/*! \mainpage Geo::GDAL

\section index_version Version

These pages document the development version of the GDAL Perl API,
which is extended from the released versions APIs. Old versions:
<a href="../1.4/index.html">1.4</a>
<a href="../1.5/index.html">1.5</a>
<a href="../1.6/index.html">1.6</a>
<a href="../1.7/index.html">1.7</a>
<a href="../1.8/index.html">1.8</a>
<a href="../1.9/index.html">1.9</a>
<a href="../1.10/index.html">1.10</a>
<a href="../1.11/index.html">1.11</a>
<a href="../2.0/index.html">2.0</a>
<a href="../2.1/index.html">2.1</a>

\section index_intro Introduction

The Geo::GDAL modules are the Perl bindings to the GDAL/OGR
library. The modules allow you to use Perl to access and manipulate
all geospatial data that the installed GDAL library is configured to
access and manipulate.

This documentation covers the Perl bindings. For more in-depth
documentation see the main documentation of <a
href="http://www.gdal.org/">GDAL</a>. This documentation also
emphasizes the recommended Perl API. Some methods and aliases to
method names are left out since they may skip some built-in usability
or other additions.

\subsection usage Usage

\code
use Geo::GDAL;
\endcode

Makes all classes available in your Perl program. Some subroutines
(Driver, Open, BuildVRT) can be imported into the main namespace with
':all'.

\code
use Geo::GDAL qw/:all/;
\endcode

\subsection arguments Arguments

Some arguments are optional and have a default value. This is
illustrated like this:

SomeMethod(arg1, arg2 = 4);

arg1 is a required argument.

arg2 may be left out and if left out, will get the value 4 (in this
case).

Only the last argument or arguments can be optional.

In some cases a method can be called in a traditional way and with
named arguments (i.e. with a hash):

\code
$object->method(1, 2, 3);
$object->method(number=>1, param=>2, other=>3);
$object->method({number=>1, param=>2, other=>3});
\endcode

Note especially the difference between the second and the third
versions. In some cases the named arguments must be given in an
anonymous hash.

In some cases a method may behave differently depending on the
parameters that it gets:

\code
$object->method($hashref); # method called with a reference to a hash
$object->method($arrayref); # method called with a reference to an array
\endcode

In some cases a method may examine the context in which it is called,
and behave differently:

\code
$object->method(); # method called in void context
$return = $object->method(); # method called in scalar context
@return = $object->method(); # method called in list context
\endcode

Many of the methods may throw an error, which can be caught by putting
the call into eval{}; and then examining the contents of $\@.

\section class_methods_vs_object_methods Modules, package subroutines, class methods, and object methods

Geo::GDAL is the module, which you `use` in your Perl program. A
module is a file with `.pm` extension. When you `use` the Geo::GDAL
module, it `use`s other modules. Geo::GDAL modules define several
packages, also known as namespaces, which are usually but not always
classes.  Subroutines within packages may either be simply
subroutines, class methods, or object methods.

Objects of a class are often created using the class method `new`

\code
$object = SomeClass->new();
\endcode

Object methods in packages are always invoked for objects

\code
$object->method();
\endcode

while class methods are invoked for classes

\code
Geo::GDAL::Class->method();
\endcode

and package subroutines are invoked as subroutines in namespaces

\code
Geo::GDAL::Class::subroutine();
\endcode

The distinction between class methods and package subroutines is
subtle but often important. The method invocation passes the class
name as the first argument while the subroutine invocation does
not. Especially constructor (new) must be called as a class method.

Similar to methods, also attributes are either class attributes or
object attributes. Class attributes are package variables and object
attributes are variables owned by each individual object. Class
attributes are used for example for enumerated values. In some cases
object attributes can be accessed as hash values

\code
$value = $object->{attribute};
$object->{attribute} = $new_value;
\endcode

but this is mostly deprecated and class and object attributes should
be accessed through methods.

Class attributes are also used for maintaining, e.g., is_a_part_of
relationships between objects. Because Band objects are a part of
Dataset objects, there is a class attribute (a hash) in Dataset, which
maintains these relationships and makes sure the Dataset object that
owns a Band object is not destroyed before the Band object. Also,
definition objects that are linked to feature and layer objects are
read-only, and this constraint is enforced using class attributes.

\section index_exceptions Exceptions

Geo::GDAL uses the Perl exception mechanism. It means that exceptions
which GDAL classifies as failures trigger a Perl exception, and an
exception that is classified as a warning triggers a Perl warning.
Fatal errors trigger a message to standard error and an abort.

All errors are confessed (except those generated by Swig, which are
croaked) in the Perl bindings, i.e., the Perl stack is included in the
Perl error message $\@. However, several error messages may be
generated by GDAL as a result of one call and only the last error
message is included in $\@. The whole stack of GDAL error messages is
stored in a package variable \@Geo::GDAL::error. Thus, $\@ and
\@Geo::GDAL::error give a different insight into the error.  It is
usually not a good idea to look directly into \@Geo::GDAL::error,
instead call Geo::GDAL::errstr, which returns the error stack chomped
and joined into one string.

Note that it is a good idea to call Geo::GDAL::errstr(); before eval
{}; to clear the error stack.

Perl exceptions can be caught by eval() and Perl warnings can be
caught by signal \_\_WARN\_\_. Examples:

\code
eval {
    my $point = Geo::OGR::Geometry->new(WKT=>"POINTXX(1 1)");
};
say STDERR Geo::GDAL->errstr if $@;
\endcode

Produces:
\code
OGR Error: Unsupported geometry type
\endcode

while

\code
$point = Geo::OGR::Geometry->new(WKT=>"POINTXX(1 1)");
\endcode

Produces:
\code
OGR Error: Unsupported geometry type at <long path>/OGR.pm line 652.
	Geo::OGR::Geometry::new("Geo::OGR::Geometry", "WKT", "POINTXX(1 1)") called at foo.pl line 18
\endcode

and

\code
BEGIN {
    $SIG{__WARN__} = sub {  print STDERR "Warning: @_"; }
}
Geo::GDAL::GetDriver('GTiff')->Create( Name => 'my.tiff',
                                       Width => 100,
                                       Height => 100,
                                       Type => 'Byte',
                                       Options => { my_unknown_option => 'b' } );
\endcode

Produces:
\code
Warning: Driver GTiff does not support my_unknown_option creation option at site/lib/Geo/GDAL.pm line 771.
\endcode

\subsection cpl_debug Environment variable CPL_DEBUG

Setting $ENV{CPL_DEBUG} to 'ON' makes GDAL verbose about many
non-fatal things. It may be useful in trying to figure out what is
going on internally. GDAL uses its normal error mechanism to report
the debug messages. This means that the messages will end up in
\@Geo::GDAL::error and can be accessed with Geo::GDAL::errstr. That
can be changed by calling Geo::GDAL::DontUseExceptions, after which
all error and debug messages are sent to standard error (STDERR). For
example

\code
$ENV{CPL_DEBUG} = 'ON';
Geo::GDAL::DontUseExceptions;
my $mp = Geo::OGR::Geometry->new(GeometryType => 'MultiPoint', Points => [[1,1]]);
$mp->AddGeometry(Geo::OGR::Geometry->new(GeometryType => 'Point'));
say $mp->As(Format => 'ISO WKT');
\endcode

Prints

OGR: OGRMultiPoint::exportToWkt() - skipping POINT EMPTY.

to STDERR, and

MULTIPOINT ((1 1))

to STDOUT.

\section index_named_params Named parameters

Quite many methods and subroutines in Geo::GDAL accept named
parameters. In all cases the named parameters are handled with the
same filter, which accepts either a list of parameters (the order is
the order in which the named parameters are described in this
documentation), single hash reference, or a list of key value pairs.
The keys are given in the documentation but the filter ignores case
and underscores. Thus, although the name of the parameter is
ProgressData, the method will happily recognize also progress_data.

\section index_progress Progress callback

Some methods accept a callback function for reporting the
progress. The progress subroutine is called with three arguments: a
number, a string, and user defined data. The user defined data
is an argument to the method.

\code
sub progress {
    my($progress, $message, $data) = @_;
    my $terminate = 0;
    ...
    return $terminate ? 0 : 1;
}
\endcode

\section index_stdout_redirection Redirecting stdout to a writer object

The Perl bindings takes advantage of the virtual file system of GDAL
and offers a way to redirect the output from a format driver to a Perl
object. The object needs to implement \c write and \c close
methods. \c write is called for each output from the driver with the
output byte string as an argument. \c close is called when the dataset
being used in the creation is destroyed. Note that the destruction of
the dataset may depend on destruction of all objects that depend on
it. For a simple example see page \ref streaming.

Due to the limitation of the redirection mechanism only one redirection
may be active at any time.

Note that the return value of the \c write method does not have any
effect. Internally the return value is set to the length of the byte
string.

\section index_refcount Reference counting

In GDAL many objects may depend on other objects to exist, for example
a geometry, which is an attribute of a feature depends on the feature
to exist when the geometry is accessed. If a feature is destroyed,
then using a geometry object, which points for its data into the
feature, will lead to memory error.

GDAL maintains its own reference counting but this is largely lost in
the bindings.

The Perl bindings attempt to maintain these dependencies by keeping
the parent objects alive under the hood, independent of their existence
in the user space. Thus it is perfectly legal to do something like

\code
my $layer = Geo::OGR::Driver('Memory')->
    Create()->
        CreateLayer(Name => 'layer',
                    Fields => [{Name => 'test', Type => 'Integer'},
                               {Name => 'geom', Type => 'Point'}]);
\endcode

or

\code
my $geometry;
{
    my $feature = $layer->InsertFeature({test => 13, geom => {WKT => 'POINT (10 20)'}});
    $geometry = $feature->Geometry('geom');
}
say $geometry->AsText;
\endcode

There is at least one caveat. Swig sets the bindings up in such a way
that the objects are tied hashes. For example a feature can used as a
hash.

\code
my $feature = $lauer->InsertFeature({test => 13, geom => {WKT => 'POINT (10 20)'}});
my $test = $feature->{test};
\endcode

Due to the mechanism of tied hashes in Perl it is not possible to
maintain the parent referencing when using this syntax. Thus do not
use this for subobjects. (In fact it will create an error.)

\code
my $feature = $lauer->InsertFeature({test => 13, geom => {WKT => 'POINT (10 20)'}});
my $geom = $feature->{geom}; # will lead to memory error!
\endcode

\section index_footer Note

This documentation is generated from files within the GDAL
distribution (directory swig/perl) with Doxygen using a Perl module <a
href="http://search.cpan.org/~jordan/Doxygen-Filter-Perl/">Doxygen::Filter::Perl</a>.
A tailor made preprocessor in the GDAL distribution is used to process
and put all Perl code and documentation into a single file (all.pm)
for Doxygen-Filter-Perl and Doxygen.

Many methods are just interfaces to non-Perl code in the bindings or
GDAL and thus their code show as blank on these pages. The bindings
are created with Swig, which adds some methods by default.

Code examples in method pages contain dots ('.') to enforce
indentation. This is due to a doxygen bug.

*/
