=pod









=head1 NAME

Image::PNG::Libpng - Perl interface to the C library "libpng".



=head1 SYNOPSIS

     use Image::PNG::Libpng;
     my $png = Image::PNG::Libpng::create_read_struct ();
     open my $file, '<:raw', 'nice.png';
     $png->init_io ($file);
     $png->read_png ();
     close $file;

Image::PNG::Libpng enables Perl to use the "libpng" library for
reading and writing files in the PNG (Portable Network Graphics)
format.

Image::PNG::Libpng consists of Perl subroutines which mirror
the C functions in libpng, plus helper subroutines to make it easier
to read and write PNG data in Perl. 

For an easy way to access PNG files, try the companion module L<Image::PNG>, which offers a simplified interface to the functions
in this module.

For those who are familiar with libpng, to know what the differences
between this module and libpng are, please go to the section
L</Differences from libpng>.

=head1 FUNCTIONS

=head2 create_read_struct

     my $png = create_read_struct ();

Create a structure for reading a PNG.

=head3 Correspondence to libpng

This function corresponds to C<png_create_read_struct> plus C<create_info_struct> (see L</No info structure>) with the error and warning handler variables set up to use
Perl's error reporting.

=head2 create_write_struct

     my $png = create_write_struct ();

Create a structure for writing a PNG.

=head3 Correspondence to libpng

This function corresponds to C<png_create_write_struct> plus C<create_info_struct> (see L</No info structure>)  with the error and warning handler variables set up to use
Perl's error reporting.

=head1 Input and output

=head2 init_io

     open my $file, "<", 'nice.png';
     $png->init_io ($file);

Set the file which C<$png> reads or writes to C<$file>. C<$file> must
be an already-opened Perl file handle. If C<$png> was created with
L<create_write_struct>, C<$file> must be opened for writing. If
C<$png> was created with L<create_read_struct>, C<$file> must be open
for reading.

Since PNG files are binary files, it is safest to specify the "raw"
pragma or use "binmode" with the file to override any default text
file encoding which Perl might be using:

     open my $file, ">:raw", 'output.png';

or

     open my $file, ">", 'output.png';
     binmode $file;

=head3 Correspondence to libpng

This function corresponds to C<png_init_io>, with a Perl
file handle substituting for the C C<FILE *>.

=head2 read_from_scalar

     my $png = create_read_struct ();
     $png->read_from_scalar ($string);

This subroutine sets the image data of C<$png> to be the contents of a
Perl scalar variable C<$string>. The first argument, C<$png>, is a PNG structure created with 
L</create_read_struct>. It reads in all the
data from the structure on being called.

This is useful when image data is stored in a Perl scalar. For example

     use LWP::Simple;
     my $image_data = get 'http://www.example.com/example.png';
     # Now $image_data contains the PNG file
     my $png = create_read_struct ();
     $png->read_from_scalar ($image_data);
     # Now $png contains the PNG information from the image.

=head3 Correspondence to libpng

This uses C<png_set_read_fn> to set the reading function of C<$png> to
a routine which reads data from Perl scalars. It then uses
C<png_read_png> to read all the data.

The C function which does this is called C<perl_png_scalar_read>, 
L<in the file C<perl-libpng.c> in the top directory of the distribution|http://cpansearch.perl.org/src/BKB/Image-PNG-0.18/perl-libpng.c>.

See also L</Input/output manipulation functions>.

=head2 write_to_scalar

    my $image_data = $png->write_to_scalar ();

This writes the PNG image data in C<$png> into a Perl scalar. So, for
example,

    my $img = $png->write_to_scalar ();
    open my $output, ">:raw", "test-write-scalar.png";
    print $output $img;
    close $output;

creates a PNG file called C<test-write-scalar.png> from the PNG data
in C<$png>.

The first argument, C<$png>, is a writeable PNG structure created with 
L</create_write_struct>. The return value of the subroutine is the Perl scalar
containing the image data.

=head3 Correspondence to libpng

This subroutine doesn't correspond directly to anything in libpng. It
uses C<png_set_write_fn> to set the writing function of C<$png> to be
its own function, which writes data to the Perl scalar.

The C function which does this is called C<perl_png_scalar_write>, 
L<in the file C<perl-libpng.c> in the top directory of the distribution|http://cpansearch.perl.org/src/BKB/Image-PNG-0.18/perl-libpng.c>.

See also L</Input/output manipulation functions>.

=head2 read_png

     $png->read_png ();

Read the entire PNG file into memory. You can provide an argument
containing transformations to apply to the image:

     use Image::PNG::Const qw/PNG_TRANSFORM_STRIP_ALPHA/;
     $png->read_png (PNG_TRANSFORM_STRIP_ALPHA);

In the absence of any third argument, the default value of
C<PNG_TRANSFORM_IDENTITY> is applied. The possible transformations
which can be applied are

=over

=item PNG_TRANSFORM_IDENTITY

No transformation

=item PNG_TRANSFORM_STRIP_16

Strip 16-bit samples to 8 bits

=item PNG_TRANSFORM_STRIP_ALPHA

Discard the alpha channel

=item PNG_TRANSFORM_PACKING

Expand 1, 2 and 4-bit samples to bytes

=item PNG_TRANSFORM_PACKSWAP

Change order of packed pixels to LSB first

=item PNG_TRANSFORM_EXPAND

Expand paletted images to RGB, grayscale to 8-bit images and tRNS chunks to alpha channels

=item PNG_TRANSFORM_INVERT_MONO

Invert monochrome images

=item PNG_TRANSFORM_SHIFT

Normalize pixels to the sBIT depth

=item PNG_TRANSFORM_BGR

Flip RGB to BGR, RGBA to BGRA

=item PNG_TRANSFORM_SWAP_ALPHA

Flip RGBA to ARGB or GA to AG

=item PNG_TRANSFORM_INVERT_ALPHA

Change alpha from opacity to transparency

=item PNG_TRANSFORM_SWAP_ENDIAN

Byte-swap 16-bit samples

=back

=head3 Correspondence to libpng

This function corresponds to C<png_read_png> with a default value for the third
argument. The fourth, unused, argument to C<png_read_png> does not
need to be supplied. See L<Unused arguments omitted>.

It does not take a second "info" argument. See L</No info structure>.

=head2 write_png

    $png->write_png ();

This writes the PNG to the file stream which was associated with it
using L</init_io>. For example,

    open my $output, ">:raw", 'out.png';
    $png->init_io ($output);
    $png->write_png ();
    close $output;

An optional argument consists of transformations to apply to the PNG
image before writing it:

    use Image::PNG::Const qw/PNG_TRANSFORM_STRIP_ALPHA/;
    $png->write_png (PNG_TRANSFORM_STRIP_ALPHA);

The transformations which can be applied are as follows:


=over

=item PNG_TRANSFORM_IDENTITY

No transformation

=item PNG_TRANSFORM_STRIP_16

Strip 16-bit samples to 8 bits

=item PNG_TRANSFORM_STRIP_ALPHA

Discard the alpha channel

=item PNG_TRANSFORM_PACKING

Expand 1, 2 and 4-bit samples to bytes

=item PNG_TRANSFORM_PACKSWAP

Change order of packed pixels to LSB first

=item PNG_TRANSFORM_EXPAND

Expand paletted images to RGB, grayscale to 8-bit images and tRNS chunks to alpha channels

=item PNG_TRANSFORM_INVERT_MONO

Invert monochrome images

=item PNG_TRANSFORM_SHIFT

Normalize pixels to the sBIT depth

=item PNG_TRANSFORM_BGR

Flip RGB to BGR, RGBA to BGRA

=item PNG_TRANSFORM_SWAP_ALPHA

Flip RGBA to ARGB or GA to AG

=item PNG_TRANSFORM_INVERT_ALPHA

Change alpha from opacity to transparency

=item PNG_TRANSFORM_SWAP_ENDIAN

Byte-swap 16-bit samples

=back


(NOTE: this list might be wrong, it is just copied from the linux lib
pages & the linux lib pages have different transformations for the
read and write png functions.)

=head3 Correspondence to libpng

This function corresponds to C<png_write_png>.

=head1 The image header

See L<http://www.w3.org/TR/PNG/#11IHDR> for information on the PNG standards for
the image header.

=head2 sig_cmp

    if (sig_cmp ($should_be_png)) {
        print "Your data does not have a PNG signature.\n";
    }

This subroutine looks at C<$should_be_png> and checks whether its
first bytes correspond to a valid PNG signature. It returns a true
value if they do not. It can also take two further arguments
consisting of a byte offset and a number of bytes to check
respectively:

    sig_cmp ($should_be_png, 0, 8);

If these arguments are not supplied, the byte offset is assumed
to be zero, and the number of bytes to check is assumed to be eight.

=head3 Correspondence to libpng

This function corresponds to C<png_sig_cmp>, with default arguments of
0 and 8 if second and third arguments are not supplied.

=head2 get_valid

    my $valid = $png->get_valid ();
    if ($valid->{oFFs}) {
        print "The PNG has valid screen offsets.\n";
    }

This function returns a hash with a key for each possible chunk which
may or may not be valid. The chunks which you can test for are

=over

=item bKGD

=item cHRM

=item gAMA

=item hIST

=item iCCP

=item IDAT

=item IHDR

=item iTXt

=item oFFs

=item pCAL

=item pHYs

=item PLTE

=item sBIT

=item sCAL

=item sPLT

=item sRGB

=item tEXt

=item tIME

=item tRNS

=item zTXt

=back

The first argument, C<$png>, is a PNG structure created with 
L</create_read_struct>.

=head3 Correspondence to libpng

This function corresponds to C<png_get_valid>, with the difference being that the
return value is a hash containing a key for each possible chunk.

=head2 get_IHDR

    my $IHDR = $png->get_IHDR ();

Read the IHDR information from the PNG file. The return value is a
reference to a hash.

The hash reference contains the following fields:

=over


=item width

The width of the image in pixels.


=item height

The height of the image in pixels.


=item bit_depth

The bit depth of the image (the number of bits used for each colour in a pixel).
This can take the values 1, 2, 4, 8, 16.

=item color_type

The colour type.
This can take the values PNG_COLOR_TYPE_GRAY, PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB, PNG_COLOR_TYPE_RGB_ALPHA.

=item interlace_method

The method of interlacing.
This can take the values PNG_INTERLACE_NONE, PNG_INTERLACE_ADAM7.




=back

So, for example, to get the width and height of an image,

    my $ihdr = $png->get_IHDR ();
    printf "Your image is %d x %d\n", $ihdr->{width}, $ihdr->{height};

=head3 Correspondence to libpng

This function corresponds to C<png_get_IHDR>, with a single Perl hash reference
used instead of the several pointers to integers used in libpng.

=head2 set_IHDR

    my $ihdr = { width => 10, height => 10, bit_depth => 8,
                 color_type => PNG_COLOR_TYPE_RGB };
    $png->set_IHDR ($ihdr);

Set the IHDR chunk (the image header) of the PNG image. 

The first argument, C<$png>, is a writeable PNG structure created with 
L</create_write_struct>. The second argument is a hash with the following values:

=over


=item width

The width of the image in pixels.


=item height

The height of the image in pixels.


=item bit_depth

The bit depth of the image (the number of bits used for each colour in a pixel).
This can have the values 1, 2, 4, 8, 16.

=item color_type

The colour type.
This can have the values PNG_COLOR_TYPE_GRAY, PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB, PNG_COLOR_TYPE_RGB_ALPHA.

=item interlace_method

The method of interlacing.
This can have the values PNG_INTERLACE_NONE, PNG_INTERLACE_ADAM7.




=back

Other fields in the hash are ignored.

=head3 Correspondence to libpng

This function corresponds to C<png_set_IHDR>, with a single Perl hash reference
used instead of the seven integers. The variables
C<compression_method>, C<filter_method>, 
in C<png_set_IHDR> can only take one possible value, so the routine
ignores them. See L<Unused arguments omitted>.

=head2 get_color_type

    my $color_type;
    $png->get_color_type (\$color_type);

This returns an integer value. If you want to get a name for the
colour type, use L</color_type_name>.

=head3 Correspondence to libpng

This function corresponds to C<png_get_color_type>.

=head1 PNG timestamps

See L<http://www.w3.org/TR/PNG/#11timestampinfo> for information on the PNG standards for
time stamp information.

=head2 get_tIME

    my $time = $png->get_tIME ();
    if ($time && $time->{year} < 2005) {
        warn "Your PNG is now getting old. Don't forget to oil it to prevent rust.";
    }

This subroutine gets the modification time value of the PNG image and
puts it into fields labelled "year", "month", "day", "hour", "minute"
and "second" of the hash reference given as the third argument. If
there is no modification time it returns an undefined value. Note that
PNG images are not required to contain a modification time, and that
this modification time is not the same thing as the modification time
of the PNG file itself.

=head3 Correspondence to libpng

This function corresponds to C<png_get_tIME>, with a Perl hash reference
substituted for the C struct C<png_timep> used in libpng.

=head2 set_tIME

    # Set the time to "now"
    $png->set_tIME ();
    # Set the time
    $png->set_tIME ({year => 1999, month => 12});

Set the modification time of the PNG to the hash value given by the
argument. If the argument is omitted, the time is set to the current
time. If any of year, month, day, hour, minute or second is omitted,
these are set to zero.

=head3 Correspondence to libpng

This function corresponds to C<png_set_tIME>, with a Perl hash reference
substituted for the C struct C<png_timep> used in libpng.

=head1 Text chunks

See L<http://www.w3.org/TR/PNG/#11textinfo> for information on the PNG standards for
text information.

=head2 get_text

    my $text_chunks = $png->get_text ();

This subroutine gets all the text chunks in the PNG image and returns
them as an array reference. Each element of the array represents one
text chunk. The element representing one chunk is a hash reference
with the text fields such as "key", "lang_key", "compression" taken
from the PNG's information.

The text data is uncompressed by libpng. If it is international text,
Image::PNG::Libpng automatically puts it into Perl's internal
Unicode encoding (UTF-8). 

Note that PNG international text is required to be in the UTF-8
encoding, and non-international text is required to contain whitespace
and printable ASCII characters only. See L</The PNG specification> for more on
the requirements of a PNG text section.

=head3 Correspondence to libpng

This function corresponds to C<png_get_text>, with a Perl array of hash
references substituted for the C array of structs used in libpng.

=head2 set_text

    $png->set_text ($text_chunks);

This sets the text chunks in an array reference C<$text_chunks>. . If it is called more than once, the chunks are not
overwritten but appended to the existing ones. (This behaviour is
copied from libpng itself.)

    $png->set_text ([{compression => PNG_TEXT_COMPRESSION_NONE,
                      keyword => "Copyright",
                      text => "Copyright (C) 1997 The Dukes of Hazzard",
              }]);

=head3 Correspondence to libpng

This function corresponds to C<png_set_text>.

=head1 Private chunks

=head2 set_keep_unknown_chunks

    use Image::PNG::Const 'PNG_HANDLE_CHUNK_ALWAYS';
    $png->set_keep_unknown_chunks (PNG_HANDLE_CHUNK_ALWAYS);

Tell libpng not to discard unknown chunks when reading the file.

=head2 get_unknown_chunks

    my $private_chunks = $png->get_unknown_chunks ();
    # Get some data from a private chunk
    my $chunk_three_data = $private_chunks->[3]->{data};
    # Get the size of the data
    print length $chunk_three_data;

This gets all of the private chunks from the image. The return value
is an array reference containing hash references. If there are no
private chunks, this returns an undefined value. There is one element
of the array for each chunk member. It is necessary to call
L</set_keep_unknown_chunks> with an appropriate value before reading
the file, otherwise libpng discards unknown chunks when reading the
file.

Each member hash reference has the following keys:

=over

=item name

The name of the unknown chunk, in the PNG chunk format (four bytes).


=item location

The location of the unknown chunk.


=item data

The data of the unknown chunk


=back

The "size" field of the PNG structure is not stored, because the
"data" member of the hash contains information on its length.

=head3 Correspondence to libpng

This function corresponds to C<png_get_unknown_chunks>

=head2 set_unknown_chunks

    $png->set_unknown_chunks ($private_chunks);

=head3 Correspondence to libpng

This function corresponds to C<png_set_unknown_chunks>

=head1 Helper functions

These helper functions assist the programmer in the transition from
libpng, which uses C conventions such as upper case macros standing
for numerical constants and C structures, to Perl's string-based
conventions.

=head2 color_type_name

    my $name = Image::PNG::Libpng::color_type_name ($color_type);

Given a numerical colour type in C<$color_type>, return the equivalent
name. The name is in upper case, with words separated by underscores,
as in C<RGB_ALPHA>.

=head3 Correspondence to libpng

This function does not correspond to anything in libpng. The names of
the colour types are taken from those defined in the libpng header
file, C<png.h>.

=head2 text_compression_name

    my $name = Image::PNG::Libpng::text_compression_name ($text->{compression});

Given a numerical text compression type, return the equivalent
name. The name is in upper case. The possible return values are

=over

=item TEXT_NONE

=item TEXT_zTXt

=item ITXT_NONE

=item ITXT_zTXt

=item an empty string

if the compression method is unknown.

=back

The compression field is also used to store the information about
whether the text is "international text" in UTF-8 or not.

=head3 Correspondence to libpng

This function does not correspond to anything in libpng. The names of
the text compression types are based on those in C<png.h>, but without
the word "COMPRESSION", so for example the libpng constant
C<PNG_ITXT_COMPRESSION_zTXt> corresponds to a return value of
C<ITXT_zTXt>.

=head1 Library version functions

=head2 get_libpng_ver

    my $libpng_version = Image::PNG::Libpng::get_libpng_ver ();

This function returns the version of the libpng library which the
module is using.

=head3 Correspondence to libpng

This function corresponds to C<png_get_libpng_ver>. However, it
doesn't require the C<png_structp> argument of the C function. See
L</About libpng>.

=head2 access_version_number

    my $libpng_version_number = Image::PNG::Libpng::access_version_number ();

This function returns the version of the libpng library which the
module is using as an integer number.

=head3 Correspondence to libpng

This function corresponds to C<png_access_version_number>.

=head1 Palettes

See L<http://www.w3.org/TR/PNG/#11PLTE> for information on the PNG standards for
the palette chunk.

=head2 get_PLTE

     my $colours = $png->get_PLTE ();
     # Get the green value of the twentieth entry in the palette.
     my $green = $colours->[20]->{green};

This function gets the palette from the PNG. The return value is an
array reference containing the palette. This array contains hash
references with the values "green", "blue" and "red" for the colour of
each pixel in the palette. If the PNG has no palette, it returns an
undefined value.

A PNG image may or may not contain a palette. To check whether the
image contains a palette, use something of the following form:

     use Image::PNG::Const ':all';
     my $color_type = $png->get_color_type ();
     if ($color_type == PNG_COLOR_TYPE_PALETTE) {
         # The PNG uses a palette.
     }

A PNG image may also contain a palette even when the "color_type" does
not indicate that. To check for that case, use L</get_valid>.

=head3 Correspondence to libpng

This function corresponds to C<png_get_PLTE>.

=head2 set_PLTE

    $png->set_PLTE ($palette);

Set the palette of C<$png>. The argument is an array reference
containing hash references. There is one hash reference for each
palette entry. The hash references contain three fields, red, green,
and blue, corresponding to the pixel value for that palette
entry. Other values in the hash references are ignored. For example,

    $png->set_PLTE ([{red => 1, green => 99, blue => 0x10},
                     {red => 0xFF, green => 0xFF, blue => 0xFF}]);

creates a palette with two entries in C<$png>.

=head3 Correspondence to libpng

This function corresponds to C<png_set_PLTE>.

=head1 Image data

=head2 get_rows

    my $rows = $png->get_rows ();
    my $pixel = substr ($rows->[10], 20, 1);

This returns the rows of the PNG image, after uncompressing and
unfiltering, as binary data. The return value, C<$rows> in the
example, is an array reference with a number of rows equal to the
height of the PNG image. Each row consists of the actual binary data,
which you will need to cut out using a routine like L<substr> or
L<unpack> to access pixel values. This binary data is likely to
contain bytes equal to zero.

You can get the number of bytes in each row using L</get_rowbytes>.

Each row is a Perl string. Perl terminates each row of data with an
extra zero byte at the end.

=head3 Correspondence to libpng

This function corresponds to C<png_get_rows>.

=head2 set_rows

    $png->set_rows (\@rows);

Set the rows of data to be written in to the PNG to C<@rows>. C<@rows>
needs to contain at least the same number of rows of data as the
height of the PNG image, and the length of each entry needs to be at
least the width of the entry times the number of bytes required for
each pixel.

Please also note that this is a bit unreliable. 
See L</set_rows is unreliable>.
I recommend that you only use this immediately before calling L</write_png> 
in order to prevent problems occuring.

=head3 Correspondence to libpng

This function corresponds to C<png_set_rows>.

=head2 get_rowbytes

    my $bytes_in_a_row = $png->get_rowbytes ();

=head3 Correspondence to libpng

This function corresponds to C<png_get_rowbytes>.

=head1 Compression and filtering

=head2 set_filter

    use Image::PNG::Const 'PNG_FILTER_NONE';
    $png->set_filter (PNG_FILTER_NONE);

This sets the filters which are allowed to be used for writing a PNG
image. The possible values are

=over

=item PNG_NO_FILTERS

=item PNG_FILTER_NONE

=item PNG_FILTER_SUB

=item PNG_FILTER_UP

=item PNG_FILTER_AVG

=item PNG_FILTER_PAETH

=item PNG_ALL_FILTERS

=back

These can be combined using C<|> (logical or):

    use Image::PNG::Const ':all';
    set_filter ($png, PNG_FILTER_UP | PNG_FILTER_AVG);

Please see L<http://www.w3.org/TR/PNG/#9Filter-types> for the meanings of these
filter types.

=head3 Correspondence to libpng

This function corresponds to C<png_set_filter> with the second (unused)
argument omitted. See L<Unused arguments omitted>.

=head1 Other chunks

=head2 get_bKGD

Get the bKGD (background) chunk of the image.

=head3 Correspondence to libpng

This function corresponds to C<png_get_bKGD> with a hash function instead of a
C<png_color> struct.

=head2 set_bKGD

Set the bKGD (background) chunk of the image.

=head3 Correspondence to libpng

This function corresponds to C<png_set_bKGD> with a hash function instead of a
C<png_color> struct.

=head2 get_cHRM

    my %cHRM = $png->get_cHRM ();

Get the cHRM chunk as a hash.

The keys of the hash are

=over

=item white_x

=item white_y

=item red_x

=item red_y

=item green_x

=item green_y

=item blue_x

=item blue_y

=back

The values of the hash are floating point numbers between 0 and 1.

See section "4.2.2.2. cHRM Primary chromaticities" of the PNG specification.

=head3 Correspondence to libpng

This function corresponds to C<png_get_cHRM> with a hash function instead of the C<double>
arguments. The hash key names correspond to the names of the C<double>
arguments in libpng.

=head2 set_cHRM

    $png->set_cHRM (\%cHRM);

Set the cHRM chunk from a hash.

The keys of the hash are

=over

=item white_x

=item white_y

=item red_x

=item red_y

=item green_x

=item green_y

=item blue_x

=item blue_y

=back

The values of the hash are floating point numbers between 0 and 1.

See section "4.2.2.2. cHRM Primary chromaticities" of the PNG specification.

=head3 Correspondence to libpng

This function corresponds to C<png_set_cHRM> with a hash function instead of the C<double>
arguments.

=head1 META FUNCTIONS

=head2 supports

    if (supports ('iTXt')) {
        print "Your libpng supports international text.\n";
    }

This function returns true or false depending on whether the version
of libpng which this was compiled with supports or does not support a
particular facility.

The possible arguments to C<supports> are

=over

=item iTXt

Does the libpng support international text?

=back

=head1 EXPORTS

Nothing is exported by default, but all the functions in this module
can be exported on request. The export tag 'all' exports everything in
the module:

    use Image::PNG::Libpng ':all';
    # Now everything in the module has been imported

=head1 Differences from libpng

The procedures in Image::PNG::Libpng are closely based on
those of libpng, with the following differences.

=head2 No info structure

This module, C<Image::PNG::Libpng> does not use the "info"
structure of libpng. Almost all libpng functions require two initial
arguments, a C<png_structp> and a C<png_infop>. However, in 
Image::PNG::Libpng, both the "png" and the "info" are contained in
the first argument to each function.

=head2 Unused arguments omitted

This module eliminates all the unevaluated arguments of libpng. For
example, libpng requires the user to pass a pointer to a C<png_struct>
before calling the library to ask for its version number (see
L</get_libpng_ver>), but the library ignores the this structure
anyway, so this module does not duplicate this. There are many similar
instances of unevaluated arguments, which have all been eliminated
from this module.

If you are interested in exactly which libpng arguments are omitted,
you can find each instance L<in the file C<perl-libpng.c> in the top directory of the distribution|http://cpansearch.perl.org/src/BKB/Image-PNG-0.18/perl-libpng.c> in the macro
C<UNUSED_ZERO_ARG>.

=head2 Function return values are used to return values

In libpng, some functions return results using references, and some
return results using the function's return value. For example
C<png_get_rows> (see L</get_rows>) uses the return value of the
function to return an array of pointers, but C<png_get_PLTE> (see
L</get_PLTE>) uses a pointer reference to return an array of pointers,
and the return value to indicate errors. 

Image::PNG::Libpng uses only the return value. Errors and
non-existence are indicated by a return value of the undefined
value. You can also choose to raise errors or print errors using the
L</raise_error> and L</print_error> options.

In libpng, some functions use the return value to indicate errors, and
some of the functions don't indicate errors but fail silently. Some of
the functions which use the return value to indicate an error use a
non-zero value to indicate an error, and some of them use a zero value
to indicate an error.

=head2 No destructors

Freeing the memory allocated by L</create_read_struct> and
L</create_write_struct> is automatically handled by Perl.

Older versions of this module (pre-0.18) had functions called
C<destroy_read_struct> and C<destroy_write_struct> corresponding to
the functions with similar names in libpng. From version 0.18, these
functions still exist, but they no longer do anything. The memory
freeing is now handled by Perl automatically.

=head2 Other unimplemented parts of libpng

=head3 Memory management functions

This module does not offer an interface to C<png_malloc> and C<png_free>.

=head3 Error handling functions

This module does not offer an interface to C<png_error> and
C<png_get_error_ptr>. It redirects the error and warning handlers to
Perl's error stream.

=head3 Input/output manipulation functions

This module does not offer a direct interface to C<png_set_write_fn>
and C<png_set_read_fn>. However, it is possible to use their
functionality to access Perl data via L</read_from_scalar> and
L</write_to_scalar>.

=head3 Partial read/write functions

This module does not yet offer an interface to the partial read and
write functions of libpng. The reason is because I don't know enough
about Perl's internal structures to be able to create a memory-safe
interface to these functions. The partial read/write functions would
rely on preserving pointers to data structures within the Perl
program's data area between calls. So this module doesn't deal with
png_write_chunk, png_write_end, png_write_info, png_write_row, or
png_write_rows.

=head1 DIAGNOSTICS

The module may produce the following error or warning messages.

=over


=item libpng error: %s\n

(F) 
An error from libpng sent via Perl's warning handler.




=item libpng warning: %s\n

(W) 
A warning from libpng sent via Perl's warning handler.




=item %s:%d: Call to calloc for %d '%s' failed: out of memory

(F) 
A request for more memory was refused. The first two parameters are the file name and line number of where this happened.




=item Memory leak detected: there are %d allocated pieces of memory which have not been freed.\n

(W) 
The module's internal check for memory errors was tripped somehow. This probably indicates a bug in the module.




=item Attempt to destroy an object of unknown type

(F) 
There was an attempt to free some corrupted memory.




=item According to its compression type, a text chunk in the current PNG file claims to be ITXT but Perl's 'is_utf8_string' says that its encoding is invalid.

(W) 



=item A language key 'lang_key' member of a 'png_text' structure in the file failed Perl's 'is_utf8_string' test, which says that its encoding is invalid.

(W) 



=item None of your text chunks was allowed

(W) 
The user tried to set some text chunks in the image but they were not allowed.




=item Trying to read from a PNG in memory but there is no PNG in memory

(F) 
Something went wrong trying to read a PNG from a Perl scalar. This probably indicates a bug in the program.




=item Request for too many bytes %d on a scalar of length %d at read position %d.\n

(F) 
There was an attempt to read some data from a Perl scalar which went beyond the expected end of the scalar in memory.




=item Attempt to write PNG without calling init_io

(F) 
write_png was called before a file handle was associated with the PNG.




=item set_IHDR: Bad values for width (%d), height (%d), or bit depth (%d)

(F) 
The user tried to set a PNG header with unacceptable values, as indicated.




=item set_PLTE: Empty array of colours in set_PLTE

(F) 
The user tried to set an empty palette of colours.




=item Too many transparencies %d supplied

(F) 
The user tried to set more than the maximum possible number of transparencies for a paletted image.




=item Image has no height

(F) 
The height of the image is zero.




=item Image has no rows

(F) 
The image does not have any rows of image data.




=item Image rows have zero length

(F) 
The rows of image data have zero length.




=item Image has zero height

(F) 
The image we are trying to read has zero height.




=item Out of memory allocating %d bytes for image

(F) 
We were refused the memory we want to read the image into.




=item Row pointers already allocated

(F) 
There was an attempt to set the rows of an image after they had already been set.




=item array has %d rows but PNG image requires %d rows

(F) 
set_rows was called with an array of the wrong size.




=item Chunk name '%s' has wrong number of characters: %d required

(F) 
We tried to set an unknown chunk with a name which doesn't have four characters.




=item Number of unknown chunks is zero

(F) 
The user tried to set an empty list of unknown chunks.




=item Illegal PNG chunk name length, chunk names must be %d characters long

(W) 
The user's name for a private chunk was not a valid length. In this case the chunk is ignored.




=item Unknown chunk location is zero, which means 'don't write the chunk'

(W) 
The user supplied a value for "location" of zero, which means "don't write the chunk".




=item undefined chunk name at offset %d in chunk list

(F) 
The chunk name was not defined.




=item chunk %i has bad length %d: should be %d in chunk list

(F) 



=item set_keep_unknown_chunks is not supported in your libpng

(F) 
libpng was compiled without this option.




=back

=head1 BUGS

This section documents some known deficiencies in the module.

=head2 set_rows is unreliable

The method L</set_rows> doesn't actually copy or write any
information. All it does is set a pointer to the pointers to the rows
in the PNG data structure. The actual data is only written when you
ask to write it. So if you "set_rows" to some data, then delete or
change that data before asking to write the png with L</write_png>,
you will get a memory error.

=head2 Conditional compilation

It is possible to compile a version of the libpng library without
support for various things. For example, it's possible to have a
libpng without support for text chunks by undefining a macro
C<PNG_TEXT_SUPPORTED>. The module supports some of the conditional
compilation choices which I've found in practice, but it does not
support every possible choice. If you encounter problems using this
Perl module because of a conditionally-compiled libpng, then please
let me know and I'll consider adding that facility to the module.

=head1 SEE ALSO

=head2 The PNG specification

L<The PNG specification|http://www.w3.org/TR/PNG/> (link to W3
consortium) explains the details of the PNG format.


=head2 The libpng documentation

=head3 "Official" documentation

The starting point is the plain text libpng manual at
L<http://libpng.org/pub/png/libpng-manual.txt> and the manual page
libpng.3, which you can read using "man 3 libpng".

Be warned that the documentation which comes with libpng is rather
sketchy. See L</Differences from libpng>. It doesn't contain full
specifications (prototypes, return values) for all of the functions in
the library. If you are considering programming in C using libpng, you
will definitely also need to look at the header file "png.h".  In some
cases you will also need to look at the source code of the library.

=head3 Unofficial documentation

There is a collection of function definitions under the title
"Interface Definitions for libpng12" at
L<http://refspecs.freestandards.org/LSB_3.1.1/LSB-Desktop-generic/LSB-Desktop-generic/libpng12man.html>
as part of the "Linux Standard Base Desktop Specification". These
contain extensive information on the prototypes and return values for
the libpng routines, something which is often only available elsewhere
by actually looking at the libpng source code. These pages are usually
the first hits on search engines if you search for a function name in
libpng.

=head2 Other Perl modules on CPAN

=head3 Image::ExifTool

L<Image::ExifTool> is a pure Perl (doesn't require a C compiler)
solution for accessing the text segments of images. It has extensive
support for PNG text segments.

=head3 Alien::PNG

L<Alien::PNG> claims to be a way of "building, finding and using PNG
binaries". It may help in installing libpng. I didn't use it as a
dependency for this module because it seems not to work in batch mode,
but stop and prompt the user. I'm interested in hearing feedback from
users whether this works or not on various platforms.

=head3 Image::PNG::Rewriter

L<Image::PNG::Rewriter> is a utility for unpacking and recompressing
the IDAT (image data) part of a PNG image. The main purpose seems to
be to recompress the image data with the module author's other module
L<Compress::Deflate7>. Unfortunately that only works with Perl
versions 5.12.

=head3 Image::Pngslimmer

L<Image::Pngslimmer> reduces the size of dynamically created PNG
images. It's very, very slow at reading PNG data, but seems to work
OK.

=head3 Image::Info

L<Image::Info> is a module for getting information out of various
types of images. It has good support for PNG and is written in pure
Perl (doesn't require a C compiler). As well as basics such as height,
width, and colour type, it can get text chunks, modification time,
palette, gamma (gAMA chunk), resolution (pHYs chunk), and significant
bits (sBIT chunk). At the time of writing (version 1.31) it doesn't
support other chunks.



=head1 EXAMPLES

There is a collection of demonstrations and example scripts online at
L<http://www.lemoda.net/image-png/>. This currently contains

=over

=item PNG inspector L<http://www.lemoda.net/png/inspect/>

This demonstration downloads a PNG file you specify from the internet
and prints out its contents.

=item PNG quantizer L<http://www.lemoda.net/png/quantize/>

This demonstration downloads a PNG file you specify from the internet
and quantizes it with as many colours as you want (from two to 256) in
order to reduce its size. The quantized image is uploaded to imgur.com
using L<WWW::Imgur>.

=back



=head1 AUTHOR

Ben Bullock, <bkb@cpan.org>

=head1 COPYRIGHT & LICENCE

The Image::PNG package and associated files are copyright (C)
2013 Ben Bullock.

You can use, copy, modify and redistribute Image::PNG and
associated files under the Perl Artistic Licence or the GNU General
Public Licence.

=head1 SUPPORT

=head2 Mailing list

There is a mailing list at
L<http://groups.google.com/group/perlimagepng>. It is not necessary to
be a member of the list to post messages to the list or participate in
discussions.

=head2 CPAN stuff

There is a bug tracker at L<https://github.com/benkasminbullock/Image-PNG/issues>.

=head1 FOR PROGRAMMERS

The distributed files are not the source code of the module. The
source code lives in the "tmpl" directory of the distribution and the
distribution is created via scripts.



=cut

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