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  <title>RMagick 0.0.0: Common Tasks</title>
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<body>
  <h6 id="header">RMagick 0.0.0 User's Guide and Reference</h6>

  <div class="nav">
    &laquo;&nbsp;<a href="optequiv.html">Prev</a> | <a href=
    "index.html">Contents</a> | <a href=
    "magick.html">Next</a>&nbsp;&raquo;
  </div>

  <h1>Common Tasks</h1>

  <div id="toc">
    <h2>Table of Contents</h2>

    <ul style="margin-left: 15px; padding-top: 1em;">
      <li><a href="#info">Getting information about an
      image</a></li>

      <li><a href="#convert">Converting an image to another
      format</a></li>

      <li><a href="#thumb">Making thumbnails</a></li>

      <li><a href="#resizing">Resizing to a maximum (or minimum)
      size</a></li>

      <li><a href="#blob">Writing to or reading from a string
      instead of a file</a></li>

      <li><a href="#gray">Converting a color image to
      grayscale</a></li>

      <li><a href="#compressing">Compressing image files</a></li>

      <li><a href="#shadow">Making a drop shadow</a></li>
    </ul>
  </div>

  <h2 id="info">Getting information about an image</h2>

  <p>One of the most fundamental operations on an image is simply
  getting basic information about the image. RMagick assigns dozens
  of <a href="imageattrs.html">attributes</a> to an image. All you
  have to do is read the image and then call the attribute methods.
  Here's a Ruby program that takes image filenames from the command
  line and then prints a variety of information about each image to
  the terminal.</p>
  <pre class="example">
require 'RMagick'
ARGV.each { |file|
    puts file
    img = Magick::Image::read(file).first
    puts " Format: #{img.format}"
    puts " Geometry: #{img.columns}x#{img.rows}"
    puts " Class: " + case img.class_type
                            when Magick::DirectClass
                                "DirectClass"
                            when Magick::PseudoClass
                                "PseudoClass"
                        end
    puts " Depth: #{img.depth} bits-per-pixel"
    puts " Colors: #{img.number_colors}"
    puts " Filesize: #{img.filesize}"
    puts " Resolution: #{img.x_resolution.to_i}x#{img.y_resolution.to_i} "+
        "pixels/#{img.units == Magick::PixelsPerInchResolution ?
        "inch" : "centimeter"}"
    if img.properties.length &gt; 0
        puts " Properties:"
        img.properties { |name,value|
            puts %Q| #{name} = "#{value}"|
        }
    end
    }
</pre>

  <h2 id="convert">Converting an image to another format</h2>

  <p>Converting an image to another format is as simple as writing
  the image to a file. ImageMagick uses the output filename
  suffix (".jpg" for JPEG, ".gif" for GIF, for example) or prefix
  ("ps:" for PostScript, for example) to determine the format of
  the output image.</p>

  <h2 id="thumb">Making thumbnails</h2>

  <p>RMagick gives you four different methods for resizing an
  image: <a href="image3.html#resize"><code>resize</code></a>,
  <a href="image3.html#sample"><code>sample</code></a>, <a href=
  "image3.html#scale"><code>scale</code></a>, and <a href=
  "image3.html#thumbnail"><code>thumbnail</code></a>. All four are
  equally easy to use. Specify the number of columns and rows you
  want the thumbnail to have, like this:</p>
  <pre class="example">
img = Image.new "bigimage.gif"
thumb = img.scale(125, 125)
thumb.write "thumb.gif"
</pre>

  <p>Alternatively, just pass a single <code>Float</code> argument
  that represents the change in size. For example, to
  proportionally reduce the size of an image to 25% of its original
  size, do this:</p>
  <pre class="example">
img = Image.new "bigimage.gif"
thumb = img.scale(0.25)
thumb.write "thumb.gif"
</pre>

  <p>The <code>resize</code> method gives you more control by
  allowing you to specify a <a href=
  "constants.html#FilterType">filter</a> to use when scaling the
  image. Some filters produce a better-looking thumbnail at the
  expense of extra processing time. You can also use a
  <code>blur</code> argument, which specifies how much blurriness
  or sharpness the resize method should introduce.</p>

  <p>The <code>sample</code> method, unlike the other two, does not
  do any color interpolation when resizing.</p>

  <p>The <code>thumbnail</code> method is faster than
  <code>resize</code> if the thumbnail is less than 10% of the size
  of the original image.</p>

  <h3>flickr-style thumbnails</h3>

  <p><a href="http://www.flickr.com">flickr</a> thumbnails are 75
  pixels wide and 75 pixels tall. If the original image isn't
  square, the thumbnail is cropped in its larger dimension so that
  the image isn't distorted. You can get make this kind of
  thumbnail with the <a href=
  "image3.html#resize_to_fill">resize_to_fill</a> method.</p>
  <pre class="example">
thumb = img.resize_to_fill(75, 75)
</pre>

  <h2 id="resizing">Resizing to a maximum (or minimum) size</h2>

  <p>Say you need to make all your thumbnails no bigger than 64x64
  but with the same aspect ratio as the original. Or, you don't
  want to resize the image if it's already smaller than 64x64. The
  <a href=
  "image1.html#change_geometry"><code>change_geometry</code></a>
  method can help.</p>

  <p>The <code>change_geometry</code> method accepts an
  ImageMagick <a href="imusage.html#geometry">geometry string</a>
  argument and a block. The geometry string specifies how to change
  the image's size: one or two numbers to specify the new size and
  optional flags to describe any constraints. The
  <code>change_geometry</code> method parses the geometry string
  and computes new width and height values. Then it calls the
  block, passing the values it computed.</p>

  <p>Within the block you can do whatever you want with the new
  values. Typically you'll call one of the resize methods mentioned
  in the previous section and make the resized image the return
  value from the block. The <code>change_geometry</code> method
  then returns that value to its caller.</p>

  <h3>Simple thumbnails</h3>

  <p>If you just want to make sure your thumbnail is no bigger than
  a certain width and height, use the <a href=
  "image3.html#resize_to_fit">resize_to_fit</a> method.</p>
  <pre class="example">
thumb = img.resize_to_fit(75, 75)
</pre>

  <h2><a id="blob" name="blob">Writing to or reading from a string
  instead of a file</a></h2>

  <p>Use the <a href="image1.html#from_blob">Image.from_blob</a>
  method to construct an Image object from a string. Use the
  <a href="image3.html#to_blob">Image#to_blob</a> method to convert
  an image to a string. A blob is simply an in-memory version of an
  image file. That is, you could use <code>File.read</code> to read
  an JPEG file into a string, then create an image by using that
  string as an argument to <code>from_blob</code>. Similarly, if
  you create a string version of an image with
  <code>to_blob</code>, then write the string to a file, any image
  viewer will be able to display it just as if you had written the
  image directly to a file. Blobs are very useful in web
  applications when you want to modify an image and then stream it
  back to the client.</p>

  <p>Use <a href=
  "image2.html#import_pixels">Image#import_pixels</a> to load pixel
  data from a string buffer into an image. The pixel data must be
  in scanline order, right-to-left and top-to-bottom. The data can
  be packed as 8-bit bytes, 16-bit halfwords, 32-bit fullwords, or
  as C floats or doubles. The reciprocal method is <a href=
  "image3.html#export_pixels_to_str">Image#export_pixels_to_str</a>.</p>

  <h2 id="gray">Converting a color image to grayscale</h2>

  <p>Use the <a href=
  "image3.html#quantize"><code>quantize</code></a> method with the
  <a href=
  "constants.html#ColorspaceType">Magick::GRAYColorspace</a>
  argument. If you want real "grayscale," quantize the image to 256
  colors. If you want to convert a color image to black-and-white,
  use 2 colors. (See the <code>demo.rb</code> example.)</p>

  <h2 id="compressing">Compressing image files</h2>

  <p>Many image formats, including JPEG, PDF, and BMP, support
  compressed image files. The type of compression used depends on
  the format. Specify the compression type by assigning a <a href=
  "constants.html#CompressionType">CompressionType</a> value to the
  <a href="info.html#compression">compression</a> optional argument
  to the <a href="image3.html#write">write</a> method.</p>

  <p>The JPEGCompression and ZipCompression types support multiple
  levels of compression. Use the <a href=
  "info.html#quality">quality</a> optional argument to the
  <code>write</code> method. The quality attribute is a number
  between 0 and 100, with 100 representing the least compression.
  When you compress an image using JPEGCompression, more
  compression usually results in a lower-quality image. When you
  compress an image using ZipCompression, more compression usually
  takes longer.</p>

  <p>For more information, see the ImageMagick documentation for
  the <code>-quality</code> option to the utility commands.</p>
  <pre class="example">
img.write("myimage.jpg") { self.quality = 50 }
</pre>

  <h2 id="shadow">Making a drop shadow</h2>

  <p>Here's one way to make a drop shadow behind text. Make the
  shadow first by drawing the text in a light gray color. Position
  the text slightly to the right and down from where the real text
  will be. Then use the <a href=
  "image1.html#blur_image"><code>blur_image</code></a> method to
  make the shadow by blurring the text. Finally, draw the text
  again in whatever color you want. <em>(Click the image to see the
  Ruby program that created it.)</em></p>

  <div id="drop_shadow">
    <a href="javascript:popup('drop_shadow.rb.html')"><img src=
    "ex/drop_shadow.gif" title="Click to see the example script"
    alt="drop shadow example" /></a>
  </div>
  <hr />

  <p class="spacer"></p>

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