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File: Image/ExifTool/README
Description: ExifTool support modules documentation
The ExifTool support modules are loaded by ExifTool to interpret various formats
of meta information.
The tables in these files are used as lookups based on the tag ID values. The
hash keys are the tag IDs (in decimal or hexadecimal if the ID is numerical as
with EXIF tables, or the tag name if the ID is ASCII as with XMP tables). In
the case of a BinaryData table, the IDs are numerical and specify offsets into
the binary data block. The corresponding hash value provides information about
the tag (explained later).
Twenty-one special keys (PROCESS_PROC, WRITE_PROC, CHECK_PROC, GROUPS, FORMAT,
FIRST_ENTRY, TAG_PREFIX, PRINT_CONV, DID_TAG_ID, WRITABLE, NOTES, IS_OFFSET,
EXTRACT_UNKNOWN, NAMESPACE, PREFERRED, PARENT, PRIORITY, WRITE_GROUP, LANG_INFO,
VARS and DATAMEMBER) are used to provide additional information about a table.
The special keys have names that are all capitalized to avoid possible conflicts
with tag keys. Below is an explanation of the meaning of each special key:
PROCESS_PROC : Reference to a function used to process the directory for this
table. If PROCESS_PROC is not given, \&Image::ExifTool::Exif::ProcessExif is
assumed. If PROCESS_PROC is set to 0, the tags are not added to the lookup.
The process proc returns 1 on success or 0 on failure, and takes 3 arguments:
0) reference to the ExifTool object, 1) reference to a directory information
hash (with the following entries:), 2) reference to the tag table hash.
Name - Tag name for this SubDirectory entry (for verbose messages)
Base - Base offset for pointers from start of file
DataPt - Reference to data block containing directory (may be undef)
DataPos - Position of data block within file (relative to Base)
DataLen - Length of data block in bytes
DirStart - Offset to start of directory from start of data block
DirLen - Length of directory data within block
DirName - Name of this directory
OutFile - Output file or scalar reference
Parent - Name of parent directory
RAF - Reference to File::RandomAccess object if available
NewDataPos - File position of new data (write proc only)
Fixup - Reference to hash of offset fixups (used in EXIF writing only)
FixBase - Flag set to attempt to fix base offsets
LastIFD - Used by WriteExif() to return offset of last IFD written
ImageData - Used by WriteExif() to avoid buffering large image data blocks
WRITE_PROC : Reference to a function to write all new tags for this directory.
The write proc returns the new directory data or undefined on error. It takes
the same arguments as the process proc above except that the second argument
(reference to directory information hash) is optional, and if specified gives
information about the source directory for tags to be copied to the output.
CHECK_PROC : Reference to a function which validates Raw values for writing.
The function takes three arguments: 0) ExifTool object reference, 1) tagInfo
reference, 2) value reference, and returns undefined (and possibly modifies
the input value) if successful, or an error message if there was a format
problem.
GROUPS : A hash lookup for the default group names for all entries in this
table. If not specified, the Group 0 and 1 names will be set automatically
according to the name of the module.
FORMAT : Specifies the default tag Format and corresponding pointer increment
for entries in a BinaryData table, and defaults to 'int8u' if not specified.
The possible values of FORMAT are:
int8s - Signed 8-bit integer (EXIF 'SBYTE')
int8u - Unsigned 8-bit integer (EXIF 'BYTE')
int16s - Signed 16-bit integer (EXIF 'SSHORT')
int16u - Unsigned 16-bit integer (EXIF 'SHORT')
int32s - Signed 32-bit integer (EXIF 'SLONG')
int32u - Unsigned 32-bit integer (EXIF 'LONG')
int64s - Signed 64-bit integer
int64u - Unsigned 64-bit integer
rational32s - Rational consisting of 2 int16s values
rational32u - Rational consisting of 2 int16u values
rational64s - Rational consisting of 2 int32s values (EXIF 'SRATIONAL')
rational64u - Rational consisting of 2 int32u values (EXIF 'RATIONAL')
fixed16s - Signed 16-bit fixed point value
fixed16u - Unsigned 16-bit fixed point value
fixed32s - Signed 32-bit fixed point value
fixed32u - Unsigned 32-bit fixed point value
float - 32-bit IEEE floating point value (EXIF 'FLOAT')
double - 64-bit IEEE floating point value (EXIF 'DOUBLE')
extended - 80-bit extended floating float
ifd - Unsigned 32-bit integer sub-IFD pointer (EXIF 'IFD')
string - Series of 8-bit ASCII characters (EXIF 'ASCII')
undef - Undefined-format binary data (EXIF 'UNDEFINED')
binary - Binary data
FIRST_ENTRY : Specifies the index for the first tag entry in a binary table.
This value is only used if the Unknown option is set to 2 or higher, and
allows the binary data to be scanned for unknown tag entries.
TAG_PREFIX : Prefix for names of unknown tags.
PRINT_CONV : Default print conversion tags where PrintConv isn't specified.
DID_TAG_ID : Used by GetTagID() as a flag to indicate that TagID's have been
defined for all tags in this table. This may be set in the table definition
to prevent GetTagID() from assigning TagID's.
WRITABLE : Indicates that all tags in this table are writable. This is the
same as setting the Writable flag for each individual tag in the table, except
for SubDirectory tags which are not made Writable.
NOTES : Notes to introduce the table in the TagNames documentation.
IS_OFFSET : Reference to list of TagID's representing offsets for binary
data tables only. Not used for EXIF tables.
EXTRACT_UNKNOWN : Used in PDF tables to specify a directory where all unknown
tags should be extracted.
NAMESPACE : Namespace prefix for tags in the XMP table. May be a reference to
a list of the namespace prefix and URI if the URI needs specifying.
PREFERRED : Set to true if the tags in this table should always be added when
writing information. Overrides the order specified by SetNewGroups().
PARENT : Used internally to store the parent table name of a user-defined tag
table so the appropriate module can be loaded as required.
PRIORITY : Default Priority for all tags in this table.
WRITE_GROUP : Default WriteGroup for all tags in the table.
LANG_INFO : Code reference to a routine which returns a reference to a
language-specific tag information hash. The routine takes two arguments: a
reference to the non-specific tagInfo hash, and the language code. Used only
in tables which support tag name language extensions.
VARS : Used to store additional module-specific variables for the table.
DATAMEMBER : BinaryData tables only. Gives a reference to a list of tag ID's
which must be extracted as data members when writing.
The remaining entries in a tag table are the EXIF tags with their associated
information. The information may exist in one of three forms: 1) A simple
scalar which is the name of the tag, 2) A reference to a hash of information
describing this tag, or 3) a reference to a list of hashes which contain
Condition expressions allowing the appropriate hash to be selected to suit the
conditions. The following is a description of possible hash entries. All
entries are optional, except for the tag Name which is required if the tag ID is
numerical.
Name : The tag name. Tag names need not be unique. If they aren't
unique, duplicate tags overwrite values of previous tags
unless the Duplicates option is set or the new tag has lower
Priority. With Duplicates set, the name of the previous tag
is changed to the form "TagName (N)" where N starts at 1 and
increments for subsequent duplicate tags. A tag should start
with an uppercase letter, and contain only the charcters in
the set [A-Za-z0-9_-].
Description : A more readable description of tag name. If a tag doesn't
specify a Description, then the tag Name is used instead, with
spaces inserted between the words.
Notes : Notes for this tag in the html TagNames documentation.
Groups : Hash lookup for group names for this tag.
Format : Only valid for BinaryData, EXIF and IPTC tables. For a Binary
or EXIF table, this gives the format that is used to convert
the binary data, and is one of the FORMAT types specified
above. For BinaryData tables, the format may have a size in
trailing brackets which is a perl expression to be evaluated.
The expression may access any of the previous table entries
through a %val hash, or the data size via $size. For example,
'string[$val{3}]' defines a string with length given by the
table entry with tag index '3'. If not specified, the format
of an EXIF entry is taken from the EXIF information, and the
format of a BinaryData entry is taken from the FORMAT
specified for the table (or int8u if FORMAT is not specified).
Count : Used when writing EXIF information to specify the number
values to write, or the number of characters in a fixed-length
string. A value of -1 indicates that the count is variable
and should be determined by the number of values provided.
Flags : Flags to specify characteristics for this tag. May be a
simple scalar flag name, a reference to a list of flag names,
or a reference to a hash of flag/value pairs. If not a hash
reference, the flag value is set to 1. Flags are expanded for
faster access at run time into members of the tagInfo hash,
and may be written directly as members if desired. The
available flag names are:
'List' - indicates that duplicate entries of this tag are
allowed, and will be accumulated in a list. Note that for XMP
information, 3 different types of lists are supported and the
value of this flag specifies the type of list: 'Bag', 'Seq' or
'Alt'.
'Priority' - gives the priority of this tag while reading. If
set to zero, this tag will not override the value of previous
tags with the same name. If the priority is greater than
zero, this tag won't be overridden by subsequent tags unless
their priority is equal to or greater than this priority. A
special feature is that Priority 0 tags are automatically
incremented to Priority 1 if they exist in the IFD of the full
resolution image (as determined by SubFileType).
'Protected' - bit mask to protect tags from writing:
Bit 0x01 indicates an 'unsafe' tag, which is not set via
SetNewValuesFromFile() unless specified explicitly.
Bit 0x02 indicates a 'protected' tag, which should not be set
directly by the user.
'PrintHex' - specifies that unknown PrintConv values should
be printed in hex (ie. "Unknown (0x0)")
'SubIFD' - used in writing to determine that the tag specifies
an offset to a sub-IFD. When this flag is set, the Group1
name gives the name of the IFD.
'IsOffset' - flag set if the tag represents an offset to some
data, and causes value will be adjusted to an absolute file
offset. If set to '2', the offset base of the parent
directory is used even if the base changes for the current
directory (only some maker notes are this messed up).
'OffsetPair' - set if the tag represents half of an offset/
byte count pair. Data for these tags must be handled
separately. Value is the tagID for the paired tag.
'DataTag' - associated tag name containing data for offset or
byte count tags.
'MakerNotes' - set if this tag is maker note data.
'DataMember' - name of exiftool data member associated with
this tag if it should be stored as a special data member when
writing information. Necessary only if the value of the tag
affects other written information.
'Permanent' - flag indicates that a tag is permanent, and
can't be added or deleted from the file. By default, all
MakerNotes tags are permanent unless otherwise specified.
'Avoid' - avoid creating this tag if possible.
'Unknown' - this is an unknown tag (only extracted when the
Unknown option is set.
'SeparateTable' - set to list PrintConv values in a separate
table in the HTML documentation. Value is 1 or the name of
the module where the table should be located.
'Drop' - set to 1 for tags that should be excluded when
rebuilding maker notes when copying all tags.
'EntryBased' - set to 1 if the offset for this value is based
on the IFD entry position. This allows individual values to
be entry-based even though some others aren't (as with the
Casio PrintIM information).
RawConv : Used to convert the Raw value at extraction time (while the
image file is still open, unlike ValueConv and PrintConv below
which are done later only if the value is requested). May be
a scalar expression using $val (the Raw tag value) and $self
(the current ExifTool object), or a code reference with $val
and $self as arguments. For Composite tags, $val is a
reference to a hash of component tag names, and @val may be
used to access the Raw values of these tags. The returned
value may be a scalar with is used as the new Raw value, a
scalar reference to a binary data value, or undefined to
indicate that the tag should be ignored. Note: RawConv should
only be used if necessary. It is preferable to use ValueConv
instead of RawConv because ValueConv is only executed if the
tag value is requested, while RawConv is executed for all
extracted tags.
ValueConv : Used to convert the Raw value to a useable form. May be a hash
reference to act as a lookup table, a scalar which is
evaluated as a Perl expression, or a code reference to a
subroutine. If a hash reference is used and the Raw value doesn't
appear as one of the keys, then the converted value is set to
"Unknown (X)", where X is the Raw value (unless a special
'BITMASK' key exists, in which case the hash referenced by
'BITMASK' is used to decode individual value bits). In an
expression, $self is a reference to the current ExifTool
object, $val is the Raw value, and $tag is the tag name. The
subroutine takes 2 arguments: the Raw value and a reference to
the current ExifTool object. The expression or subroutine is
evaluated when and if the tag value is requested (ie. only
after all extraction is complete), so if necessary at this
time the value of all other tags are available via calls to
$self->GetValue("Tag","Raw"). (Note: In theory, types other
than "Raw" may be accessed, but they are slower and may lead
to cyclical dependencies so they should be avoided). When
evaluated, the expression or subroutine returns a scalar for
the converted value, or a scalar reference to a binary data
value. The return value should always be defined -- use
RawConv instead to return undef if it is necessary to test the
value for validity, otherwise an undef tag may hide a
previously defined value when the Duplicates option is not
enabled. Composite tags which Require or Desire other tags
may access the ValueConv, PrintConv and Raw values of these
tags through the elements of the @val, @prt and @raw lists
respectively. If ValueConv is not specified, the Raw value is
not converted.
PrintConv : This entry is similar to ValueConv above, except that it is
used to further convert the tag value to a human readable
form. It can be either a hash lookup, a scalar Perl
expression or a code reference. In this expression, $self,
$val and $tag may be used as with ValueConv, but the returned
value should always be defined. Note that the print
conversion is only done if the PrintConv option is enabled
(which it is by default), and if the result of the ValueConv
is not a scalar reference.
ValueConvInv : The inverse of ValueConv. Only necessary if ValueConv is
specified and this tag is Writable. Note that DataMember tags
may NOT be used in the inverse conversions because these
conversions are done before the input file is parsed.
Instead, a Condition must be used.
PrintConvInv : The inverse of PrintConv. Only necessary if PrintConv is
specified and this tag is Writable.
Condition : If given, specifies scalar which is evaluated as a Perl
expression at extraction time to decide whether the tag is
valid. If used in a list of alternate tag definitions, the
first list entry with a true condition is taken. If no
condition exists, then a 'true' condition is assumed. The
expression may use $self to access the ExifTool object, and
$oldVal to access the previous Raw value of the tag (if it
exists). The first 48 bytes of the raw data value are
accessible through the reference $valPt for EXIF and Jpeg2000
tags only. Note that if the value is writable and $valPt is
used, the tag must have a Format (unless 'undef' or 'string'),
and a Count (unless 1 or length of the 'undef' or 'string'),
so the raw data may be generated for evaluating the Condition.
Require : [Composite tags only] Specifies a list of tag names for values
which are required in order to calculate the composite value.
The values are stored in a @val list in the positions
specified by the Require keys, and can be used in a ValueConv
or PrintConv expression. The composite value is only
calculated if the values for all Require'd tags are defined.
Require and Desire tag names may be prefixed by an optional
group family 0 or 1 name followed by a colon. Case is
significant.
Desire : [Composite tags only] This is the same as Require except that
the value is still calculated even if the specified tags don't
exist. Beware that the elements of @val, @prt and @raw may be
undefined for Desire'd tags. Both Require and Desire may
exist together, however be sure to specify unique keys for
each tag since they are used as indices into the @val, @prt
and @raw lists.
Shift : [Writable tags only] Specifies type of shift to apply if this
value may be shifted. Set to 'Time' for shifting date/time
tags.
Writable : Indicates this tag can be written (or not written if value is
set to zero), and for EXIF-type tables gives format for
writing. Writable may be set to 1 for MakerNotes information
because the existing format is always used, however providing
a format is desireable because it is used in validating the
value. For EXIF tables, the Writable flag may be different
than the Format flag, in which case Format is used for
converting the binary value and Writable specifies the format
code written to the EXIF IFD.
WriteAlso : Used for writable tag to specify other tags to write when this
tag is written. The value is a hash reference. The hash is
keys are the names of the tags that must be written, and the
values are evaluated to return the ValueConv value of the
specified tag (or returns undef to delete the tag). In the
eval, $val may be used to represent the value of the parent
tag.
WriteCheck : If given, specifies a scalar which is evaluated as a Perl
expression for a one-time validatation the value being
written. The expression has access to 3 variables: $val is
the value to be written, $self is the ExifTool object, and
$tagInfo is the tag information hash reference. It returns an
error string or undef if the value is good.
WriteCondition: [Writable EXIF tags only] Specifies a condition to be
evaluated before the tag can be written to a specific file.
The condition may use $self to reference the ExifTool object,
and returns true if it is OK for writing. Unlike WriteCheck
which is done only once when the new value is set, this
condition is evaluated before the information is written in
each file.
WriteGroup : [Required for writable EXIF tags] Specifies the IFD where the
information gets written by default.
OffsetPair : Used in EXIF table to specify the tagID for the corresponding
offset or length tag.
DataTag : Used in EXIF table to specify the tag name of the data
associated with offset/length tags.
Struct : [XMP tags only] Used by XMP writer code to specify name of XMP
structure so the PropertyPath can be determined.
TagID : [reserved] This entry is used internally by GetTagID() to
cache tag ID numbers for speed.
Table : [reserved] Reference to parent tag table.
PropertyPath : [reserved] Used internally by XMP writer to save property path
name.
SubDirectory { If it exists, this specifies the start of a new subdirectory.
It contains a collection of variables which specify the type
and location of the subdirectory. These variables are
described below:
TagTable : Specifies the name of the tag table lookup for the new
subdirectory. If not specified, the parent tag table is used.
Start : The offset to the start of the subdirectory relative to the
current Base. This is a Perl expression which may use
$valuePtr to represent the location of the tag value in the
file, or $val for the value itself. If not specified, a Start
of '$valuePtr' is assumed.
OffsetPt : If specified, this is a Perl expression that gives the
position of a 32-bit word in the current directory that is
added to the Start position to get the position of the new
subdirectory. The expression should use the position of the
current tag ($valuePtr).
Base : This specifies the base offset for all pointers in the
subdirectory. This need not be specified if the offset is the
same as the current directory, which is normally the case.
May use $start to represent the subdirectory start location
relative to the current base.
EntryBased : Flag indicating that the offsets are based on the indidual
directory entry position, so offsets are incremented by 12
times the corresponding entry index.
MaxSubdirs : Maximum number of subdirectories specified by the current tag
(if the tag specifies multiple values). If not specified, the
tag value ($val) is used as-is. If MaxSubdirs is specified,
then one subdirectory is parsed for each value found up to the
maximum number specified.
ByteOrder : Specifies byte ordering if different than than the rest of the
file. Must be either BigEndian, LittleEndian or Unknown. If
Unknown is specified, the byte order will be determined from
the directory count (however, this can not be done if OffsetPt
is specified).
Validate : If given, specifies Perl expression which is used to validate
the subdirectory data. The following variables may be used in
the expression: $val (value of the tag), $dirData (reference
to directory data), $subdirStart (offset to subdirectory
start) and $size (size of subdirectory).
ProcessProc: If given, specifies processing procedure used to decode this
subdirectory data. This overrides the default procedure
specified by PROCESS_PROC in the tag table.
DirName : Name of this subdirectory. If not specified, the name is
taken from the tag name. DirName is important because it is
used when writing to compare against names in the directory
map to determine which directories need to be edited.
FixBase : Flag set if base offsets should be fixed. Used to add a
constant to maker notes offsets to fix damage done by some
image editing utilities.
}
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