Open Source 2D CAD
Important Classes

This section introduces some important part of the QCAD Application Framework API.

The Drawing Document

The document that is being viewed or edited is a central part for most applications. Within the QCAD Application Framework, documents can exist independent of any user interface components. Think of a document as a complete CAD drawing.


The following ECMAScript code creates an empty drawing document:

// Creating the storage underneath the document:
var storage = new RMemoryStorage();
// Creating the spatial index:
var spatialIndex = new RSpatialIndexNavel();
// Creating the document:
var document = new RDocument(storage, spatialIndex);
// Creating the document interface
// (required for import / export, graphics scenes and views):
var documentInterface = new RDocumentInterface(document);

Storage and spatial index are required for lower level data storage but usually only act behind the scenes.


The interaction between a user and an application typically consists of a user input, followed by an output of the application to indicate the result of the user input. The most important output when interacting with a CAD system is the visualization of the CAD drawing.


The visualization of a drawing is handled in the QCAD Application Framework through one or multiple graphics views (classes derived from RGraphicsView). A graphics view is typically a visible widget or window. Each graphics view is attached to a graphics scene (RGraphicsScene) which manages the visual representation of the drawing entities. The graphics scene is connected to a document (RDocument) via a document interface (RDocumentInterface).

While the document knows what entities are and how they are geometrically defined, the graphics scene knows how each entity looks like and the graphics view knows how to display that visual representation of an entity to the user. The document interface connects a document to one or multiple graphics scenes and has some other important responsibilities that are outlined later on.

This structure makes it possible to visualize a drawing in multiple views. Each view can have its own scene or share a scene with other views. The following diagram shows the relationship between a document and its graphics scenes and views. Note that any changes to the document automatically trigger an update to all scenes and views attached to it.


Multiple scenes can be attached to a drawing interface to provide different visualizations, for example a 2D visualization and a 3D visualization of the same document.

Multiple views can be attached to each scene to show the scene at a different zoom factor and offset or at a different angle.

The following ECMAScript snippet attaches a graphics scene with one graphics view to a document interface. Note that RGraphicsViewQt is a complete implementation of a graphics view, based on a Qt QWidget. RGraphicsSceneQt is the graphics scene implementation that handles the visualization of the underlying document using QPainterPath objects.

var graphicsScene = new RGraphicsSceneQt(documentInterface)
var graphicsView = new RGraphicsViewQt();

User Input

In the QCAD Application Framework, user input and drawing visualization share the same structures. The document interface is at the heart of both visualization and interaction.

Users typically interact with a graphics view, for example by moving the mouse, clicking a mouse button or pressing a key while a graphics view has the focus. The graphics view receives all such low level events from input devices and translates them into more useful, higher level events. For example the click of the left mouse button at a view coordinate in pixels is translated into a higher level event (RMouseEvent) that also stores the model coordinates of the click. These higher level events are then forwarded to the graphics scene to which the view is attached.

The graphics scene simply forwards the events to the document interface.

The document interface always knows what action (or tool) is currently active. For example if the user is drawing a line, the current action is a class derived from RAction that handles all user interaction to draw a line (e.g. Line2P). The document interface forwards all events it receives to the current action.

The current action is responsible for handling the user input, typically by reacting to it and visualizing the result.

This flow of user input and application output is shown in the figure below:



Whenever a document has to be produced from existing data, a class that is derived from RImporter should be used. If the source is a file, a specialized base class RFileImporter can be used.

The QCAD Application Framework provides an importer for the DXF format.


When a drawing is exported, an exporter, derived from RExporter is used. Graphics scenes are also derived from RExporter, since they are used to 'export' a drawing into a graphical representation (e.g. painter paths or an OpenGL context). For exporters that export to a file, the specialized class RFileExporter can be used as a base class.

The QCAD Application Framework provides an exporter for the DXF format.