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<!-- ##### SECTION Title ##### -->
Engines
<!-- ##### SECTION Short_Description ##### -->
Language-specific and rendering-system-specific processing
<!-- ##### SECTION Long_Description ##### -->
<para>
Pango utilizes a module architecture in which the language-specific
and render-system-specific components are provided by loadable
modules. Each loadable module supplies one or more
<firstterm>engines</firstterm>. Each <firstterm>engine</firstterm>
has an associated <firstterm>engine type</firstterm> and
<firstterm>render type</firstterm>. These two types are represented by
strings.
</para>
<para>
Each dynamically-loaded module exports several functions which provide
the public API. These functions are script_engine_list(),
script_engine_load() and script_engine_unload(). The latter two
functions are used for loading and unloading modules, while the first
function is used when building a catalog of all available modules.
</para>
<!-- ##### SECTION See_Also ##### -->
<para>
</para>
<!-- ##### STRUCT PangoEngineInfo ##### -->
<para>
The #PangoEngineInfo structure contains information about a particular
engine. It contains the following fields:
</para>
@id: a unique string ID for the engine.
@engine_type: a string identifying the engine type.
@render_type: a string identifying the render type.
@ranges: a pointer to an array of #PangoEngineRange
structures. Each structure contains information
about a range of Unicode code points that
this engine handles.
@n_ranges: the number of elements in <structfield>ranges</structfield>.
<!-- ##### STRUCT PangoEngineRange ##### -->
<para>
The PangoEngineRange structure contains
information about a range of Unicode code
points. It contains the following fields:
</para>
@start: the first code point in the range.
@end: the last code point for the range.
@langs: a semicolon separated list of languages that this
engine handles for this range. This may be empty,
in which case the engine is saying that it is a
fallback choice for all languages for this range,
but should not be used if another engine
indicates that it is specific for the language for
a given code point. An entry in this list of "*"
indicates that this engine is specific to all
languages for this range.
<!-- ##### STRUCT PangoEngine ##### -->
<para>
The #PangoEngine structure contains basic
information common to all script engines. It
contains the following fields:
</para>
@id: a unique string ID for this language engine.
@type: the "type" of the engine. (Is this engine type or render type??).
@length: the length of the entire structure in bytes. This is
provided so that new functions can be added at the
end of subtypes of #PangoEngine without breaking
older modules.
<!-- ##### STRUCT PangoEngineLang ##### -->
<para>
The #PangoEngineLang structure extends the
basic #PangoEngine structure to engines that
deal with the rendering-system independent part
of of the rendering pipeline. It contains the following fields:
</para>
@engine: a nested structure containing basic engine data.
@script_break: a function that provides an implementation for pango_break().
<!-- ##### STRUCT PangoEngineShape ##### -->
<para>
The #PangoEngineShape structure extends the
basic #PangoEngine structure to engines that
deal with the rendering-system dependent part
of of the rendering pipeline. It contains the following fields:
</para>
@engine: a nested structure containing basic engine data.
@script_shape: a function that provides an implementation for pango_shape.
@get_coverage:
<!-- ##### MACRO PANGO_ENGINE_TYPE_LANG ##### -->
<para>
A string constant defining the engine type
for <firstterm>language engines</firstterm>.
These engines have a engine structure of
type #PangoEngineLang.
</para>
<!-- ##### MACRO PANGO_ENGINE_TYPE_SHAPE ##### -->
<para>
A string constant defining the engine type
for <firstterm>shaping engines</firstterm>.
These engines have a engine structure of
type #PangoEngineShape.
</para>
<!-- ##### MACRO PANGO_RENDER_TYPE_NONE ##### -->
<para>
A string constant defining the render type
for engines that are not rendering-system
specific.
</para>
<!-- ##### FUNCTION script_engine_list ##### -->
<para>
Function to be provided by a module to list the engines that the
module supplies. The function stores a pointer to an array
of #PangoEngineInfo structures and the length of that array in
the given location.
</para>
@engines: location to store a pointer to an array of engines.
@n_engines: location to store the number of elements in @engines.
<!-- ##### FUNCTION script_engine_load ##### -->
<para>
Function to be provided by a module to load a particular engine.
</para>
@id: the ID from the #PangoEngineInfo structure of the
module to load.
@Returns: the newly created script engine.
<!-- ##### FUNCTION script_engine_unload ##### -->
<para>
Function to be provided by a module to unload an engine loaded
with script_engine_load().
</para>
@engine: the engine to unload.
<!-- Local Variables: -->
<!-- sgml-parent-document: ("../pango-docs.sgml" "book" "refsect2") -->
<!-- End: -->
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