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<div class="section" id="concepts">
<h1>Concepts<a class="headerlink" href="#concepts" title="Permalink to this headline">¶</a></h1>
<div class="section" id="image-classes">
<h2>Image classes<a class="headerlink" href="#image-classes" title="Permalink to this headline">¶</a></h2>
<p>An image can either be a file, an image loaded into an underlying library or a
simple buffer of pixels. Each of these states has its own Python class
(subclass of <code class="xref py py-class docutils literal"><span class="pre">willow.image.Image</span></code>).</p>
<p>For example <code class="docutils literal"><span class="pre">JPEGImageFile</span></code>, <code class="docutils literal"><span class="pre">PillowImage</span></code> and <code class="docutils literal"><span class="pre">RGBAImageBuffer</span></code> are three
of the image classes in Willow.</p>
</div>
<div class="section" id="operations">
<h2>Operations<a class="headerlink" href="#operations" title="Permalink to this headline">¶</a></h2>
<p>These are functions that perform actions on an image in a particular state. For
example, <code class="docutils literal"><span class="pre">resize</span></code> and <code class="docutils literal"><span class="pre">crop</span></code>.</p>
<p>Operations can either be defined as methods on the image class or as functions
registered separately.</p>
<p>All operations are registered in a central registry and will appear as a method
on all other image classes. If it’s called from a class that doesn’t implement
the operation, the image will be automatically converted to the nearest image
class that supports it and the operation is run on that.</p>
<p>Operations that alter an image return a new image object instead of altering the
source one. This also means that if a conversion took place, the new image’s
class would be different.</p>
</div>
<div class="section" id="converters">
<h2>Converters<a class="headerlink" href="#converters" title="Permalink to this headline">¶</a></h2>
<p>These are functions that convert an image between two image classes. For
example, a converter from <code class="docutils literal"><span class="pre">JPEGImageFile</span></code> to <code class="docutils literal"><span class="pre">PillowImage</span></code> would simply be a
function that calls <code class="docutils literal"><span class="pre">PIL.Image.open</span></code> on the underlying file to get a Pillow
image.</p>
<p>Like operations, these can either be methods on the image class or registered
separately.</p>
<p>Each converter has a cost which helps Willow decide which is the best available
image library to use for a particular file format.</p>
</div>
<div class="section" id="registry">
<h2>Registry<a class="headerlink" href="#registry" title="Permalink to this headline">¶</a></h2>
<p>The registry is where all image classes, operations and converters are
registered. It contains methods to allow you to register new items and even
override existing ones.</p>
<p>It also is responsible for finding operations and planning routes between image
classes.</p>
</div>
<div class="section" id="plugins">
<h2>Plugins<a class="headerlink" href="#plugins" title="Permalink to this headline">¶</a></h2>
<p>These are used to group related image classes, operations and converters
together allowing them to be registered as a single unit.</p>
<p>The convention within Willow is to create a single plugin for each underlying
library. The default ones are “pillow”, “wand” and “opencv”.</p>
<p>Plugins can be registered even if the underlying library is not installed. This
allows Willow to generate a useful error message if an operation is requested
that only exists in a plugin without an underlying library.</p>
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