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.. _viewport-section:
Viewport
================================
When establishing new viewport by elements **svg**, **symbol** (instanced by **use**) or **image**
several attributes must be processed (**x**, **y**, **width**, **height**, **preserveAspectRatio**, **viewbox**)
to get the new user coordinate system, viewport and clipping path.
SVG++ may do it itself and present result in convenient way.
This behavior is configured by *Viewport Policy*.
*Viewport Events Policy* defines how viewport information is passed to the user code.
**marker** element establish new viewport similar way, when it is instances in path vertices,
so **marker** elements may also be handled by *Viewport Policy* and *Viewport Events Policy*.
Viewport Policy Concept
--------------------------------
::
struct viewport_policy_concept
{
static const bool calculate_viewport = /* true or false */;
static const bool calculate_marker_viewport = /* true or false */;
static const bool calculate_pattern_viewport = /* true or false */;
static const bool viewport_as_transform = /* true or false */;
};
``calculate_viewport = true``
Enables handling of viewport attributes for **svg** and **symbol** elements.
Values will be passed by calls to ``set_viewport`` or
``set_viewbox_transform`` of *Viewport Events Policy*.
``calculate_marker_viewport = true``
The same for **marker** elements.
``calculate_pattern_viewport = true``
The same for **pattern** elements.
``viewport_as_transform = true``
Is checked only if ``calculate_viewport = true``.
``set_viewbox_transform`` method of *Viewport Events Policy* isn't used,
new coordinate system is set by :ref:`Transform Events Policy <transform-section>` instead.
:ref:`Named class template parameter <named-params>` for *Viewport Policy* is ``viewport_policy``.
File ``svgpp/policy/viewport.hpp`` contains some predefined *Viewport Policies*.
``policy::viewport::raw``, used by default, turns off viewport processing in SVG++.
``policy::viewport::as_transform`` sets all boolean members to ``true``,
thus enabling viewport attributes handling by SVG++ and
passing coordinate system changes by *Transform Events Policy*.
Viewport Events Policy Concept
--------------------------------
::
struct viewport_events_policy_concept
{
static void set_viewport(context_type & context, number_type viewport_x, number_type viewport_y,
number_type viewport_width, number_type viewport_height);
static void set_viewbox_transform(context_type & context, number_type translate_x, number_type translate_y,
number_type scale_x, number_type scale_y, bool defer);
static void set_viewbox_size(context_type & context, number_type viewbox_width, number_type viewbox_height);
static void get_reference_viewport_size(context_type & context,
number_type & viewport_width, number_type & viewport_height);
static void disable_rendering(context_type & context);
};
``set_viewport``
Sets new viewport position.
``set_viewbox_transform``
Passes offset and scale that are set by combination of **viewbox** and **preserveAspectRatio**.
Not used if ``viewport_as_transform = true``.
``set_viewbox_size``
Passes size of viewbox, set by **viewbox** attribute. Called only if **viewbox** attribute is present.
It can be used to set viewport size in *Length Factory*.
``get_reference_viewport_size``
See :ref:`viewport_referenced_element`.
``disable_rendering``
SVG Specification says that "[**width** or **height** attribute] value of zero disables rendering of this element".
``disable_rendering`` will be called in such cases, so that application can behave in proper way.
*Document Traversal Control Policy* may be used to skip processing of element content.
:ref:`Named class template parameter <named-params>` for *Viewport Events Policy* is ``viewport_events_policy``.
.. _viewport_referenced_element:
Referenced element
------------------------
When processing **svg** element referenced by **use** or **image**, **symbol** referenced by **use**
values of width and height of referencing element are required.
To pass to SVG++ information that element is referenced by **use** or **image**,
tag of referenced element must be passed as :ref:`referencing_element <referencing_element>` parameter
of ``document_traversal`` class.
In this case ``get_reference_viewport_size`` method of *Viewport Events Policy* will be called.
Implementation of ``get_reference_viewport_size`` must set ``viewport_width`` to
value of **width** attribute of referenced element, if this attribute is set.
And set ``viewport_height`` value of **height** attribute of referenced element, if this attribute is set.
Processed attributes
--------------------------------
If ``calculate_viewport = true`` or ``calculate_pattern_viewport = true`` in *Viewport Policy*,
then SVG++ intercepts and processes attributes,
listed in ``traits::viewport_attributes``::
namespace traits
{
typedef boost::mpl::set6<
tag::attribute::x,
tag::attribute::y,
tag::attribute::width,
tag::attribute::height,
tag::attribute::viewBox,
tag::attribute::preserveAspectRatio
> viewport_attributes;
}
If ``calculate_marker_viewport = true`` in *Viewport Policy*, then SVG++
intercepts and processes marker attributes listed in ``traits::marker_viewport_attributes``::
namespace traits
{
typedef boost::mpl::set6<
tag::attribute::refX,
tag::attribute::refY,
tag::attribute::markerWidth,
tag::attribute::markerHeight,
tag::attribute::viewBox,
tag::attribute::preserveAspectRatio
> marker_viewport_attributes;
}
Processing of this attributes must be :ref:`enabled <processed_attributes>` by the programmer.
Order of viewport processing
-------------------------------------
Viewport attributes will be processed and result will be passed by *Viewport Events Policy*
after all SVG element attributes are processed or when :ref:`notification <notify_in_priority>`
with tag ``tag::event::after_viewport_attributes`` arrives.
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