he unpack code for ICO/CUR file handling could extend 32-bit unsigned values to 64...

[imager.git] / lib / Imager / ImageTypes.pod

=head1 NAME

Imager::ImageTypes - image models for Imager

=head1 SYNOPSIS

  use Imager;

  $img = Imager->new(); #  Empty image (size is 0 by 0)
  $img->open(file=>'lena.png',type=>'png'); # Read image from file

  $img = Imager->new(xsize=>400, ysize=>300); # RGB data

  $img = Imager->new(xsize=>400, ysize=>300,  # Grayscale
                     channels=>1);            #

  $img = Imager->new(xsize=>400, ysize=>300,  # RGB with alpha
                     channels=>4);            #
                                              
  $img = Imager->new(xsize=>200, ysize=>200,  
                     type=>'paletted');       # paletted image
                                              
  $img = Imager->new(xsize=>200, ysize=>200,  
                     bits=>16);               # 16 bits/channel rgb
                                              
  $img = Imager->new(xsize=>200, ysize=>200,  
                     bits=>'double');         # 'double' floating point
                                              #  per channel

  $img->img_set(xsize=>500, ysize=>500,       # reset the image object
                channels=>4);


  # Example getting information about an Imager object

  print "Image information:\n";
  print "Width:        ", $img->getwidth(),    "\n";
  print "Height:       ", $img->getheight(),   "\n";
  print "Channels:     ", $img->getchannels(), "\n";
  print "Bits/Channel: ", $img->bits(),        "\n";
  print "Virtual:      ", $img->virtual() ? "Yes" : "No", "\n";
  my $colorcount = $img->getcolorcount(maxcolors=>512);
        print "Actual number of colors in image: ";
  print defined($colorcount) ? $colorcount : ">512", "\n";
  print "Type:         ", $img->type(),        "\n";

  if ($img->type() eq 'direct') {
    print "Modifiable Channels: ";
    print join " ", map {
      ($img->getmask() & 1<<$_) ? $_ : ()
    } 0..$img->getchannels();
    print "\n";
  
  } else {
    # palette info
    my $count = $img->colorcount;  
    @colors = $img->getcolors();
    print "Palette size: $count\n";
    my $mx = @colors > 4 ? 4 : 0+@colors;
    print "First $mx entries:\n";
    for (@colors[0..$mx-1]) {
      my @res = $_->rgba();
      print "(", join(", ", @res[0..$img->getchannels()-1]), ")\n";
    }
  }
  
  my @tags = $img->tags();
  if (@tags) {
    print "Tags:\n";
    for(@tags) {
      print shift @$_, ": ", join " ", @$_, "\n";
    }
  } else {
    print "No tags in image\n";
  }
  
=head1 DESCRIPTION

Imager supports two basic models of image:

=over

=item *

direct color - all samples are stored for every pixel.  eg. for an
8-bit/sample RGB image, 24 bits are stored for each pixel.

=item *

paletted - an index into a table of colors is stored for each pixel.

=back

Direct color or paletted images can have 1 to 4 samples per color
stored.  Imager treats these as follows:

=over

=item *

1 sample per color - gray scale image.

=item *

2 samples per color - gray scale image with alpha channel, allowing
transparency.

=item *

3 samples per color - RGB image.

=item *

4 samples per color - RGB image with alpha channel, allowing
transparency.

=back

Direct color images can have sample sizes of 8-bits per sample,
16-bits per sample or a double precision floating point number per
sample (64-bits on many systems).

Paletted images are always 8-bits/sample.

To query an existing image about it's parameters see the C<bits()>,
C<type()>, C<getwidth()>, C<getheight()>, C<getchannels()> and
C<virtual()> methods.

The coordinate system in Imager has the origin in the upper left
corner, see L<Imager::Draw> for details.

The alpha channel when one is present is considered unassociated -
ie the color data has not been scaled by the alpha channel.  Note
that not all code follows this (recent) rule, but will over time.

=head2 Creating Imager Objects

=over

=item new()
X<new(), Imager methods>

  $img = Imager->new();
  $img->read(file=>"alligator.ppm") or die $img->errstr;

Here C<new()> creates an empty image with width and height of zero.
It's only useful for creating an Imager object to call the read()
method on later.

  %opts = (xsize=>300, ysize=>200);
  $img = Imager->new(%opts); # create direct mode RGBA image
  $img = Imager->new(%opts, channels=>4); # create direct mode RGBA image

You can also read a file from new():

  $img = Imager->new(file => "someimage.png");

The parameters for new are:

=over

=item *

C<xsize>, C<ysize> - Defines the width and height in pixels of the
image.  These must be positive.

If not supplied then only placeholder object is created, which can be
supplied to the C<read()> or C<img_set()> methods.

=item *

C<channels> - The number of channels for the image.  Default 3.  Valid
values are from 1 to 4.

=item *

C<model> - this overrides the value, if any, supplied for C<channels>.
This can be one of C<gray>, C<graya>, C<rgb> or C<rgba>.

=item *

C<bits> - The storage type for samples in the image.  Default: 8.
Valid values are:

=over

=item *

C<8> - One byte per sample.  256 discrete values.

=item *

C<16> - 16-bits per sample, 65536 discrete values.

=item *

C<double> - one C double per sample.

=back

Note: you can use any Imager function on any sample size image.

Paletted images always use 8 bits/sample.

=item *

C<type> - either C<'direct'> or C<'paletted'>.  Default: C<'direct'>.

Direct images store color values for each pixel.  

Paletted images keep a table of up to 256 colors called the palette,
each pixel is represented as an index into that table.

In most cases when working with Imager you will want to use the
C<direct> image type.

If you draw on a C<paletted> image with a color not in the image's
palette then Imager will transparently convert it to a C<direct>
image.

=item *

C<maxcolors> - the maximum number of colors in a paletted image.
Default: 256.  This must be in the range 1 through 256.

=item *

C<file>, C<fh>, C<fd>, C<callback>, C<readcb>, or C<io> - specify a
file name, filehandle, file descriptor or callback to read image data
from.  See L<Imager::Files> for details.  The typical use is:

  my $im = Imager->new(file => $filename);

=item *

C<filetype> - treated as the file format parameter, as for C<type>
with the read() method, eg:

  my $im = Imager->new(file => $filename, filetype => "gif");

In most cases Imager will detect the file's format itself.

=back

In the simplest case just supply the width and height of the image:

  # 8 bit/sample, RGB image
  my $img = Imager->new(xsize => $width, ysize => $height);

or if you want an alpha channel:

  # 8 bits/sample, RGBA image
  my $img = Imager->new(xsize => $width, ysize => $height, channels=>4);

Note that it I<is> possible for image creation to fail, for example if
channels is out of range, or if the image would take too much memory.

To create paletted images, set the 'type' parameter to 'paletted':

  $img = Imager->new(xsize=>200, ysize=>200, type=>'paletted');

which creates an image with a maximum of 256 colors, which you can
change by supplying the C<maxcolors> parameter.

For improved color precision you can use the bits parameter to specify
16 bit per channel:

  $img = Imager->new(xsize=>200, ysize=>200,
                     channels=>3, bits=>16);

or for even more precision:

  $img = Imager->new(xsize=>200, ysize=>200,
                     channels=>3, bits=>'double');

to get an image that uses a double for each channel.

Note that as of this writing all functions should work on images with
more than 8-bits/channel, but many will only work at only
8-bit/channel precision.

If you want an empty Imager object to call the read() method on, just
call new() with no parameters:

  my $img = Imager->new;
  $img->read(file=>$filename)
    or die $img->errstr;

Though it's much easier now to just call new() with a C<file>
parameter:

  my $img = Imager->new(file => $filename)
    or die Imager->errstr;

If none of C<xsize>, C<ysize>, C<file>, C<fh>, C<fd>, C<callback>,
C<readcb>, C<data>, C<io> is supplied, and other parameters I<are> supplied
C<< Imager->new >> will return failure rather than returning an empty
image object.

=item img_set()
X<img_set>

img_set destroys the image data in the object and creates a new one
with the given dimensions and channels.  For a way to convert image
data between formats see the C<convert()> method.

  $img->img_set(xsize=>500, ysize=>500, channels=>4);

This takes exactly the same parameters as the new() method, excluding
those for reading from files.

=back

=head2 Image Attribute functions

These return basic attributes of an image object.

=over

=item getwidth()

  print "Image width: ", $img->getwidth(), "\n";

The C<getwidth()> method returns the width of the image.  This value
comes either from C<new()> with C<xsize>, C<ysize> parameters or from
reading data from a file with C<read()>.  If called on an image that
has no valid data in it like C<Imager-E<gt>new()> returns, the return
value of C<getwidth()> is undef.

=item getheight()

  print "Image height: ", $img->getheight(), "\n";

Same details apply as for L</getwidth()>.

=item getchannels()
X<getchannels() method>X<methods, getchannels()>

  print "Image has ",$img->getchannels(), " channels\n";

Returns the number of channels in an image.

Note: previously the number of channels in an image mapped directly to
the color model of the image, ie a 4 channel image was always RGBA.
This may change in a future release of Imager.

Returns an empty list if the image object is not initialized.

=item colorchannels()
X<colorchannels() method>X<methods, colorchannels()>

Returns the number of color channels.

Currently this is always 1 or 3, but may be 0 for some rare images in
a future version of Imager.

Returns an empty list if the image object is not initialized.

=item colormodel()
X<colormodel method>X<methods, colormodel>

Returns the color model of the image, including whether there is an
alpha channel.

By default this is returned as a string, one of C<unknown>, C<gray>,
C<graya>, C<rgb> or C<rgba>.

If you call C<colormodel()> with a true numeric parameter:

  my $model = $img->colormodel(numeric => 1);

then the color model is returned as a number, mapped as follows:

  Numeric  String
  -------  ------
      0    unknown
      1    gray
      2    graya
      3    rgb
      4    rgba

=item alphachannel()
X<alphachannel() method>X<methods, alphachannel()>

Returns the channel index of the alpha channel of the image.

This is 1 for grayscale images with alpha, 3 for RGB images with alpha
and will return C<undef> for all other images.

Returns an empty list if the image object is not initialized.

=item bits()

The bits() method retrieves the number of bits used to represent each
channel in a pixel, 8 for a normal image, 16 for 16-bit image and
'double' for a double/channel image.

  if ($img->bits eq 8) {
    # fast but limited to 8-bits/sample
  }
  else {
    # slower but more precise
  }

Returns an empty list if the image object is not initialized.

=item type()

The type() method returns either 'direct' for direct color images or
'paletted' for paletted images.

  if ($img->type eq 'paletted') {
    # print the palette
    for my $color ($img->getcolors) {
      print join(",", $color->rgba), "\n";
    }
  }

Returns an empty list if the image object is not initialized.

=item virtual()

The virtual() method returns non-zero if the image contains no actual
pixels, for example masked images.

=for stopwords SDL

This may also be used for non-native Imager images in the future, for
example, for an Imager object that draws on an SDL surface.

=item is_bilevel()

Tests if the image will be written as a monochrome or bi-level image
for formats that support that image organization.

In scalar context, returns true if the image is bi-level.

In list context returns a list:

  ($is_bilevel, $zero_is_white) = $img->is_bilevel;

An image is considered bi-level, if all of the following are true:

=over

=item *

the image is a paletted image

=item *

the image has 1 or 3 channels

=item *

the image has only 2 colors in the palette

=item *

those 2 colors are black and white, in either order.

=back

If a real bi-level organization image is ever added to Imager, this
function will return true for that too.

Returns an empty list if the image object is not initialized.

=back

=head2 Direct Type Images

Direct images store the color value directly for each pixel in the
image.

=over

=item getmask()

  @rgbanames = qw( red green blue alpha );
  my $mask = $img->getmask();
  print "Modifiable channels:\n";
  for (0..$img->getchannels()-1) {
    print $rgbanames[$_],"\n" if $mask & 1<<$_;
  }

=for stopwords th

C<getmask()> is used to fetch the current channel mask.  The mask
determines what channels are currently modifiable in the image.  The
channel mask is an integer value, if the C<i-th> least significant bit
is set the C<i-th> channel is modifiable.  eg. a channel mask of 0x5
means only channels 0 and 2 are writable.

=item setmask()

  $mask = $img->getmask();
  $img->setmask(mask=>8);     # modify alpha only

    ...

  $img->setmask(mask=>$mask); # restore previous mask

C<setmask()> is used to set the channel mask of the image.  See
L</getmask()> for details.

=back

=head2 Palette Type Images

Paletted images keep an array of up to 256 colors, and each pixel is
stored as an index into that array.

In general you can work with paletted images in the same way as RGB
images, except that if you attempt to draw to a paletted image with a
color that is not in the image's palette, the image will be converted
to an RGB image.  This means that drawing on a paletted image with
anti-aliasing enabled will almost certainly convert the image to RGB.

Palette management takes place through C<addcolors()>, C<setcolors()>,
C<getcolors()> and C<findcolor()>:

=over

=item addcolors()

You can add colors to a paletted image with the addcolors() method:

   my @colors = ( Imager::Color->new(255, 0, 0), 
                  Imager::Color->new(0, 255, 0) );
   my $index = $img->addcolors(colors=>\@colors);

The return value is the index of the first color added, or undef if
adding the colors would overflow the palette.

The only parameter is C<colors> which must be a reference to an array
of Imager::Color objects.

=item setcolors()

  $img->setcolors(start=>$start, colors=>\@colors);

Once you have colors in the palette you can overwrite them with the
C<setcolors()> method:  C<setcolors()> returns true on success.

Parameters:

=over

=item *

start - the first index to be set.  Default: 0

=item *

colors - reference to an array of Imager::Color objects.

=back

=item getcolors()

To retrieve existing colors from the palette use the getcolors() method:

  # get the whole palette
  my @colors = $img->getcolors();
  # get a single color
  my $color = $img->getcolors(start=>$index);
  # get a range of colors
  my @colors = $img->getcolors(start=>$index, count=>$count);

=item findcolor()

To quickly find a color in the palette use findcolor():

  my $index = $img->findcolor(color=>$color);

which returns undef on failure, or the index of the color.

Parameter:

=over

=item *

color - an Imager::Color object.

=back

=item colorcount()

Returns the number of colors in the image's palette:

  my $count = $img->colorcount;

=item maxcolors()

Returns the maximum size of the image's palette.

  my $maxcount = $img->maxcolors;

=back

=head2 Color Distribution

=over

=item getcolorcount()

Calculates the number of colors in an image.

The amount of memory used by this is proportional to the number of
colors present in the image, so to avoid using too much memory you can
supply a maxcolors() parameter to limit the memory used.

Note: getcolorcount() treats the image as an 8-bit per sample image.

=over

=item *

X<maxcolors!getcolorcount>C<maxcolors> - the maximum number of colors to
return.  Default: unlimited.

=back

  if (defined($img->getcolorcount(maxcolors=>512)) {
    print "Less than 512 colors in image\n";
  }

=item getcolorusagehash()

Calculates a histogram of colors used by the image.

=over

=item *

X<maxcolors!getcolorusagehash>C<maxcolors> - the maximum number of colors
to return.  Default: unlimited.

=back

Returns a reference to a hash where the keys are the raw color as
bytes, and the values are the counts for that color.

The alpha channel of the image is ignored.  If the image is gray scale
then the hash keys will each be a single character.

  my $colors = $img->getcolorusagehash;
  my $blue_count = $colors->{pack("CCC", 0, 0, 255)} || 0;
  print "#0000FF used $blue_count times\n";

=item getcolorusage()

Calculates color usage counts and returns just the counts.

=over

=item *

X<maxcolors!getcolorusage>C<maxcolors> - the maximum number of colors
to return.  Default: unlimited.

=back

Returns a list of the color frequencies in ascending order.

  my @counts = $img->getcolorusage;
  print "The most common color is used $counts[0] times\n";

=back

=head2 Conversion Between Image Types

Warning: if you draw on a paletted image with colors that aren't in
the palette, the image will be internally converted to a normal image.

=over

=item to_paletted()

You can create a new paletted image from an existing image using the
to_paletted() method:

 $palimg = $img->to_paletted(\%opts)

where %opts contains the options specified under L</Quantization options>.

  # convert to a paletted image using the web palette
  # use the closest color to each pixel
  my $webimg = $img->to_paletted({ make_colors => 'webmap' });

  # convert to a paletted image using a fairly optimal palette
  # use an error diffusion dither to try to reduce the average error
  my $optimag = $img->to_paletted({ make_colors => 'mediancut',
                                    translate => 'errdiff' });

=item to_rgb8()

You can convert a paletted image (or any image) to an 8-bit/channel
RGB image with:

  $rgbimg = $img->to_rgb8;

No parameters.

=item to_rgb16()

Convert a paletted image (or any image) to a 16-bit/channel RGB image.

  $rgbimg = $img->to_rgb16;

No parameters.

=item to_rgb_double()

Convert a paletted image (or any image) to an double/channel direct
color image.

  $rgbimg = $img->to_rgb_double;

No parameters.

=item masked()

Creates a masked image.  A masked image lets you create an image proxy
object that protects parts of the underlying target image.

In the discussion below there are 3 image objects involved:

=over

=item *

the masked image - the return value of the masked() method.  Any
writes to this image are written to the target image, assuming the
mask image allows it.

=item *

the mask image - the image that protects writes to the target image.
Supplied as the C<mask> parameter to the masked() method.

=item *

the target image - the image you called the masked() method on.  Any
writes to the masked image end up on this image.

=back

Parameters:

=over

=item *

mask - the mask image.  If not supplied then all pixels in the target
image are writable.  On each write to the masked image, only pixels
that have non-zero in channel 0 of the mask image will be written to
the original image.  Default: none, if not supplied then no masking is
done, but the other parameters are still honored.

=item *

left, top - the offset of writes to the target image.  eg. if you
attempt to set pixel (x,y) in the masked image, then pixel (x+left,
y+top) will be written to in the original image.

=item *

bottom, right - the bottom right of the area in the target available
from the masked image.

=back

Masked images let you control which pixels are modified in an
underlying image.  Where the first channel is completely black in the
mask image, writes to the underlying image are ignored.

For example, given a base image called $img:

  my $mask = Imager->new(xsize=>$img->getwidth, ysize=>$img->getheight,
                         channels=>1);
  # ... draw something on the mask
  my $maskedimg = $img->masked(mask=>$mask);

  # now draw on $maskedimg and it will only draw on areas of $img 
  # where $mask is non-zero in channel 0.

You can specify the region of the underlying image that is masked
using the left, top, right and bottom options.

If you just want a subset of the image, without masking, just specify
the region without specifying a mask.  For example:

  # just work with a 100x100 region of $img
  my $maskedimg = $img->masked(left => 100, top=>100,
                               right=>200, bottom=>200);

=item make_palette()

This doesn't perform an image conversion, but it can be used to
construct a common palette for use in several images:

  my @colors = Imager->make_palette(\%opts, @images);

You must supply at least one image, even if the C<make_colors>
parameter produces a fixed palette.

On failure returns no colors and you can check C<< Imager->errstr >>.

=back

=head2 Tags

Image tags contain meta-data about the image, ie. information not
stored as pixels of the image.

At the perl level each tag has a name or code and a value, which is an
integer or an arbitrary string.  An image can contain more than one
tag with the same name or code, but having more than one tag with the
same name is discouraged.

You can retrieve tags from an image using the tags() method, you can
get all of the tags in an image, as a list of array references, with
the code or name of the tag followed by the value of the tag.

Imager's support for fairly limited, for access to pretty much all
image metadata you may want to try L<Image::ExifTool>.

=over

=item tags()

Retrieve tags from the image.

With no parameters, retrieves a list array references, each containing
a name and value: all tags in the image:

  # get a list of ( [ name1 => value1 ], [ name2 => value2 ] ... )
  my @alltags = $img->tags;
  print $_->[0], ":", $_->[1], "\n" for @all_tags;

  # or put it in a hash, but this will lose duplicates
  my %alltags = map @$_, $img->tags;

in scalar context this returns the number of tags:

  my $num_tags = $img->tags;

or you can get all tags values for the given name:

  my @namedtags = $img->tags(name => $name);

in scalar context this returns the first tag of that name:

  my $firstnamed = $img->tags(name => $name);

or a given code:

  my @tags = $img->tags(code=>$code);

=item addtag()

You can add tags using the addtag() method, either by name:

  my $index = $img->addtag(name=>$name, value=>$value);

or by code:

  my $index = $img->addtag(code=>$code, value=>$value);

=item deltag()

You can remove tags with the deltag() method, either by index:

  $img->deltag(index=>$index);

or by name:

  $img->deltag(name=>$name);

or by code:

  $img->deltag(code=>$code);

In each case deltag() returns the number of tags deleted.

=item settag()

settag() replaces any existing tags with a new tag.  This is
equivalent to calling deltag() then addtag().

=back

=head2 Common Tags

Many tags are only meaningful for one format.  GIF looping information
is pretty useless for JPEG for example.  Thus, many tags are set by
only a single reader or used by a single writer.  For a complete list
of format specific tags see L<Imager::Files>.

Since tags are a relatively new addition their use is not wide spread
but eventually we hope to have all the readers for various formats set
some standard information.

=over

=item *

X<i_xres tag>X<i_yres tag>X<tags, i_xres>X<tags, i_yres>C<i_xres>, C<i_yres>
- The spatial resolution of the image in pixels per inch.  If the
image format uses a different scale, eg. pixels per meter, then this
value is converted.  A floating point number stored as a string.

  # our image was generated as a 300 dpi image
  $img->settag(name => 'i_xres', value => 300);
  $img->settag(name => 'i_yres', value => 300);

  # 100 pixel/cm for a TIFF image
  $img->settag(name => 'tiff_resolutionunit', value => 3); # RESUNIT_CENTIMETER
  # convert to pixels per inch, Imager will convert it back
  $img->settag(name => 'i_xres', value => 100 * 2.54);
  $img->settag(name => 'i_yres', value => 100 * 2.54);

=item *

X<i_aspect_only tag>X<tags, i_aspect_only>C<i_aspect_only> - If this is
non-zero then the values in i_xres and i_yres are treated as a ratio
only.  If the image format does not support aspect ratios then this is
scaled so the smaller value is 72 DPI.

=item *

X<i_incomplete tag>X<tags, i_incomplete>C<i_incomplete> - If this tag is
present then the whole image could not be read.  This isn't
implemented for all images yet, and may not be.

=item *

X<i_lines_read tag>X<tags, i_lines_read>C<i_lines_read> - If
C<i_incomplete> is set then this tag may be set to the number of
scan lines successfully read from the file.  This can be used to decide
whether an image is worth processing.

=item *

X<i_format tag>X<tags, i_format>i_format - The file format this file
was read from.

=item *

X<i_background>X<tags, i_background>i_background - used when writing
an image with an alpha channel to a file format that doesn't support
alpha channels.  The C<write> method will convert a normal color
specification like "#FF0000" into a color object for you, but if you
set this as a tag you will need to format it like
C<color(>I<red>C<,>I<green>C<,>I<blue>C<)>, eg color(255,0,0).

=item *

X<i_comment>C<i_comment> - used when reading or writing several image
formats.  If the format has only one text field it will be read into
the C<i_comment> tag or written to the file.

=back

=head2 Quantization options

These options can be specified when calling
L<Imager::ImageTypes/to_paletted()>, write_multi() for GIF files, when
writing a single image with the C<gifquant> option set to C<gen>, or for
direct calls to i_writegif_gen() and i_writegif_callback().

=over

=item *

C<colors> - An arrayref of colors that are fixed.  Note that some
color generators will ignore this.  If this is supplied it will be
filled with the color table generated for the image.

=item *

C<transp> - The type of transparency processing to perform for images
with an alpha channel where the output format does not have a proper
alpha channel (eg. GIF).  This can be any of:

=over

=item *

C<none> - No transparency processing is done. (default)

=item *

C<threshold> - pixels more transparent than C<tr_threshold> are
rendered as transparent.

=item *

C<errdiff> - An error diffusion dither is done on the alpha channel.
Note that this is independent of the translation performed on the
color channels, so some combinations may cause undesired artifacts.

=item *

C<ordered> - the ordered dither specified by tr_orddith is performed
on the alpha channel.

=back

This will only be used if the image has an alpha channel, and if there
is space in the palette for a transparency color.

=item *

C<tr_threshold> - the highest alpha value at which a pixel will be
made transparent when C<transp> is 'threshold'. (0-255, default 127)

=item *

C<tr_errdiff> - The type of error diffusion to perform on the alpha
channel when C<transp> is C<errdiff>.  This can be any defined error
diffusion type except for custom (see C<errdiff> below).

=item *

C<tr_orddith> - The type of ordered dither to perform on the alpha
channel when C<transp> is 'ordered'.  Possible values are:

=over

=item *

C<random> - A semi-random map is used.  The map is the same each time.

=item *

C<dot8> - 8x8 dot dither.

=item *

C<dot4> - 4x4 dot dither

=item *

C<hline> - horizontal line dither.

=item *

C<vline> - vertical line dither.

=item *

C</line>, C<slashline> - diagonal line dither

=item *

C<\line>, C<backline> - diagonal line dither

=item *

C<tiny> - dot matrix dither (currently the default).  This is probably
the best for displays (like web pages).

=item *

C<custom> - A custom dither matrix is used - see C<tr_map>.

=back

=item *

C<tr_map> - When tr_orddith is custom this defines an 8 x 8 matrix of
integers representing the transparency threshold for pixels
corresponding to each position.  This should be a 64 element array
where the first 8 entries correspond to the first row of the matrix.
Values should be between 0 and 255.

=item *

C<make_colors> - Defines how the quantization engine will build the
palette(s).  Currently this is ignored if C<translate> is C<giflib>,
but that may change.  Possible values are:

=over

=item *

C<none> - only colors supplied in 'colors' are used.

=item *

C<webmap> - the web color map is used (need URL here.)

=item *

C<addi> - The original code for generating the color map (Addi's code) is
used.

=item *

C<mediancut> - Uses a median-cut algorithm, faster than C<addi>, but not
as good a result.

=item *

C<mono>, C<monochrome> - a fixed black and white palette, suitable for
producing bi-level images (eg. facsimile)

=item *

C<gray>, C<gray4>, C<gray16> - make fixed gray palette with 256, 4 or
16 entries respectively.

=back

Other methods may be added in the future.

=item *

C<colors> - an arrayref containing Imager::Color objects, which
represents the starting set of colors to use in translating the
images.  C<webmap> will ignore this.  On return the final colors used
are copied back into this array (which is expanded if necessary.)

=item *

C<max_colors> - the maximum number of colors to use in the image.

=item *

C<translate> - The method used to translate the RGB values in the
source image into the colors selected by make_colors.  Note that
make_colors is ignored when C<translate> is C<giflib>.

Possible values are:

=over

=item *

C<giflib> - this is a historical equivalent for C<closest> that also
forces C<make_colors> to C<mediancut>.

=item *

C<closest> - the closest color available is used.

=item *

C<perturb> - the pixel color is modified by C<perturb>, and the
closest color is chosen.

=item *

C<errdiff> - an error diffusion dither is performed.  If the supplied
(or generated) palette contains only grays the source colors are
converted to gray before error diffusion is performed.

=back

It's possible other C<translate> values will be added.

=item *

C<errdiff> - The type of error diffusion dither to perform.  These
values (except for custom) can also be used in tr_errdif.

=for stopwords Floyd-Steinberg Jarvis Judice Ninke Stucki

=over

=item *

C<floyd> - Floyd-Steinberg dither

=item *

C<jarvis> - Jarvis, Judice and Ninke dither

=item *

C<stucki> - Stucki dither

=item *

C<custom> - custom.  If you use this you must also set C<errdiff_width>,
C<errdiff_height> and C<errdiff_map>.

=back

=item *

C<errdiff_width>, C<errdiff_height>, C<errdiff_orig>, C<errdiff_map> -
When C<translate> is C<errdiff> and C<errdiff> is C<custom> these
define a custom error diffusion map.  C<errdiff_width> and
C<errdiff_height> define the size of the map in the arrayref in
C<errdiff_map>.  C<errdiff_orig> is an integer which indicates the
current pixel position in the top row of the map.

=item *

C<perturb> - When translate is C<perturb> this is the magnitude of the
random bias applied to each channel of the pixel before it is looked
up in the color table.

=back

=head1 INITIALIZATION

This documents the Imager initialization function, which you will
almost never need to call.

=over

=item init()

This is a function, not a method.

This function is a mess, it can take the following named parameters:

=over

=item *

C<log> - name of a log file to log Imager's actions to.  Not all
actions are logged, but the debugging memory allocator does log
allocations here.  Ignored if Imager has been built without logging
support.  Preferably use the open_log() method instead.

=item *

C<loglevel> - the maximum level of message to log.  Default: 1.

=item *

C<warn_obsolete> - if this is non-zero then Imager will warn when you
attempt to use obsoleted parameters or functionality.  This currently
only includes the old GIF output options instead of tags.

=item *

C<t1log> - if non-zero then T1lib will be configured to produce a log
file.  This will fail if there are any existing T1lib font objects.

=back

Example:

  Imager::init(log => 'trace.log', loglevel => 9);

=back

=head1 LOGGING METHODS

Imager can open an internal log to send debugging information to.
This log is extensively used in Imager's tests, but you're unlikely to
use it otherwise.

If Imager has been built with logging disabled, the methods fail
quietly.

=over

=item open_log()

Open the Imager debugging log file.

=over

=item *

C<log> - the file name to log to.  If this is undef logging information
is sent to the standard error stream.

=item *

C<loglevel> the level of logging to produce.  Default: 1.

=back

Returns a true value if the log file was opened successfully.

  # send debug output to test.log
  Imager->open_log(log => "test.log");

  # send debug output to stderr
  Imager->open_log();

=item close_log()

Close the Imager debugging log file and disable debug logging.

No parameters.

  Imager->close_log();

=item log()

 Imager->log($message)
 Imager->log($message, $level)

This method does not use named parameters.

The default for C<$level> is 1.

Send a message to the debug log.

  Imager->log("My code got here!");

=item is_logging()

Returns a true value if logging is enabled.

=back

=head1 AUTHOR

Tony Cook <tonyc@cpan.org>, Arnar M. Hrafnkelsson

=head1 SEE ALSO

Imager(3), Imager::Files(3), Imager::Draw(3),
Imager::Color(3), Imager::Fill(3), Imager::Font(3),
Imager::Transformations(3), Imager::Engines(3), Imager::Filters(3),
Imager::Expr(3), Imager::Matrix2d(3), Imager::Fountain(3)

=cut