Arnar Mar Hrafnkelsson > Imager > Imager::Transformations

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NAME ^

Imager::Transformations - Simple transformations of one image into another.

SYNOPSIS ^

  use Imager;

  $newimg = $img->copy();

  $newimg = $img->scale(xpixels=>400);
  $newimg = $img->scale(xpixels=>400, ypixels=>400);
  $newimg = $img->scale(xpixels=>400, ypixels=>400, type=>'min');
  $newimg = $img->scale(scalefactor=>0.25);

  $newimg = $img->crop(left=>50, right=>100, top=>10, bottom=>100); 
  $newimg = $img->crop(left=>50, top=>10, width=>50, height=>90);

  $dest->paste(left=>40,top=>20,img=>$logo);

  $img->rubthrough(src=>$srcimage,tx=>30, ty=>50);
  $img->rubthrough(src=>$srcimage,tx=>30, ty=>50,
                   src_minx=>20, src_miny=>30,
                   src_maxx=>20, src_maxy=>30);


  $img->flip(dir=>"h");       # horizontal flip
  $img->flip(dir=>"vh");      # vertical and horizontal flip
  $newimg = $img->copy->flip(dir=>"v"); # make a copy and flip it vertically

  my $rot20 = $img->rotate(degrees=>20);
  my $rotpi4 = $img->rotate(radians=>3.14159265/4);


  # Convert image to gray
  $new = $img->convert(preset=>'grey');          

  # Swap red/green channel  
  $new = $img->convert(matrix=>[ [ 0, 1, 0 ],
                                 [ 1, 0, 0 ],
                                 [ 0, 0, 1 ] ]);

  # image composition
  $img->compose(img=>$source, mask=>$mask, left=>100, top=>50)
    or die $img->errstr;

  # limit the range of red channel from 0..255 to 0..127
  @map = map { int( $_/2 } 0..255;
  $img->map( red=>\@map );

  # Apply a Gamma of 1.4
  my $gamma = 1.4;
  my @map = map { int( 0.5 + 255*($_/255)**$gamma ) } 0..255;
  $img->map(all=>\@map);  # inplace conversion

DESCRIPTION ^

The methods described in Imager::Transformations fall into two categories. Either they take an existing image and modify it in place, or they return a modified copy.

Functions that modify inplace are flip(), paste() and rubthrough(). If the original is to be left intact it's possible to make a copy and alter the copy:

  $flipped = $img->copy()->flip(dir=>'h');

Image copying/resizing/cropping/rotating

A list of the transformations that do not alter the source image follows:

copy

To create a copy of an image use the copy() method. This is usefull if you want to keep an original after doing something that changes the image.

  $newimg = $orig->copy();
scale

To scale an image so porportions are maintained use the $img->scale() method. if you give either a xpixels or ypixels parameter they will determine the width or height respectively. If both are given the one resulting in a larger image is used, unless you set the type parameter to 'min'. example: $img is 700 pixels wide and 500 pixels tall.

  $newimg = $img->scale(xpixels=>400); # 400x285
  $newimg = $img->scale(ypixels=>400); # 560x400

  $newimg = $img->scale(xpixels=>400,ypixels=>400); # 560x400
  $newimg = $img->scale(xpixels=>400,ypixels=>400,type=>'min'); # 400x285

  $newimg = $img->scale(scalefactor=>0.25); 175x125 
  $newimg = $img->scale(); # 350x250

if you want to create low quality previews of images you can pass qtype=>'preview' to scale and it will use nearest neighbor sampling instead of filtering. It is much faster but also generates worse looking images - especially if the original has a lot of sharp variations and the scaled image is by more than 3-5 times smaller than the original.

If you need to scale images per axis it is best to do it simply by calling scaleX and scaleY. You can pass either 'scalefactor' or 'pixels' to both functions.

crop

Another way to resize an image is to crop it. The parameters to crop are the edges of the area that you want in the returned image, where the right and bottom edges are non-inclusive. If a parameter is omitted a default is used instead.

  # the first two produce the same image
  $newimg = $img->crop(left=>50, right=>100, top=>10, bottom=>100); 
  $newimg = $img->crop(left=>50, top=>10, width=>50, height=>90);
  $newimg = $img->crop(left=>50, right=>100); # top 

You can also specify width and height parameters which will produce a new image cropped from the center of the input image, with the given width and height.

  $newimg = $img->crop(width=>50, height=>50);

The width and height parameters take precedence over the left/right and top/bottom parameters respectively.

rotate

Use the rotate() method to rotate an image. This method will return a new, rotated image.

To rotate by an exact amount in degrees or radians, use the 'degrees' or 'radians' parameter:

  my $rot20 = $img->rotate(degrees=>20);
  my $rotpi4 = $img->rotate(radians=>3.14159265/4);

Exact image rotation uses the same underlying transformation engine as the matrix_transform() method.

To rotate in steps of 90 degrees, use the 'right' parameter:

  my $rotated = $img->rotate(right=>270);

Rotations are clockwise for positive values.

Image pasting/flipping/

A list of the transformations that alter the source image follows:

paste

To copy an image to onto another image use the paste() method.

  $dest->paste(left=>40,top=>20,img=>$logo);

That copies the entire $logo image onto the $dest image so that the upper left corner of the $logo image is at (40,20).

rubthrough

A more complicated way of blending images is where one image is put 'over' the other with a certain amount of opaqueness. The method that does this is rubthrough.

  $img->rubthrough(src=>$overlay,
                   tx=>30,       ty=>50,
                   src_minx=>20, src_miny=>30,
                   src_maxx=>20, src_maxy=>30);

That will take the sub image defined by $overlay and [src_minx,src_maxx)[src_miny,src_maxy) and overlay it on top of $img with the upper left corner at (30,50). You can rub 2 or 4 channel images onto a 3 channel image, or a 2 channel image onto a 1 channel image. The last channel is used as an alpha channel. To add an alpha channel to an image see convert().

flip

An inplace horizontal or vertical flip is possible by calling the flip() method. If the original is to be preserved it's possible to make a copy first. The only parameter it takes is the dir parameter which can take the values h, v, vh and hv.

  $img->flip(dir=>"h");       # horizontal flip
  $img->flip(dir=>"vh");      # vertical and horizontal flip
  $nimg = $img->copy->flip(dir=>"v"); # make a copy and flip it vertically

Color transformations

You can use the convert method to transform the color space of an image using a matrix. For ease of use some presets are provided.

The convert method can be used to:

The currently defined presets are:

gray
grey

converts an RGBA image into a grayscale image with alpha channel, or an RGB image into a grayscale image without an alpha channel.

This weights the RGB channels at 22.2%, 70.7% and 7.1% respectively.

noalpha

removes the alpha channel from a 2 or 4 channel image. An identity for other images.

red
channel0

extracts the first channel of the image into a single channel image

green
channel1

extracts the second channel of the image into a single channel image

blue
channel2

extracts the third channel of the image into a single channel image

alpha

extracts the alpha channel of the image into a single channel image.

If the image has 1 or 3 channels (assumed to be grayscale of RGB) then the resulting image will be all white.

rgb

converts a grayscale image to RGB, preserving the alpha channel if any

addalpha

adds an alpha channel to a grayscale or RGB image. Preserves an existing alpha channel for a 2 or 4 channel image.

For example, to convert an RGB image into a greyscale image:

  $new = $img->convert(preset=>'grey'); # or gray

or to convert a grayscale image to an RGB image:

  $new = $img->convert(preset=>'rgb');

The presets aren't necessary simple constants in the code, some are generated based on the number of channels in the input image.

If you want to perform some other colour transformation, you can use the 'matrix' parameter.

For each output pixel the following matrix multiplication is done:

  | channel[0] |   | $c00, ...,  $c0k |   | inchannel[0] |
  |    ...     | = |       ...        | x |     ...      |
  | channel[k] |   | $ck0, ...,  $ckk |   | inchannel[k] |
                                                          1
Where C<k = $img-E<gt>getchannels()-1>.

So if you want to swap the red and green channels on a 3 channel image:

  $new = $img->convert(matrix=>[ [ 0, 1, 0 ],
                                 [ 1, 0, 0 ],
                                 [ 0, 0, 1 ] ]);

or to convert a 3 channel image to greyscale using equal weightings:

  $new = $img->convert(matrix=>[ [ 0.333, 0.333, 0.334 ] ])

Color Mappings

You can use the map method to map the values of each channel of an image independently using a list of lookup tables. It's important to realize that the modification is made inplace. The function simply returns the input image again or undef on failure.

Each channel is mapped independently through a lookup table with 256 entries. The elements in the table should not be less than 0 and not greater than 255. If they are out of the 0..255 range they are clamped to the range. If a table does not contain 256 entries it is silently ignored.

Single channels can mapped by specifying their name and the mapping table. The channel names are red, green, blue, alpha.

  @map = map { int( $_/2 } 0..255;
  $img->map( red=>\@map );

It is also possible to specify a single map that is applied to all channels, alpha channel included. For example this applies a gamma correction with a gamma of 1.4 to the input image.

  $gamma = 1.4;
  @map = map { int( 0.5 + 255*($_/255)**$gamma ) } 0..255;
  $img->map(all=> \@map);

The all map is used as a default channel, if no other map is specified for a channel then the all map is used instead. If we had not wanted to apply gamma to the alpha channel we would have used:

  $img->map(all=> \@map, alpha=>[]);

Since [] contains fewer than 256 element the gamma channel is unaffected.

It is also possible to simply specify an array of maps that are applied to the images in the rgba order. For example to apply maps to the red and blue channels one would use:

  $img->map(maps=>[\@redmap, [], \@bluemap]);
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