Bachelor Thesis - Computer Graphics Group

only. Now that the application has been finished, it allows adjustment of advanced
parameters, which are not accessible from the user interface. To achieve
the concept, classes that need to retain persistent state have to implement:
• constructor accepting an XML element (in some cases a separate method
loadFromXml is provided)
• a method returning the actual state represented by a hierarchy of XML
elements (toXmlElement / saveToXml)
Hierarchical object structure used in the application maps perfectly to the
tree-like structure of XML documents. State of an object corresponds to an
XML element, which is a sub-tree in the document. The root of the document
represents the state of the whole application. While saving or loading the
state, parent objects delegate the XML parsing / assembling to their child
elements. It is also possible to store only a part of the configuration to a
separate document. This is used in the user interface to implement export and
import functions.
5.2 Gesture capture library wrapper
The gesture capture library communicates with the main application using
callback functions. This is a low-level interface provided by the library. The
capture library has been designed to be independent of any frameworks or
toolkits that could be used in the hosting application. The callback interface
had to be wrapped for two main reasons:
• it is reasonable to translate the low-level library events into Qt events, to
allow further processing using standard Qt event loop mechanism. This
also makes the dependency on the capture library modular. The rest of
the application would not be affected, if we decided to replace the gesture
capture library. Only the wrapping code would have to be modified
• the callback function is executed in the context of the message thread
created by the capture library. By posting an event to the default Qt
event loop running in the main thread, we switch the context in which
the event is being actually handled to the main thread. Thus, we prevent
possible synchronization problems and the limitation of Qt, which does
not allow certain operations to be performed outside the main thread
Only the gesture-event callback function posts events to the main event loop.
The window-ignored callback has to be processed immediately. As the operation
is constant and thread-safe, the decision can be delegated directly to the
ActionMapping class, mentioned later.
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