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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="id-1.2.4"></a>Recommendations for Applications</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="ch02.html#sizing">How sizing works in SVG</a></span></dt>
<dd><dl><dt><span class="section"><a href="ch02.html#id-1.2.4.2.6">How librsvg computes document sizes</a></span></dt></dl></dd>
<dt><span class="section"><a href="recommendations-assets.html">Recommendations for applications with SVG assets</a></span></dt>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sizing"></a>How sizing works in SVG</h2></div></div></div>
<p>
SVG documents are <span class="emphasis"><em>scalable</em></span>. The
conventional way to position SVG documents, which comes from the
web platform, is to consider a <span class="emphasis"><em>viewport</em></span> in
which to place the SVG document — that is, a rectangular region
to where the SVG will be scaled and positioned.
</p>
<p>
SVG renderers are supposed to use the viewport provided by the
application, plus the SVG document's <code class="literal">width</code>
and <code class="literal">height</code> attributes, and its
<code class="literal">viewBox</code> attribute, to compute the position
and size for the rendered document.
</p>
<p>
Ideally, the toplevel <code class="literal"><svg></code> element of
an SVG document will contain <code class="literal">width</code> and
<code class="literal">height</code> attributes, that indicate the
proportions and "natural size" of the document. When those
attributes are present, the SVG renderer can unambiguously
figure out the natural aspect ratio of the document, and can
also suggest a natural size for the document. Since SVGs are
scalable, it is not mandatory to actually use its natural size;
it can be scaled arbitrarily. Of course, it is up to each
application how an SVG document will be scaled: a web browser
would want to consider the semantics of embedding images in
HTML, which may be different from a GUI toolkit loading SVG
assets with hard-coded sizes.
</p>
<p>
If an SVG document's toplevel <code class="literal"><svg></code>
element does not have <code class="literal">width</code> and
<code class="literal">height</code> attributes, then the SVG renderer can
try to figure out the document's aspect ratio from the
<code class="literal">viewBox</code> attribute. If there is no
<code class="literal">viewBox</code> either, then the SVG renderer cannot
easily figure out the natural size of the document. It can
either set a 1:1 scaling matrix within the application's
viewport and render the SVG there, or it can actually try to
compute the size of each object in the SVG document to figure
out the size. The latter is a moderately expensive operation,
and can be avoided by having the SVG document specify
<code class="literal">width</code> and <code class="literal">height</code>
attributes.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.2.4.2.6"></a>How librsvg computes document sizes</h3></div></div></div>
<p>
Librsvg looks for the <code class="literal">width</code> and
<code class="literal">height</code> attributes in the toplevel
<code class="literal"><svg></code> element. If they are present,
librsvg uses them for the "natural" size of the SVG, and this
also defines the aspect ratio. The size has actual units
(pixels, centimeters, etc.) depending on the value of the
<code class="literal">width</code> and <code class="literal">height</code>
attributes.
</p>
<p>
If there are no <code class="literal">width</code> or
<code class="literal">height</code> attributes in the toplevel
<code class="literal"><svg></code>, librsvg looks for the
<code class="literal">viewBox</code> attribute. If present, this
defines the aspect ratio and a "natural" size in pixels.
</p>
<p>
In both cases above (with
<code class="literal">width</code>/<code class="literal">height</code> and/or
<code class="literal">viewBox</code>), librsvg can determine the
"natural" size and aspect ratio of an SVG document immediately
after loading.
</p>
<p>
Otherwise, if none of those attributes are present in the
toplevel <code class="literal"><svg></code> element, librsvg must
actually compute the coverage of all the graphical elements in
the SVG. This is a moderately expensive operation, and
depends on the complexity of the document.
</p>
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