++ed by:

2 non-PAUSE users.

Stefano Bettelli

NAME

Image::MetaData::JPEG - Perl extension for showing/modifying JPEG (meta)data.

SYNOPSIS

    use Image::MetaData::JPEG;

    # Create a new JPEG file structure object
    my $file = new Image::MetaData::JPEG($filename);
    die 'Error: ' . Image::MetaData::JPEG::Error() unless $file;

    # Get a list of references to JPEG segments
    my @segments = $file->get_segments($regex, $do_indexes);

    # Get the JPEG picture dimensions
    my ($dim_x, $dim_y) = $file->get_dimensions();

    # Show all JPEG segments and their content
    print $file->get_description();

    # Modify the DateTime tag for the main image
    $file->set_Exif_data({'DateTime' => '1994:07:23 12:14:51'},
                         'IMAGE_DATA', 'ADD');

    # Delete all meta-data segments (please, don't)
    $file->drop_segments('METADATA');

    # Rewrite file to disk after your modifications
    $file->save('new_file_name.jpg');

    # ... and a lot more methods for viewing/modifying meta-data, which
    # are accessed through the $file or $segments[$index] references.

DESCRIPTION

The purpose of this module is to read/modify/rewrite meta-data segments in JPEG (Joint Photographic Experts Group format) files, which can contain comments, thumbnails, Exif information (photographic parameters), IPTC information (editorial parameters) and similar data.

Each JPEG file is made of consecutive segments (tagged data blocks), and the actual row picture data. Most of these segments specify parameters for decoding the picture data into a bitmap; some of them, namely the COMment and APPlication segments, contain instead meta-data, i.e., information about how the photo was shot (usually added by a digital camera) and additional notes from the photograph. These additional pieces of information are especially valuable for picture databases, since the meta-data can be saved together with the picture without resorting to additional database structures. See the appendix about the structure of JPEG files for technical details.

This module works by breaking a JPEG file into individual segments. Each file is associated to an Image::MetaData::JPEG structure object, which contains one Image::MetaData::JPEG::Segment object for each segment. Segments with a known format are then parsed, and their content can be accessed in a structured way for display. Some of them can even be modified and then rewritten to disk.

$JPEG::show_warnings

This package variable must be used to inhibit the printing of warnings: if it is false, warnings are silently ignored. Otherwise, warning messages come with a detailed back-trace and description of the warning location.

    $Image::MetaData::JPEG::show_warnings = undef;

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.

    $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)] 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. The following method calls are a proposal open to discussion with the end user (if he/she can find a cleaner interface with an acceptable cost for the developer...). 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. Let us discuss the guidelines for the Exif setter method(s).

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
    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:

ROOT_DATA

The only modifiable item is the 'Endianness' (and it can only be set to big-endian, 'MM', or little-endian, 'II'); everything else is rejected (see the APP1 structure for further details). This only influences how the image is written back to disk (the in-memory representation is always native).

IMAGE_DATA

By specifying this target one can address the IFD0_DATA and SUBIFD_DATA targets at once. First, all records are tried in the IFD0, then, rejected records are tried into SubIFD (then, they are definitively rejected).

IFD0_DATA

See the canonical, additional and company-assigned tags' sections in the appendixes (this target refers to the primary image). The 'XResolution', 'YResolution', 'ResolutionUnit', and 'YCbCrPositioning' records are forced if not present (to [1,72], [1,72], 2 and 1 respectively). Note that the situation would be more complicated if we were dealing with uncompressed (TIFF) primary images.

SUBIFD_DATA

See the private Exif section in the appendixes. The 'ExifVersion', 'ComponentsConfiguration', 'FlashpixVersion', 'ColorSpace', and 'Pixel[XY]Dimension' records are forced if not present (to '0220', '1230', '0100', 1 and 0x0 respectively). Image dimensions can be retrieved from the SOF segment with the JPEG structure object's method get_dimensions() and set explicitly by the user if necessary (this cannot be done from within the APP1 segment, because it does not link back to its parent); however, the horizontal field in the SubIFD should not include data padding, while that in the SOF segment does, so the meaning is slightly different and these fields cannot be automatically calculated.

THUMB_DATA (or its alias IFD1_DATA)

See the canonical, additional and company-related tag lists' sections in the appendixes (this target refers to thumbnail properties). The 'XResolution', 'YResolution', 'ResolutionUnit', 'YCbCrSubSampling', 'PhotometricInterpretation' and 'PlanarConfiguration' records are forced if not present (to [1,72], [1,72], 2, [2,1], 2 and 1 respectively). Note that some of these records are not necessary for all types of thumbnails, but JPEG readers will probably skip unnecessary information without problems.

GPS_DATA

See the GPS tags section in the appendixes. The 'GPSVersionID' record is forced, if it is not present at the end of the process, because it is mandatory (ver 2.2 is chosen). There are some record inter-correlations which are still neglected here (for instance, the 'GPSAltitude' record can be inserted without providing the corresponding 'GPSAltitudeRef' record).

INTEROP_DATA
JPEG::forge_interoperability_IFD

[arguments: none] See the Interoperability directory section in the appendixes. The 'InteroperabilityIndex' and 'InteroperabilityVersion' records are forced, if they are not present at the end of the process, because they are mandatory ('R98' and ver 1.0 are chosen). Note that an Interoperability subIFD should be made as standard as possible: if you just want to add it to the file, it is better to use the forge_interoperability_IFD method, which takes care of all values ('RelatedImageFileFormat' is set to 'Exif JPEG Ver. 2.2', and the dimensions are taken from get_dimensions()).

MAKERNOTE_DATA

See the appendix on MakerNotes for a detailed discussion on how the content of a MakerNote is managed. If there is an error during the parsing of the MakerNote, only those tags which could be fully decoded before the error are returned. Note that MakerNote structures are often partially known, so many tags will likely be translated as 'Unknown_tag_...'. MakerNotes cannot be currently modified.

THUMBNAIL

$data must be a reference to a scalar containing the new thumbnail; if it points to an empty string, the thumbnail is erased (the undefined value DOES NOT erase the thumbnail, it generates instead an error). All thumbnail specific records (see the canonical tags section) are removed, and only those corresponding to the newly inserted thumbnail are calculated and written back. Currently, it is not possible to insert an uncompressed thumbnail (this will probably happen in the form of a TIFF image), only JPEG ones are accepted (automatic records contain the type, length and offset).

Photoshop and IPTC data ("APP13" segments)

The 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...; this includes editorial information modelled on IPTC/NAA recommendations (have a look at the appendices about valid Photoshop-style and IPTC tags for further details). 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 interface is intentionally similar to that for Exif data.

The structure of the IPTC data block is managed in detail and separately from the rest, although this block is a sort of "sub-case" of Photoshop information; all public methods have a $what argument, which can be only 'PHOTOSHOP' or 'IPTC' [default], selecting which part of the APP13 segment you are working with. If $what is invalid, an exception is always raised.

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; so, specifically, if $what is 'IPTC', there must be a non-void IPTC data block, while if $what is 'PHOTOSHOP' at least a non-IPTC data block must be present. 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 non-IPTC information from the $index-th APP13 Photoshop-style Segment, use the remove_app13_info method with $what set to 'PHOTOSHOP'. Conversely, if you want to remove IPTC information from the $index-th APP13 IPTC-enabled Segment, set $what to 'IPTC'. 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');

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, $what is 'IPTC' of course). It returns a reference to a hash containing a copy of the list of IPTC records, if present, undef otherwise: each element of the hash is a pair (key, arrayref), where arrayref points to an array with the real values (some IPTC records are repeatable so multiple values are possible). The record keys can be the native numeric keys ($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.

    my $hash_ref = $segment->get_app13_data('TEXTUAL', 'IPTC');

An example of a possible output from this call is the following:

    $hash_ref = { 'DateCreated'        => [ '19890207' ],
                  'ByLine'             => [ 'Interesting picture', 'really' ],
                  'Category'           => [ 'POL' ],
                  '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 $action, the 'RecordVersion' record (dataset 0) is still undefined, it is added (with version = 2), because it is mandatory according to the IPTC standard. The return value is a reference to a hash containing the rejected key-values entries. The entries of %$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 = {
        ObjectName => 'prova',
        ByLine     => 'ciao',
        Keywords   => [ 'donald', 'duck' ],
        SupplementalCategory => ['arte', 'scienza', 'diporto'] };
    $segment->set_app13_data($hashref, 'REPLACE', 'IPTC');
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'. 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'.

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');

CURRENT STATUS

A lot of other routines for modifying other meta-data could be added in the future. The following is a list of the current status of various meta-data Segments (only APP and COM Segments).

    Segment  Possible content           Status

    * COM    User comments              parse/read/write
    * APP0   JFIF data (+ thumbnail)    parse/read
    * APP1   Exif or XMP data           parse/read[Exif]/write[Exif]
    * APP1   Maker notes                parse/read
    * APP2   FPXR data or ICC profiles  parse
    * APP3   additional Exif-like data  parse
    * APP4   HPSC                       nothing
    * APP12  PreExif ASCII meta         parse[devel.]
    * APP13  IPTC and PhotoShop data    parse/read/write
    * APP14  Adobe tags                 parse

KNOWN BUGS

USE WITH CAUTION! THIS IS EXPERIMENTAL SOFTWARE!

This module is still experimental, and not yet finished. In particular, it is far from being well tested, and some interfaces could change depending on user feedback. The ability to modify maker notes is not yet implemented (moreover, have a look at the MakerNote appendix for a general note on the problem of MakerNote corruption). APP13 data spanning multiple Segments are not correctly read/written. Most of APP12 Segments do not fit the structure parsed by parse_app12(), probably there is some standard I don't know.

OTHER PACKAGES

Other packages are available in the free software arena, with a feature set showing a large overlap with that found in this package; a probably incomplete list follows. However, none of them is completely satisfactory with respect to the package's objectives, which are: being a single package dealing with all types of meta-information in read/write mode in a JPEG (and possibly TIFF) file; depending on the least possible number of non standard packages and/or external programs or libraries; being open-source and written in Perl. Of course, most of these objectives are far from being reached ....

"Image::ExifTool" by Phil Harvey

ExifTool is a Perl script that extracts meta information from various image file types; it can read Exif, IPTC, XPM and GeoTIFF formatted data as well as the maker notes of many digital cameras. This script is just a command-line interface to the Image::ExifTool module, a very complete library, highly customisable and capable of organising the results in various ways, but not designed to modify file data (it only reads).

"Image::IPTCInfo" by Josh Carter

This is a CPAN module for for extracting IPTC image meta-data. It allows reading IPTC data (there is an XML and also an HTML output feature) and manipulating them through native Perl structures. This library does not implement a full parsing of the JPEG file, so I did not consider it as a good base for the development of a full-featured module. Moreover, I don't like the separate treatment of keywords and supplemental categories.

"JPEG::JFIF" by Marcin Krzyzanowski, "Image::EXIF" by Sergey Prozhogin and "exiftags" by Eric M. Johnston

JPEG::JFIF is a very small CPAN module for reading meta-data in JFIF/JPEG format files. In practice, it only recognises a subset of the IPTC tags in APP13, and the parsing code is not suitable for being reused for a generic JPEG segment. Image::Exif is just a Perl wrapper around exiftags, which is a program parsing the APP1 section in JPEG files for Exif meta-data (it supports a variety of MakerNotes). exiftags can also rewrite comments and date and time tags.

"Image::Info" by Gisle Aas

This CPAN module extracts meta information from a variety of graphic formats (including JPEG and TIFF). So, it is not specifically about JPEG segments: reported information includes media type, extension, width, height, colour type, comments, Interlace, Compression, Gamma, and LastModificationTime. For JPEG files, it additionally reports from JFIF (APP0) and Exif (APP1) segments (including MakerNotes). This module does not allow for editing.

"exif" by Martin Krzywinski and "exifdump.py" by Thierry Bousch

These are two basic scripts to extract Exif information from JPEGs. The first script is written in Perl and targets Canon pictures. The second one is written in Python, and it only works on JPEG files beginning with an APP1 section after the SOI. So, they are much simpler than all other programs/libraries described here. Of course, they cannot modify Exif data.

"jhead" by Matthias Wandel

The jhead program (written in C) is used to display JPEG comments and Exif data, and to perform limited manipulation of Exif headers (such as changing the internal time-stamps, removing the thumbnail, or transferring headers back into edited images) and comments. Exif header data modification is very limited, as jhead's internal implementation of the file system contained in the Exif header is read-only; there, for instance, no way to replace the thumbnail in the Exif header with another.

"exifprobe" by Duane H. Hesser

This is a C program which examines and reports the contents and structure of JPEG and TIFF image files. It recognises all standard JPEG markers and reports the contents of any properly structured TIFF IFD encountered, even when entry tags are not recognised. Camera MakerNotes are included. GPS and GeoTIFF tags are recognised and entries printed in "raw" form, but are not expanded. The output is nicely formatted, with indentation and colouration; this program is a great tool for inspecting a JPEG/TIFF structure while debugging.

"libexif" by Lutz Müller

This is a library, written in C, for parsing, editing, and saving Exif data. All Exif tags described in Exif standard 2.1 are supported. Libexif can only handle some maker notes, and even those not very well. It is used by a number of front-ends, including: Exif (read-only command-line utility), gexif (a GTK+ front-end for editing Exif data), gphoto2 (command-line front-end to libgphoto2, a library to access digital cameras), gtkam (a GTK+ front-end to libgphoto2), thirdeye (a digital photos organiser and driver for eComStation).

"jpegrdf" by Norman Walsh

This is a Java application for manipulating (read/write) RDF meta-data in the comment sections of JPEG images (is this the same thing which can be found in APP1 segments in XMP format?). It can also access and convert into RDF the Exif tags and a few other general properties. However, I don't want to rely on a Java environment being installed in order to be able to access these properties.

"OpenExif" by Eastman Kodak Company

This is an object-oriented interface written in C++ to Exif formatted JPEG image files. It is very complete and sponsored by a large company, so it is to be considered a sort of reference. The toolkit allows creating, reading, and modifying the meta-data in the Exif file. It also provides means of getting and setting the main image and the thumbnail image. OpenExif is also extensible, and Application segments can be added.

AUTHOR

Stefano Bettelli, bettelli@cpan.org

COPYRIGHT AND LICENSE

Copyright (C) 2004,2005 by Stefano Bettelli

This library is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License. See the COPYING and LICENSE file for the license terms.

SEE ALSO

Have a look at the technical appendixes of the Image::MetaData::JPEG module [M in the following], packaged as separate documents: they contain a description of segment structures [M::Structures], and lists of valid tags [M::TagLists], including a tentative description of some MakerNote formats [M::MakerNotes]. See also your current perl(1) documentation, an explanation for the General Public License and the manual pages of the following optional Perl modules: Image::ExifTool(3pm), Image::IPTCInfo(3pm), JPEG::JFIF(3pm), Image::EXIF(3pm) and Image::Info(3pm).

1 POD Error

The following errors were encountered while parsing the POD:

Around line 1401:

Non-ASCII character seen before =encoding in 'Müller'. Assuming ISO8859-1