a parser routine is executed at the same time. The name member of a Segment object identifies the "nature" of the Segment (e.g. 'APP0', ..., 'APP15' or 'COM'). If any error occurs (in the Segment or in an underlying class), the parsing of that Segment is interrupted at some point and remains therefore incomplete: the error member of the relevant Segment object is then set to a meaningful error message. If no error occurs, the same variable is left undefined.

    printf 'Invalid %s!\n', $segment->{name} if $segment->{error};
    

JPEG::Segment::records

The reference to the Segment object is returned in any case. In this way, a faulty Segment cannot inhibit the creation of a JPEG structure object; faulty segments cannot be edited or modified, basically because their structure could not be fully understood. They are always rewritten to disk unmodified, so that a file with corrupted or non-standard Segments can be partially edited without fearing of damaging it. Once a Segment has successfully been built, its parsed information can be accessed directly through the records member: this is a reference to an array of JPEG::Record objects, an internal class modelled on Exif records (see the subsection about record management for further details).

    my $records = $segment->{records};
    printf '%s has %d records\n', $segment->{name}, scalar @$records;
    

JPEG::Segment::search_record

JPEG::Segment::search_record_value

[arguments: "([$dirref], $keys ...)"] If a specific record is needed, it can be selected with the help of the search_record method, which searches for a record with a given key (see "JPEG::Record::key") in a given record directory, returning a reference to the record if the search was fruitful, the undefined value otherwise. The algorithm for the search is as follows: 1) a start directory is chosen by looking at the last argument: if it is an ARRAY ref it is popped out and used, otherwise the top-level directory is selected; 2) a string is created by joining all remaining arguments on '@', then it is exploded into a list of keys on the same character (all undefined or "false" arguments are simply discarded); 3) these keys are used for an iterative search starting from the initially chosen directory: all but the last key must correspond to $REFERENCE records. If $key is exactly "FIRST_RECORD" / "LAST_RECORD", the first/last record in the current dir is used.

    my $segments = $file->get_segments('APP0');
    my $segment = $$segments[0];
    print "I found it!\n" if $segment->search_record('Identifier');
    

If you are interested only in the Record's value, you can use the search_record_value method, a simple wrapper around search_record(): it returns the record value (with "JPEG::Record::get_value") if the search is successful, undef otherwise.

    print "Its value is: ", $segment->search_record_value('Identifier');
    

Nota bene: the returned record is initialised with a "fake" $REFERENCE record pointing to the records member of the current segment; this record is therefore returned if search_record is invoked without arguments. For the same reason, search_record_value invoked without arguments returns the records member:

    $segment->search_record_value() eq $this->{records} || print "error!";
    

JPEG::Segment::update

[arguments: none] If a Segment's content (i.e. its Records' values) is modified, it is necessary to dump it into the private binary data area of the Segment in order to have the modification written to disk at "JPEG::save" time. This is accomplished by invoking the update method (necessary only if you changed record values "by hand"; all "high-level" methods for changing a Segment's content in fact call "update" on their own). However, only Segments without errors can be updated (don't try to undef the Segment's error flag, unless you know what you are doing!); trying to update a segment with errors throws an exception. The same happens when trying to update a segment without update support or without records (this catches segments created with the 'NOPARSE' flag). In practise, never use this method unless you are writing an extension for this module.

Note that this method preliminarly saves a reference to the old segment data area and restores it if the update process fails (if this happens, a warning is generated). One wonders wheather there are there cleverer ways to handle this case (any suggestion is welcome). It is however better to have a corrupt object in memory, than a corrupt object written over the original. Currently, this is restricted to the possibility that an updated segment becomes too large.

    $segment->update();
    

JPEG::Segment::reparse_as

[arguments: "($new_name)"] The reparse_as method re-executes the parsing of a Segment after changing the Segment name. This is very handy if you have a JPEG file with a "correct" application Segment exception made for its name. I used it the first time for a file having an ICC_profile Segment (normally in APP2) stored as APP13. Note that the name of the Segment is permanently changed, so, if the Segment is updated and the file is rewritten to disk, it will be "correct".

    for my $segment ($file->get_segments('APP13')) {
        $segment->reparse_as('APP2') if $segment->{error} &&
             $segment->search_record('Identifier') =~ 'ICC_PROFILE';
        $segment->update(); }
    

JPEG::Segment::output_segment_data

[arguments: none] The current in-memory data area of a Segment can be output to a file through the output_segment_data method (exception made for entropy coded Segments, this includes the initial two bytes with the Segment identifier and the two bytes with the length if present); the argument is a file handle (this is likely to become more general in the future). If there are problems at output time (e.g., the segment content is too large), an exception is thrown

    eval { $segment->output_segment_data($output_handle) } ||
        print "A terrible output error occurred! Help me.\n";
    

JPEG::Segment::get_description

JPEG::Segment::size

[arguments: none] A string describing the parsed content of the Segment is obtained through the get_description method (this is the same string used by the get_description method of a JPEG structure object). If the Segment parsing stage was interrupted, this string includes the relevant error. The size method returns the size of the internal data area of a Segment object. This can be different from the length of the scalar returned by get_segment_data, because the identifier and the length is not included.

    print $segment->get_description();
    print 'Size is 4 + ' . $segment->size();
    

Managing a JPEG Record object¶

JPEG::Record::key

JPEG::Record::type

JPEG::Record::values

JPEG::Record::extra

The JPEG::Record class is an internal class for storing parsed information about a JPEG Segment, inspired by Exif records. A Record is made up by four fields: key, type, values and extra. The key is the record's identifier; it is either numeric or textual (numeric keys can be translated with the help of the %JPEG_lookup function in Tables.pm, included in this package). The type is obviously the type of stored info (like unsigned integers, ASCII strings and so on ...). extra is a helper field for storing additional information. Last, values is an array reference to the record content (almost always there is just one value). For instance, for a non-IPTC Photoshop record in APP13:

    printf 'The numeric key 0x%04x means %s',
      $record->{key}, JPEG_lookup('APP13@Photoshop_RECORDS', $record->{key});
    printf 'This record contains %d values\n', scalar @{$record->{values}};
    

A Record's type can be one among the following predefined constants:

         0  $NIBBLES    two 4-bit unsigned integers (private)
         1  $BYTE       An 8-bit unsigned integer
         2  $ASCII      A variable length ASCII string
         3  $SHORT      A 16-bit unsigned integer
         4  $LONG       A 32-bit unsigned integer
         5  $RATIONAL   Two LONGs (numerator and denominator)
         6  $SBYTE      An 8-bit signed integer
         7  $UNDEF      A generic variable length string
         8  $SSHORT     A 16-bit signed integer
         9  $SLONG      A 32-bit signed integer (2's complement)
        10  $SRATIONAL  Two SLONGs (numerator and denominator)
        11  $FLOAT      A 32-bit float (a single float)
        12  $DOUBLE     A 64-bit float (a double float)
        13  $REFERENCE  A Perl list reference (internal)
    

$UNDEF is used for not-better-specified binary data. A record of a numeric type can have multiple elements in its @{values} list ($NIBBLES implies an even number); an $UNDEF or $ASCII type record instead has only one element, but its length can vary. Last, a $REFERENCE record holds a single Perl reference to another record list: this allows for the construction of a sort of directory tree in a Segment.

JPEG::Record::get_category

[arguments: none] The category of a record can be obtained with the get_category method, which returns 'p' for Perl references, 'I' for integer types, 'S' for $ASCII and $UNDEF, 'R' for rational types and 'F' for floating point types.

    for my $record (@{$segment->{records}}) {
        print "Subdir found\n" if $record->get_category() eq 'p'; }
    

JPEG::Record::get_description

[arguments: "($names)"] A human-readable description of a Record's content is the output of the get_description method. Its argument is a reference to an array of names, which are to be used as successive keys in a general hash keeping translations of numeric tags. No argument is needed if the key is already non-numeric (see the example of get_value for more details). In the output of get_description unreasonably long strings are trimmed and non-printing characters are replaced with their hexadecimal representation. Strings are then enclosed between delimiters, and null-terminated $ASCII strings have their last character chopped off (but a dot is added after the closing delimiter). $ASCII strings use a " as delimiter, while $UNDEF strings use '.

    print $record->get_description($names);
    

JPEG::Record::get_value

[arguments: "($index)"] In absence of "high-level" routines for collecting information, a Record's content can be read directly, either by accessing the values member or by calling the get_value method: it returns the $index-th value in the value list; if the index is undefined (not supplied), the sum/concatenation of all values is returned. The index is checked for out-of-bound errors. The following code, an abridged version of get_description, shows how to proficiently use these methods and members.

    sub show_directory {
      my ($segment, $records, $names) = @_;
      my @subdirs = ();
      for my $record (@$records) {
        print $record->get_description($names);
        push @subdirs, $record if $record->get_category() eq 'p'; }
      foreach my $subdir (@subdirs) {
        my $directory = $subdir->get_value();
        push @$names, $subdir->{key};
        printf 'Subdir %s (%d records)', $names, scalar @$directory;
        show_directory($segment, $directory, $names);
        pop @$names; } }
    show_directory($segment, $segment->{records}, [ $segment->{name} ]);
    

JPEG::Record::get

[arguments: "($endianness)"] If the Record structure is needed in detail, one can resort to the get method; in list context this method returns (key, type, count, dataref). The data reference points to a packed scalar, ready to be written to disk. In scalar context, it returns the dereferenced dataref. This is tricky (but handy for other routines). The argument specifies an endianness (this defaults to big endian).

    my ($key, $type, $count, $dataref) = $record->get();
    

Comments ("COM" segments)¶

JPEG::get_number_of_comments

JPEG::get_comments

[arguments: none] Each "COM" Segment in a JPEG file contains a user comment, whose content is free format. There is however a limitation, because a JPEG Segment cannot be longer than 64KB; this limits the length of a comment to $max_length = (2^16 - 3) bytes. The number of comment Segments in a file is returned by get_number_of_comments, while get_comments returns a list of strings (each string is the content of a COM Segment); if no comments are present, they return zero and the empty list respectively.

    my $number = $file->get_number_of_comments();
    my @comments = $file->get_comments();
    

JPEG::add_comment

[arguments: "($string)"] A comment can be added with the add_comment method, whose only argument is a string. Indeed, if the string is too long, it is broken into multiple strings with length smaller or equal to $max_length, and multiple comment Segments are added to the file. If there is already at least one comment Segment, the new Segments are created right after the last one. Otherwise, the standard position search of find_new_app_segment_position
is applied.

    $file->add_comment('a' x 100000);
    

JPEG::set_comment

[arguments: "($index, $string)"] An already existing comment can be replaced with the set_comment method. Its two arguments are an $index and a $string: the $index-th comment Segment is replaced with one or more new Segments based on $string (the index of the first comment Segment is 0). If $string is too big, it is broken down as in add_comment. If $string is undefined, the selected comment Segment is erased. If $index is out-of-bound a warning is printed out.

    $file->set_comment(0, 'This is the new comment');
    

JPEG::remove_comment

JPEG::remove_all_comments

[arguments: "($index)" for remove_comment] However, if you only need to erase the comment, you can just call remove_comment with just the Segment $index. If you want to remove all comments, just call remove_all_comments.

    $file->remove_comment(0);
    $file->remove_all_comments();
    

JPEG::join_comments

[arguments: "($separation, @selection)"] It is known that some JPEG comment readers out there do not read past the first comment. So, the join_comments method, whose goal is obvious, can be useful. This method creates a string from joining all comments selected by the @selection index list (the $separation scalar is a string inserted at each junction point), and overwrites the first selected comment while deleting the others. A exception is thrown for each illegal comment index. Similar considerations as before on the string length apply. If no separation string is provided, it defaults to \n. If no index is provided in @selection, it is assumed that the method must join all the comments into the first one, and delete the others.

    $file->join_comments('---', 2, 5, 8);
    

JFIF data ("APP0" segments)¶

APP0 Segments are written by older cameras adopting the JFIF (JPEG File Interchange Format), or one of its extensions, for storing images. JFIF files use the APP0 application Segment for inserting configuration data and a JPEG or RGB packed thumbnail image. The format is described in the appendix about the APP0 structure, including the names of all possible tags. It is of course possible to access each APP0 Segment individually by means of the get_segments and search_record_value methods. A snippet of code for doing this is the following:

    for my $segment ($file->get_segments('APP0')) {
        my $iden = $segment->search_record_value('Identifier');
        my $xdim = $segment->search_record_value('Xthumbnail');
        my $ydim = $segment->search_record_value('Ythumbnail');
        printf 'Segment type: %s; dimensions: %dx%d\n',
                substr($iden, 0, -1), $xdim, $ydim;
        printf '%15s => %s\n', $_->{key}, $_->get_value()
                for $segment->{records}; }

JPEG::get_app0_data

[arguments: none] However, if you want to avoid to deal directly with Segments, you can use the get_app0_data method, which returns a reference to a hash with a plain translation of the content of the first interesting APP0 segment (this is the first 'JFXX' APP0 segment, if present, the first 'JFIF' APP0 segment otherwise). Segments with errors are excluded. An empty hash means that no valid APP0 segment is present.

    my $data = $file->get_app0_data();
    printf '%15s => %s\n', $_, (($_=~/..Thumbnail/)?'...':$$data{$_});
    

Exif data ("APP1" segments)¶

The DCT Exif (Exchangeable Image File format) standard provides photographic meta-data in the APP1 section. Various tag-values pairs are stored in groups called IFDs (Image File Directories), where each group refers to a different kind of information; one can find data about how the photo was shot, GPS data, thumbnail data and so on ... (see the appendix about the APP1 segment structure for more details). This module provides a number of methods for managing Exif data without dealing with the details of the low level representation. Note that, given the complicated structure of an Exif APP1 segment (where extensive use of "pointers" is made), some digital cameras and graphic programs decide to leave some unused space in the JPEG file. The dump routines of this module, on the other hand, leave no unused space, so just calling update() on an Exif APP1 segment even without modifying its content can give you a smaller file (some tens of kilobytes can be saved).

JPEG::retrieve_app1_Exif_segment

[arguments: "($index)"] In order to work on Exif data, an Exif APP1 Segment must be selected. The retrieve_app1_Exif_Segment method returns a reference to the $index-th such Segment (the first Segment if the index is undefined). If no such Segment exists, the method returns the undefined reference. If $index is (-1), the routine returns the number of available APP1 Exif Segments (which is non negative).

    my $num = $file->retrieve_app1_Exif_segment(-1);
    my $ref = $file->retrieve_app1_Exif_segment($num - 1);
    

JPEG::provide_app1_Exif_segment

[arguments: none] If you want to be sure to have an Exif APP1 Segment, use the provide_app1_Exif_segment method instead, which forces the Segment to be present in the file, and returns its reference. The algorithm is the following: 1) if at least one Segment with this properties is already present, we are done; 2) if [1] fails, an APP1 segment is added and initialised with a big-endian Exif structure (its position is chosen by find_new_app_segment_position, as usual). Note that there is no $index argument here.

    my $ref = $file->provide_app1_Exif_segment();
    

JPEG::remove_app1_Exif_info

[arguments: "($index)"] If you want to eliminate the $index-th Exif APP1 Segment from the JPEG file segment list use the remove_app1_Exif_info method. As usual, if $index is (-1), all Exif APP1 Segments are affected at once; if $index is undefined, it defaults to -1, so both (-1) and undef cause all Exif APP1 segments to be removed. Be aware that the file won't be a valid Exif file after this.

    $file->remove_app1_Exif_info(-1);
    

How to inspect your Exif data

JPEG::Segment::get_Exif_data

JPEG::get_Exif_data

[arguments: "($what, $type)"] Once you have a Segment reference pointing to your favourite Exif Segment, you may want to have a look at the records it contains, by using the get_Exif_data method: it accepts two arguments ($what and $type) and returns the content of the APP1 segment packed in various forms. Error conditions (invalid $what's and $type's) manifest themselves through an undefined return value.

All Exif records are natively identified by numeric tags (keys), which can be "translated" into a human-readable form by using the Exif standard docs; only a few fields in the Exif APP1 preamble (they are not Exif records) are always identified by this module by means of textual tags. The $type argument selects the output format for the record keys (tags):

    * NUMERIC: record tags are native numeric keys
    * TEXTUAL: record tags are human-readable (default)
    

Of course, record values are never translated. If a numeric Exif tag is not known, a custom textual key is created with "Unknown_tag_" followed by its numerical value (this solves problems with non-standard tags). The subset of Exif tags returned by this method is determined by the value of $what, which can be one of:

    $what          returned info                         returned type
    ---------------------------------------------------------------------
    ALL            (default) everything but THUMBNAIL    ref. to hash of hashes
    IMAGE_DATA     a merge of IFD0_DATA and SUBIFD_DATA  ref. to flat hash
    THUMB_DATA     this is an alias for IFD1_DATA        ref. to flat hash
    THUMBNAIL      the actual (un)compressed thumbnail   ref. to scalar
    ROOT_DATA      header records (TIFF and similar)     ref. to flat hash
    IFD0_DATA      primary image TIFF tags               ref. to flat hash
    SUBIFD_DATA    Exif private tags                     ref. to flat hash
    MAKERNOTE_DATA MakerNote tags (if struct. is known)  ref. to flat hash
    GPS_DATA       GPS data of the primary image         ref. to flat hash
    INTEROP_DATA   interoperability data                 ref. to flat hash
    IFD1_DATA      thumbnail-related TIFF tags           ref. to flat hash
    

Setting $what equal to 'ALL' returns a reference to a hash of hashes, whose top-level hash contains the following keys: ROOT_DATA, IFD0_DATA, SUBIFD_DATA, GPS_DATA, INTEROP_DATA, MAKERNOTE_DATA and IFD1_DATA; each key corresponds to a second-level hash containing a copy of all Exif records present in the IFD (sub)directory corresponding to the key (if this directory is not present or contains no records, the second-level hash exists and is empty). Note that the Exif record values' format is not checked to be valid according to the Exif standard. This is, in some sense, consistent with the fact that also "unknown" tags are included in the output. This complicated structure is more easily explained by showing an example (see also the section about valid Exif tags for details on possible records):

    my $hash_ref = $segment->get_Exif_data('ALL', 'TEXTUAL');
                         can give
    $hash_ref = {
           'ROOT_DATA' => 
                { 'Signature'               => [ 42             ],
                  'Endianness'              => [ 'MM'           ],
                  'Identifier'              => [ "Exif\000\000" ],
                  'ThumbnailData'           => [ ... image ...  ], },
           'IFD1_DATA' =>
                { 'ResolutionUnit'          => [ 2              ],
                  'JPEGInterchangeFormatLength' => [ 3922       ],
                  'JPEGInterchangeFormat'   => [ 2204           ],
                  'Orientation'             => [ 1              ],
                  'XResolution'             => [ 72, 1          ],
                  'Compression'             => [ 6              ],
                  'YResolution'             => [ 72, 1          ], },
           'SubIFD_DATA' =>
                { 'ApertureValue'           => [ 35, 10         ],
                  'PixelXDimension'         => [ 2160           ],
                    etc., etc. ....
                  'ExifVersion'             => [ '0210'         ], },
           'MAKERNOTE_DATA' => {},
           'IFD0_DATA' =>
                { 'Model' => [ "KODAK DX3900 ZOOM DIGITAL CAMERA\000" ],
                  'ResolutionUnit'          => [ 2              ],
                    etc., etc. ...
                  'YResolution'             => [ 230, 1         ], },
           'GPS_DATA' => {},
           'INTEROP_DATA' =>
                { 'InteroperabilityVersion' => [ '0100'         ],
                  'InteroperabilityIndex'   => [ "R98\000"      ], }, };
    

Setting $what equal to '*_DATA' returns a reference to a flat hash, corresponding to one or more IFD (sub)dirs. For instance, 'IMAGE_DATA' is a merge of 'IFD0_DATA' and 'SUBIFD_DATA': this interface is simpler for the end-user, because there is only one dereference level; also, he/she does not need to be aware of the partition of records related to the main image into two IFDs. If the (sub)directory is not present or contains no records, the returned hash exists and is empty. With reference to the previous example:

    my $hash_ref = $segment->get_Exif_data('IMAGE_DATA', 'TEXTUAL');
                         gives
    $hash_ref = {
           'ResolutionUnit'              => [ 2      ],
           'JPEGInterchangeFormatLength' => [ 3922   ],
           'JPEGInterchangeFormat'       => [ 2204   ],
           'Orientation'                 => [ 1      ],
           'XResolution'                 => [ 72, 1  ],
           'Compression'                 => [ 6      ],
           'YResolution'                 => [ 72, 1  ],
           'ApertureValue'               => [ 35, 10 ],
           'PixelXDimension'             => [ 2160   ],
              etc., etc. ....
           'ExifVersion'                 => [ '0210' ], };
    

Last, setting $what to 'THUMBNAIL' returns a reference to a copy of the actual Exif thumbnail image (this is not included in the set returned by 'THUMB_DATA'); if there is no thumbnail, a reference to the empty string is returned (the undefined value cannot be used, because it is assumed that it corresponds to an error condition here). Note that the pointed scalar may be quite large (~ 10^1 KB). If the thumbnail is in JPEG format (this corresponds to the 'Compression' property, in IFD1, set to 6), you can create another JPEG picture object from it, like in the following example:

    my $data_ref = $segment->get_Exif_data('THUMBNAIL');
    my $thumb = new Image::MetaData::JPEG($data_ref);
    print $thumb->get_description();
    

If you are only interested in reading Exif data in a standard configuration, you can skip the segment-search calls and use directly JPEG::get_Exif_data (a method of the JPEG class, so you only need a JPEG structure object). This is an interface to the method with the same name in the Segment class, acting on the first Exif APP1 Segment (if no such segment is present, the undefined value is returned) and passing the arguments through. Note that most JPEG files with Exif data contain at most one Exif APP1 segment, so you are not going to loose anything here. A snippet of code for visualising Exif data looks like this:

    while (my ($d, $h) = each %{$image->get_Exif_data('ALL')}) {
      while (my ($t, $a) = each %$h) {
        printf '%-25s\t%-25s\t-> ', $d, $t;
        s/([\000-\037\177-\377])/sprintf '\\%02x',ord($1)/ge,
        $_ = (length $_ > 30) ? (substr($_,0,30) . ' ... ') : $_,
        printf '%-5s', $_ for @$a; print "\n"; } }
    

How to modify your Exif data

JPEG::Segment::set_Exif_data

JPEG::set_Exif_data

[arguments: "($data, $what, $action)"]

Similarly to the getter case, there is a set_Exif_data method callable from a picture object, which does nothing more than looking for the first Exif APP1 segment (creating it, if there is none) and invoke the method with the same name in the Segment class, passing its arguments through. So, the remaining of this section will concentrate on the Segment method. The problem of setting a new thumbnail or erasing it is dealt with in the last paragraphs of this section. (The APP1 Exif structure is quite complicated, and the number of different possible cases when trying to modify it is very large; therefore, designing a clean and intuitive interface for this task is not trivial. Fell free to suggest improvements and cleaner interfaces).

Exif records are usually characterised by a numeric key (a tag); this was already discussed in the "getter" section. Since these keys, for valid records, can be translated from numeric to textual form and back, the end user has the freedom to use whichever form better fits his needs. The two forms can even be mixed in the same "setter" call: the method will take care to translate textual tags to numeric tags when possible, and reject the others; then, it will proceed as if all tags were numeric from the very beginning. Records with unknown textual or numeric tags are always rejected.

The arguments to set_Exif_data are $data, $what and $action. The $data argument must be a hash reference to a flat hash, containing the key - record values pairs supplied by the user. The "value" part of each hash element can be an array reference (containing a list of values for the record, remember that some records are multi-valued) or a single scalar (this is internally converted to a reference to an array containing only the supplied scalar). If a record value is supposed to be a null terminated string, the user can supply a Perl scalar without the final null character (it will be inserted automatically).

The $what argument must be a scalar, and it selects the portion of the Exif APP1 segment concerned by the set_Exif_data call. So, obviously, the end user can modify only one section at a time; this is a simplification (for the developer of course) but also for the end user, because trying to set all Exif-like values in one go would require an offensively complicated data structure to specify the destination of each record (note that some records in different sections can have the same numerical tag, so a plain hash would not trivially work). Valid values for $what are (MakerNote data are not currently modifiable):

    $what         modifies ...                          $data type
    --------------------------------------------------------------------
    IMAGE_DATA    as IFD0_DATA and SUBIFD_DATA          ref. to flat hash
    THUMB_DATA    this is an alias for IFD1_DATA        ref. to flat hash
    THUMBNAIL     the actual (un)compressed thumbnail   ref. to scalar/object
    ROOT_DATA     header records (endianness)           ref. to flat hash
    IFD0_DATA     primary image TIFF tags               ref. to flat hash
    SUBIFD_DATA   Exif private tags                     ref. to flat hash
    GPS_DATA      GPS data of the primary image         ref. to flat hash
    INTEROP_DATA  interoperability data in SubIFD       ref. to flat hash
    IFD1_DATA     thumbnail-related TIFF tags           ref. to flat hash
    

The $action argument controls whether the setter adds ($action = 'ADD') records to a given data directory or replaces ($action = 'REPLACE') them. In the first case, each user-supplied record replaces the existing version of that record if present, and simply inserts the record if it was not already present; however, existing records with no counterpart in the user supplied $data hash remain untouched. In the second case, the record directory is cleared before inserting user data. Note that, since Exif and Exif-like records are non-repeatable in nature, there is no need of an 'UPDATE' action, like for IPTC (see the IPTC section).

The set_Exif_data routine first checks that the concerned segment is of the appropriate type (Exif APP1), that $data is a hash reference (a scalar reference for the thumbnail), and that $action and $what are valid. If $action is undefined, it defaults to 'REPLACE'. Then, an appropriate (sub)IFD is created, if absent, and all user-supplied records are checked for consistency (have a look at the appendixes for this). Last, records are set in increasing (numerical) tag order, and mandatory data are added, if not present. The return value of the setter routine is always a hash reference; in general it contains records rejected by the specialised routines. If an error occurs in a very early stage of the setter, this reference contains a single entry with key='ERROR' and value set to some meaningful error message. So, returning a reference to an empty hash means that everything was OK. An example, concerning the much popular task of changing the DateTime record, follows:

    $dt = '1994:07:23 12:14:51';
    $hash = $image->set_Exif_data({'DateTime' => $dt}, 'IMAGE_DATA', 'ADD');
    print "DateTime record rejected\n" if %$hash;
    

Depending on $what, some of the following notes apply:

XMP data ("APP1" segments)¶

XMP (eXtensible Metadata Platform) is a technology, conceived by Adobe Systems, to tag graphic files with metadata, and to manage them during a lifetime made of multiple processing steps. Its serialisation (the actual way metadata are saved in the file) is based on RDF (Resource Description Framework) implemented as an application of XML. Its flexibility allows to accomodate existing, future and private metadata schemas. In a JPEG file, XMP information is included alongside Exif and IPTC data, and is stored in an APP1 segment on its own starting with the XMP namespace URI and followed by the actual XMP packet (see XMP APP1 segment structure for more details).

XMP was introduced in 2001 as part of Adobe Acrobat version 5.01. Adobe has a trademark on XMP, and retains control over its specification. Source code for the XMP software-development kit was released by Adobe, but with a custom license, whose compatibility with the GNU public license and open-source nature altogether is questioned.

Photoshop and IPTC data ("APP13" segments)¶

Adobe's Photoshop program, a de-facto standard for image manipulation, has, since long, used the APP13 segment for storing non-graphical information, such as layers, paths, ecc..., including editorial information modelled on IPTC/NAA recommendations. This module provides a number of methods for managing Photoshop/IPTC data without dealing with the details of the low level representation (although sometimes this means taking some decisions for the end user ....). The structure of the IPTC data block(s) is managed in detail and separately from the rest, although this block is a sort of "sub-case" of Photoshop information. The interface is intentionally similar to that for Exif data.

All public methods have a $what argument selecting which part of the APP13 segment you are working with. The default is 'IPTC'. If $what is invalid, an exception is always raised. The kind of information you can access with different values of $what is explained in the following (have a look at the appendices about valid Photoshop-style and IPTC tags for further details):

    $what:        Concerned pieces of information:
    -----------   --------------------------------
    'IPTC' or     Editorial information like caption, abstract, author,
    'IPTC_2'      copyright notice, byline, shot site, user defined keywords,
                  and many more; in practise, all what is covered by the IPTC
                  Application Record 2. This is the most common option; the
                  default value of $what, 'IPTC', is a synonym for 'IPTC_2'
                  for backward compatibility (NOT a merge of 'IPTC_1/2').
    'IPTC_1'      This refers to more obscure pieces of information, contained
                  in the IPTC Envelope Record 1. One is rarely interested by
                  this, exception made for the "Coded Character Set" tag,
                  which is necessary to define a character set different
                  from ASCII (i.e., when you don't write or read in English).
    'PHOTOSHOP'   Alpha channels, colour information, transfer functions,
    or 'PS_8BIM'  and many other details concerning the visual rendering of
    or 'PS_8BPS'  the picture. These fields are most often only modified by
    or 'PS_PHUT'  an image manipulation program, and not directly by the user.
                  Recent versions of Photoshop (>= 4.0) use a resource data
                  block type equal to '8BIM', and this is the default in
                  this module (so, 'PHOTOSHOP' and 'PS_8BIM' are synonyms).
                  However, some other older or undocumented resource data
                  block types are also allowed.

JPEG::retrieve_app13_segment

[arguments: "($index, $what)"] In order to work on Photoshop/IPTC data, a suitable Photoshop-style APP13 Segment must first be selected. The retrieve_app13_segment method returns a reference to the $index-th Segment (the first Segment if the $index is undefined) which contains information matching the $what argument. If such Segment does not exist, the method returns the undefined reference. If $index is (-1), the routine returns the number of available suitable APP13 Segments (which is non negative). Beware, the meaning of $index is influenced by the value of $what.

    my $num_IPTC = $file->retrieve_app13_segment(-1, 'IPTC');
    my $ref_IPTC = $file->retrieve_app13_segment($num - 1, 'IPTC');
    

JPEG::provide_app13_segment

[arguments: "($what)"] If you want to be sure to have an APP13 Segment suitable for the kind of information you want to write, use the provide_app13_segment method instead, which forces the Segment to be present in the file, and returns its reference. If at least one segment matching $what is already present, the first one is returned. Otherwise, the first Photoshop-like APP13 is adapted by inserting an appropriate subdirectory record (update is called automatically). If no such segment exists, it is first created and inserted (the "Photoshop 3.0\000" identifier is used). Note that there is no $index argument here.

    my $ref_Photoshop = $file->provide_app13_segment('PHOTOSHOP');
    

JPEG::remove_app13_info

[arguments: "($index, $what)"] If you want to remove all traces of some flavour of APP13 information from the $index-th APP13 Photoshop-style Segment, use the remove_app13_info method with $what set to the appropriate value. If, after this, the segment is empty, it is eliminated from the list of segments in the file. If $index is (-1), all APP13 Segments are affected at once. Beware, the meaning of $index is influenced by the value of $what.

    $file->remove_app13_info(3, 'PHOTOSHOP');
    $file->remove_app13_info(-1, 'IPTC');
    $file->remove_app13_info(0, 'IPTC_1');
    

How to inspect and modify your IPTC data

JPEG::Segment::get_app13_data

[arguments: "($type, $what)"]

Once you have a Segment reference pointing to your favourite IPTC-enabled APP13 Segment, you may want to have a look at the records it contains. Use the get_app13_data method for this: its behaviour is controlled by the $type and $what argument (here, $what is 'IPTC_1' or 'IPTC_2' alias 'IPTC', of course). It returns a reference to a hash containing a copy of the list of the appropriate IPTC records, if present, undef otherwise: each element of the hash is a pair (key, arrayref), where arrayref points to an array with the real values (some IPTC records are repeatable so multiple values are possible). The record keys can be the native numeric keys ($type eq 'NUMERIC') or translated textual keys ($type eq 'TEXTUAL', default); in any case, the record values are untranslated. If a numeric key stored in the JPEG file is unknown, and a textual translation is requested, the name of the key becomes "Unknown_tag_$tag". Note that there is no check on the validity of IPTC records' values: their format is not checked and one or multiple values can be attached to a single tag independently of its repeatability. This is, in some sense, consistent with the fact that also "unknown" tags are included in the output. If $type or $what is invalid, an exception is thrown out. An example of how to extract and display IPTC data is given here:

    my $hash_ref = $segment->get_app13_data('TEXTUAL', 'IPTC');
    while (my ($key, $vals) = each %$hash_ref) {
       printf "# %20s =", $key; print " '$_'" for @$vals; print "\n"; }
    ### This could print:
    # DateCreated         = '19890207'
    # ByLine              = 'Interesting picture' 'really'
    # Category            = 'POL'
    # Keywords            = 'key-1' 'key-2' 'key-99'
    # OriginatingProgram  = 'Mapivi'
    

JPEG::Segment::set_app13_data

[arguments: "($data, $action, $what)"] The hash returned by get_app13_data can be edited and reinserted with the set_app13_data method, whose arguments are $data, $action and, as usual, $what. If $action or $what is invalid, an exception is generated. This method accepts IPTC data in various formats and updates the corresponding subdirectory in the segment. The key type of each entry in the input hash can be numeric or textual, independently of the others (the same key can appear in both forms, the corresponding values will be put together). The value of each entry can be an array reference or a scalar (you can use this as a shortcut for value arrays with only one value). The $action argument can be:

   - ADD : new records are added and nothing is deleted; however, if you
           try to add a non-repeatable record which is already present,
           the newly supplied value ejects (replaces) the pre-existing value.
   - UPDATE : new records replace those characterised by the same tags,
           but the others are preserved. This makes it possible to modify
           some repeatable IPTC records without deleting the other tags.
   - REPLACE : all records present in the IPTC subdirectory are deleted
           before inserting the new ones (this is the default action).
    

If, after implementing the changes required by $action, any mandatory dataset (according to the IPTC standard), is still undefined, it is added automatically. This often concerns version datasets, with numeric index 0.

The return value is a reference to a hash containing the rejected key-values entries. The entries of %$data are not modified. An entry in the %$data hash can be rejected for various reasons (you might want to have a look at appendix about valid IPTC tags for further information): a) the tag is undefined or not known; b) the entry value is undefined or points to an empty array; c) the non-repeatability constraint is violated; d) the tag is marked as invalid; e) a value is undefined f) the length of a value is invalid; g) a value does not match its mandatory regular expression.

    $segment->set_app13_data($additional_data, 'ADD', 'IPTC');
    

A snippet of code for changing IPTC data looks like this:

    my $segment = $file->provide_app13_segment('IPTC');
    my $hashref_1 = { CodedCharacterSet => "\033\045G" }; # UTF-8
    my $hashref_2 = { ObjectName => 'prova',
                      ByLine     => 'ciao',
                      Keywords   => [ 'donald', 'duck' ],
                      SupplementalCategory => ['arte', 'scienza', 'diporto'] };
    $segment->set_app13_data($hashref_2, 'REPLACE', 'IPTC');
    $segment->provide_app13_subdir('IPTC_1');
    $segment->set_app13_data($hashref_1, 'ADD', 'IPTC_1');
    

JPEG::get_app13_data

[arguments: "($type, $what)"] If you are only interested in reading IPTC data in a standard configuration, you can skip most of the previous calls and use directly JPEG::get_app13_data (a method in the JPEG class, so you only need a JPEG structure object). This is an interface to the method with the same name in the Segment class, acting on the first relevant APP13 Segment (if no such segment is present, the undefined value is returned) and passing the arguments through. Note that most JPEG files with Photoshop/IPTC data contain at most one APP13 segment, so you are not going to "loose" anything here. A snippet of code for visualising IPTC data looks like this:

    my $hashref = $file->get_app13_data('TEXTUAL', 'IPTC');
    while (my ($tag, $val_arrayref) = each %$hashref) {
        printf '%25s --> ', $tag;
        print "$_ " for @$val_arrayref; print "\n"; }
    

JPEG::set_app13_data

[arguments: "($data, $action, $what)"] There is, of course, a symmetric JPEG::set_app13_data method, which writes data to the JPEG object without asking the user to bother about Segments: it uses the first available suitable Segment; if this is not possible, a new Segment is created and initialised (because the method uses "JPEG::provide_app13_segment" internally, and not "JPEG::retrieve_app13_segment" as "JPEG::get_app13_data" does).

    $file->set_app13_data($hashref, 'UPDATE', 'IPTC');
    

How to inspect and modify your Photoshop data

The procedure of inspecting and modifying Photoshop data (i.e., non-IPTC data in a Photoshop-style APP13 segment) is analogous to that for IPTC data, but with $what set to 'PHOTOSHOP' (alias 'PS_8BIM'), or to the seldom used 'PS_8BPS' and 'PS_PHUT'. The whole description will not be repeated here, have a look at the IPTC section for it: this section takes only care to point out differences. If you are not acquainted with the structure of an APP13 segment and its terminology (e.g., "resource data block"), have a look at the Photoshop-style tags' section.

About get_app13_data, it should only be pointed out that resource block names are appended to the list of values for each tag (even if they are undefined), so the list length is alway even. Things are more complicated for set_app13_data: non-IPTC Photoshop specifications are less uniform than IPTC ones, and checking the correctness of user supplied data would be an enumerative task. Currently, this module does not perform any syntax check on non-IPTC data, but this could change in the future (any contribution is welcome); only tags (or, how they are called in this case, "resource block identifiers") are checked for being in the allowed tags list (see the Photoshop-style tags' table for details). The IPTC/NAA tag is of course rejected: IPTC data must be inserted with $what set to 'IPTC' or its siblings.

Although not explicitly stated, it seems that non-IPTC Photoshop tags are non-repeatable (let me know if not so), so two resource blocks with the same tag shouldn't exist. For this reason, the 'UPDATE' action is changed internally to 'ADD'. Moreover, since the resource block structure is not explored, all resource blocks are treated as single-valued and the value type is $UNDEF. So, in the user-supplied data hash, if a tag key returns a data array reference, only the first element (which cannot be undefined) of the array is used as resource block value: if a second element is present, it is used as resource block name (which is otherwise set to the null string). Suppling more than two elements is an error and causes the record to be rejected.

    my $segment = $file->provide_app13_segment('PHOTOSHOP');
    my $hashref = {
        GlobalAngle    => pack('N', 0x1e),
        GlobalAltitude => pack('N', 0x1e),
        CopyrightFlag  => "\001",
        IDsBaseValue   => [ pack('N', 1), 'Layer ID Generator Base' ] };
    $segment->set_app13_data($hashref, 'ADD', 'PHOTOSHOP');

On the subject of year specification in a date¶

There are currently eight fields whose purpose is to store a date in a JPEG picture, namely 'DateTime', 'DateTimeOriginal' and 'DateTimeDigitized' (in IFD0/1 or SubIFD), 'GPSDateStamp' (in the GPS section), and 'ReleaseDate', 'ExpirationDate', 'DateCreated' and 'DigitalCreationDate' (in the IPTC section). Most of these dates refer to some electronic treatment of images, a kind of process which was not available before the late twentieth century. Two of them refer to release and expiration dates in the IPTC standard, and should therefore not be set to a date before the introduction of the standard itself. However, there exist users who want to use some of these fields in a non-conventional way to refer to dates when analog photography but not digital photography was available. For this reason, all tags (but one) can be written with a year starting from 1800 (and not from 1900 as in earlier releases). Users are however advised to check the "specifications" for these tags before setting the date and take responsibility for their non-conventionality.

There is one notable exception to the previous considerations, that is the IPTC 'DateCreated' dataset, which should explicitly refer to the creation date of the object represented in the picture, which can be many centuries in the past. For this dataset a special regular expression is provided which allows a date in the full ISO-8601 YYYY-MM-DD format (however, it should be noted that even ISO-8601 does not allow a date before 0AD, so not all masterworks from ancient Greece can be tagged in this way ... let me know if I am wrong). I am, of course, still open to suggestions and reconsiderations on this subject.

On the problem of MakerNote corruption and ways to overcome it¶

A widespread problem with Exif maker notes is that there is no common standard for how to parse and rewrite the information in the MakerNote data area. This is the reason why most programs dealing with Exif JPEG files corrupt the MakerNote on saving, or decide to drop it altogether (be aware that there existed programs known to hang when they try to read a corrupt maker note).

In fact, many maker notes contain a non-standard IFD structure, with some tags storing file offsets (see the documentation page describing the IFD structure). Therefore, saving a maker note without regard for internal offsets' adjustment reduces the note mostly to garbage. Re-dumping a maker note after changing the Exif APP1 segment endianness incurs the same problem, because no internal byte-swap is performed.

A few countermeasures have been introduced in this package to try to cure some maker note problems. The first one concerns the correct byte order (the endianness, which is not always the same used in the Exif segment), which needs not to be known in advance; it is in fact determined by using the fact that, if the note is IFD-like (even non-standard), the number of tags is always in the range [1,255], so the two-bytes tag count has always the most significant byte set to zero, and the least significant byte set to non-zero.

There is also a prediction and correction mechanism for the offsets in the interoperability arrays, based on the simple assumption that the absolute value of offsets can be wrong, but their differences are always right, so, if one can get the first one right ... a good bet is the address of the byte immediately following the next_IFD link (or the tag list, if this link is absent). If the parsing process does not end successfully, this mechanism is enabled and its "corrected" findings are stored instead of the original ones if it is able to cure the problems (i.e., if the second try at parsing the note is successful).