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<h6 id="header">RMagick 2.13.1 User's Guide and Reference</h6>
<div class="nav">
« <a href="optequiv.html">Prev</a> | <a href=
"index.html">Contents</a> | <a href=
"magick.html">Next</a> »
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<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 > 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>
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