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- <div class="section" id="extending">
- <h1>Extending</h1>
- <div class="contents local topic" id="contents">
- <ul class="simple">
- <li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
- <li><a class="reference internal" href="#defining-new-color-spaces" id="id2">Defining new color spaces</a></li>
- <li><a class="reference internal" href="#defining-new-channel-types" id="id3">Defining new channel types</a></li>
- <li><a class="reference internal" href="#overloading-color-conversion" id="id4">Overloading color conversion</a></li>
- <li><a class="reference internal" href="#defining-new-image-views" id="id5">Defining new image views</a></li>
- </ul>
- </div>
- <div class="section" id="overview">
- <h2><a class="toc-backref" href="#id1">Overview</a></h2>
- <p>You can define your own pixel iterators, locators, image views,
- images, channel types, color spaces and algorithms. You can make
- virtual images that live on the disk, inside a jpeg file, somewhere on
- the internet, or even fully-synthetic images such as the Mandelbrot
- set. As long as they properly model the corresponding concepts, they
- will work with any existing GIL code. Most such extensions require no
- changes to the library and can thus be supplied in another module.</p>
- </div>
- <div class="section" id="defining-new-color-spaces">
- <h2><a class="toc-backref" href="#id2">Defining new color spaces</a></h2>
- <p>Each color space is in a separate file. To add a new color space, just
- copy one of the existing ones (like rgb.hpp) and change it
- accordingly. If you want color conversion support, you will have to
- provide methods to convert between it and the existing color spaces
- (see color_convert.h). For convenience you may want to provide useful
- typedefs for pixels, pointers, references and images with the new
- color space (see typedefs.h).</p>
- </div>
- <div class="section" id="defining-new-channel-types">
- <h2><a class="toc-backref" href="#id3">Defining new channel types</a></h2>
- <p>Most of the time you don’t need to do anything special to use a new
- channel type. You can just use it:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="k">typedef</span> <span class="n">pixel</span><span class="o"><</span><span class="kt">double</span><span class="p">,</span><span class="n">rgb_layout_t</span><span class="o">></span> <span class="n">rgb64_pixel_t</span><span class="p">;</span> <span class="c1">// 64 bit RGB pixel</span>
- <span class="k">typedef</span> <span class="n">rgb64_pixel</span><span class="o">*</span> <span class="n">rgb64_pixel_ptr_t</span><span class="p">;</span><span class="c1">// pointer to 64-bit interleaved data</span>
- <span class="k">typedef</span> <span class="n">image_type</span><span class="o"><</span><span class="kt">double</span><span class="p">,</span><span class="n">rgb_layout_t</span><span class="o">>::</span><span class="n">type</span> <span class="n">rgb64_image_t</span><span class="p">;</span> <span class="c1">// 64-bit interleaved image</span>
- </pre></div>
- </div>
- <p>If you want to use your own channel class, you will need to provide a
- specialization of <code class="docutils literal"><span class="pre">channel_traits</span></code> for it (see channel.hpp). If you
- want to do conversion between your and existing channel types, you
- will need to provide an overload of <code class="docutils literal"><span class="pre">channel_convert</span></code>.</p>
- </div>
- <div class="section" id="overloading-color-conversion">
- <h2><a class="toc-backref" href="#id4">Overloading color conversion</a></h2>
- <p>Suppose you want to provide your own color conversion. For example,
- you may want to implement higher quality color conversion using color
- profiles. Typically you may want to redefine color conversion only in
- some instances and default to GIL’s color conversion in all other
- cases. Here is, for example, how to overload color conversion so that
- color conversion to gray inverts the result but everything else
- remains the same:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// make the default use GIL's default</span>
- <span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcColorSpace</span><span class="p">,</span> <span class="k">typename</span> <span class="n">DstColorSpace</span><span class="o">></span>
- <span class="k">struct</span> <span class="nl">my_color_converter_impl</span>
- <span class="p">:</span> <span class="k">public</span> <span class="n">default_color_converter_impl</span><span class="o"><</span><span class="n">SrcColorSpace</span><span class="p">,</span><span class="n">DstColorSpace</span><span class="o">></span> <span class="p">{};</span>
- <span class="c1">// provide specializations only for cases you care about</span>
- <span class="c1">// (in this case, if the destination is grayscale, invert it)</span>
- <span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcColorSpace</span><span class="o">></span>
- <span class="k">struct</span> <span class="n">my_color_converter_impl</span><span class="o"><</span><span class="n">SrcColorSpace</span><span class="p">,</span><span class="n">gray_t</span><span class="o">></span>
- <span class="p">{</span>
- <span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">DstP</span><span class="o">></span> <span class="c1">// Model PixelConcept</span>
- <span class="kt">void</span> <span class="k">operator</span><span class="p">()(</span><span class="k">const</span> <span class="n">SrcP</span><span class="o">&</span> <span class="n">src</span><span class="p">,</span> <span class="n">DstP</span><span class="o">&</span> <span class="n">dst</span><span class="p">)</span> <span class="k">const</span>
- <span class="p">{</span>
- <span class="n">default_color_converter_impl</span><span class="o"><</span><span class="n">SrcColorSpace</span><span class="p">,</span><span class="n">gray_t</span><span class="o">></span><span class="p">()(</span><span class="n">src</span><span class="p">,</span><span class="n">dst</span><span class="p">);</span>
- <span class="n">get_color</span><span class="p">(</span><span class="n">dst</span><span class="p">,</span><span class="n">gray_color_t</span><span class="p">())</span><span class="o">=</span><span class="n">channel_invert</span><span class="p">(</span><span class="n">get_color</span><span class="p">(</span><span class="n">dst</span><span class="p">,</span><span class="n">gray_color_t</span><span class="p">()));</span>
- <span class="p">}</span>
- <span class="p">};</span>
- <span class="c1">// create a color converter object that dispatches to your own implementation</span>
- <span class="k">struct</span> <span class="n">my_color_converter</span>
- <span class="p">{</span>
- <span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">DstP</span><span class="o">></span> <span class="c1">// Model PixelConcept</span>
- <span class="kt">void</span> <span class="k">operator</span><span class="p">()(</span><span class="k">const</span> <span class="n">SrcP</span><span class="o">&</span> <span class="n">src</span><span class="p">,</span><span class="n">DstP</span><span class="o">&</span> <span class="n">dst</span><span class="p">)</span> <span class="k">const</span>
- <span class="p">{</span>
- <span class="k">typedef</span> <span class="k">typename</span> <span class="n">color_space_type</span><span class="o"><</span><span class="n">SrcP</span><span class="o">>::</span><span class="n">type</span> <span class="n">SrcColorSpace</span><span class="p">;</span>
- <span class="k">typedef</span> <span class="k">typename</span> <span class="n">color_space_type</span><span class="o"><</span><span class="n">DstP</span><span class="o">>::</span><span class="n">type</span> <span class="n">DstColorSpace</span><span class="p">;</span>
- <span class="n">my_color_converter_impl</span><span class="o"><</span><span class="n">SrcColorSpace</span><span class="p">,</span><span class="n">DstColorSpace</span><span class="o">></span><span class="p">()(</span><span class="n">src</span><span class="p">,</span><span class="n">dst</span><span class="p">);</span>
- <span class="p">}</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>GIL color conversion functions take the color converter as an
- optional parameter. You can pass your own color converter:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">color_converted_view</span><span class="o"><</span><span class="n">gray8_pixel_t</span><span class="o">></span><span class="p">(</span><span class="n">img_view</span><span class="p">,</span><span class="n">my_color_converter</span><span class="p">());</span>
- </pre></div>
- </div>
- </div>
- <div class="section" id="defining-new-image-views">
- <h2><a class="toc-backref" href="#id5">Defining new image views</a></h2>
- <p>You can provide your own pixel iterators, locators and views,
- overriding either the mechanism for getting from one pixel to the next
- or doing an arbitrary pixel transformation on dereference. For
- example, let’s look at the implementation of <code class="docutils literal"><span class="pre">color_converted_view</span></code>
- (an image factory method that, given any image view, returns a new,
- otherwise identical view, except that color conversion is performed on
- pixel access). First we need to define a model of
- <code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code>; a function object that will be
- called when we dereference a pixel iterator. It will call
- <code class="docutils literal"><span class="pre">color_convert</span></code> to convert to the destination pixel type:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcConstRefP</span><span class="p">,</span> <span class="c1">// const reference to the source pixel</span>
- <span class="k">typename</span> <span class="n">DstP</span><span class="o">></span> <span class="c1">// Destination pixel value (models PixelValueConcept)</span>
- <span class="k">class</span> <span class="nc">color_convert_deref_fn</span>
- <span class="p">{</span>
- <span class="k">public</span><span class="o">:</span>
- <span class="k">typedef</span> <span class="n">color_convert_deref_fn</span> <span class="n">const_t</span><span class="p">;</span>
- <span class="k">typedef</span> <span class="n">DstP</span> <span class="n">value_type</span><span class="p">;</span>
- <span class="k">typedef</span> <span class="n">value_type</span> <span class="n">reference</span><span class="p">;</span> <span class="c1">// read-only dereferencing</span>
- <span class="k">typedef</span> <span class="k">const</span> <span class="n">value_type</span><span class="o">&</span> <span class="n">const_reference</span><span class="p">;</span>
- <span class="k">typedef</span> <span class="n">SrcConstRefP</span> <span class="n">argument_type</span><span class="p">;</span>
- <span class="k">typedef</span> <span class="n">reference</span> <span class="n">result_type</span><span class="p">;</span>
- <span class="k">static</span> <span class="kt">bool</span> <span class="k">constexpr</span> <span class="n">is_mutable</span> <span class="o">=</span> <span class="nb">false</span><span class="p">;</span>
- <span class="n">result_type</span> <span class="nf">operator</span><span class="p">()(</span><span class="n">argument_type</span> <span class="n">srcP</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
- <span class="n">result_type</span> <span class="n">dstP</span><span class="p">;</span>
- <span class="n">color_convert</span><span class="p">(</span><span class="n">srcP</span><span class="p">,</span><span class="n">dstP</span><span class="p">);</span>
- <span class="k">return</span> <span class="n">dstP</span><span class="p">;</span>
- <span class="p">}</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>We then use the <code class="docutils literal"><span class="pre">add_deref</span></code> member struct of image views to construct the
- type of a view that invokes a given function object (<code class="docutils literal"><span class="pre">deref_t</span></code>) upon
- dereferencing. In our case, it performs color conversion:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">SrcView</span><span class="p">,</span> <span class="k">typename</span> <span class="n">DstP</span><span class="o">></span>
- <span class="k">struct</span> <span class="n">color_converted_view_type</span>
- <span class="p">{</span>
- <span class="k">private</span><span class="o">:</span>
- <span class="k">typedef</span> <span class="k">typename</span> <span class="n">SrcView</span><span class="o">::</span><span class="n">const_t</span><span class="o">::</span><span class="n">reference</span> <span class="n">src_pix_ref</span><span class="p">;</span> <span class="c1">// const reference to pixel in SrcView</span>
- <span class="k">typedef</span> <span class="n">color_convert_deref_fn</span><span class="o"><</span><span class="n">src_pix_ref</span><span class="p">,</span> <span class="n">DstP</span><span class="o">></span> <span class="n">deref_t</span><span class="p">;</span> <span class="c1">// the dereference adaptor that performs color conversion</span>
- <span class="k">typedef</span> <span class="k">typename</span> <span class="n">SrcView</span><span class="o">::</span><span class="k">template</span> <span class="n">add_deref</span><span class="o"><</span><span class="n">deref_t</span><span class="o">></span> <span class="n">add_ref_t</span><span class="p">;</span>
- <span class="k">public</span><span class="o">:</span>
- <span class="k">typedef</span> <span class="k">typename</span> <span class="n">add_ref_t</span><span class="o">::</span><span class="n">type</span> <span class="n">type</span><span class="p">;</span> <span class="c1">// the color converted view type</span>
- <span class="k">static</span> <span class="n">type</span> <span class="nf">make</span><span class="p">(</span><span class="k">const</span> <span class="n">SrcView</span><span class="o">&</span> <span class="n">sv</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="n">add_ref_t</span><span class="o">::</span><span class="n">make</span><span class="p">(</span><span class="n">sv</span><span class="p">,</span> <span class="n">deref_t</span><span class="p">());</span> <span class="p">}</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>Finally our <code class="docutils literal"><span class="pre">color_converted_view</span></code> code simply creates color-converted view
- from the source view:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">DstP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">View</span><span class="o">></span> <span class="kr">inline</span>
- <span class="k">typename</span> <span class="n">color_converted_view_type</span><span class="o"><</span><span class="n">View</span><span class="p">,</span><span class="n">DstP</span><span class="o">>::</span><span class="n">type</span> <span class="n">color_convert_view</span><span class="p">(</span><span class="k">const</span> <span class="n">View</span><span class="o">&</span> <span class="n">src</span><span class="p">)</span>
- <span class="p">{</span>
- <span class="k">return</span> <span class="n">color_converted_view_type</span><span class="o"><</span><span class="n">View</span><span class="p">,</span><span class="n">DstP</span><span class="o">>::</span><span class="n">make</span><span class="p">(</span><span class="n">src</span><span class="p">);</span>
- <span class="p">}</span>
- </pre></div>
- </div>
- <p>(The actual color convert view transformation is slightly more
- complicated, as it takes an optional color conversion object, which
- allows users to specify their own color conversion methods). See the
- GIL tutorial for an example of creating a virtual image view that
- defines the Mandelbrot set.</p>
- </div>
- </div>
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