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# Creating an interactive UI for viewing graphs: the code

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In a previous entry I wrote about my journey to find a suitable library for creating and interactive UI for viewing and modifying graphs which lead me to the Open Graph Drawing Framework to layout my graph for drawing with Qt. So here is the code for the prototype that I wrote:

# Setting up

First we set up a simple graph consisting of a parent with six children. I link child[0] to child[1] for a bit more complexity.//------------------------------------------------------------------------------// Creating example nodes and edges//------------------------------------------------------------------------------ogdf::Graph g; // Root data container for OGDFauto parent = g.newNode(); for(uint i = 0; i < 6; i++){ g.newNode(); g.newEdge(parent, g.lastNode());}// An edge between two nodes for more complexity in the layout// and to force ogdf to create bent edges g.newEdge(g.firstNode()->succ(), g.firstNode()->succ()->succ());
Then I set up QGraphicsView which is straight forward from the documentation. For simplicity I draw every node as a square of 40x40 pixels. It is also possible to set individual dimensions (and other attributes) or elliptical shapes for single nodes using the GraphAttributes class. But for simplicity I leave it as it is.QGraphicsView* contentWidget = new QGraphicsView();QGraphicsScene* graphicsScene = new QGraphicsScene();contentWidget->resize(500, 500);

# Arranging the graph

After the set up, we call the layouting of OGDF. The spatial information of the graph among other attributes is stored in the GraphAttributes class. There are quite a few layout algorithms provided by OGDF, but I found the PlanarizationLayout the easiest to use, which yielded good results without too much tweaking.//------------------------------------------------------------------------------// Layouting//------------------------------------------------------------------------------// The GraphAttributes will contain the position and other layouting informationogdf::GraphAttributes ga(g, ogdf::GraphAttributes::nodeGraphics | ogdf::GraphAttributes::edgeGraphics | ogdf::GraphAttributes::nodeTemplate | ogdf::GraphAttributes::nodeStyle | ogdf::GraphAttributes::edgeStyle);ga.setAllHeight(40); // Set the dimensions of all nodes; ga.setAllWidth(40);// Create and apply a layout on the graphogdf::PlanarizationLayout layout; layout.call(ga);// resize the graphicsScene to the bounding box that was calculated in the layoutgraphicsScene->setSceneRect(QRect(0, 0, ga.boundingBox().width(), ga.boundingBox().height()));

# Drawing

Drawing of the nodes is straight forward. We iterate over all nodes in the graph, retrieve their positions from the GraphAttributes and then draw a square on the QGraphicsScene.// Draw the nodes QPolygonF square; // create a square of size 40, 40 to use for displaying latersquare << QPointF(-20, -20) << QPointF(-20, 20) << QPointF(20, 20) << QPointF(20, -20) << QPointF(-20, -20);//it is also possible to use the macro forall_nodes(iterateNode, g) from OGDF for(ogdf::node iterateNode = g.firstNode(); iterateNode; iterateNode = iterateNode->succ()){ double x = ga.x(iterateNode) ; double y = ga.y(iterateNode); QGraphicsPolygonItem* squareItem = new QGraphicsPolygonItem(); // Create a QtPolygon for the node squareItem->setPolygon(square); squareItem->setFlags(squareItem->flags() | QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemSendsGeometryChanges); squareItem->setBrush(QColor(1, 1, 0, 1)); squareItem->setPos( x, y); graphicsScene->addItem(squareItem);}
Drawing of the edges is a bit more complex, as the edge-paths can be a combination of straight lines and cubic bezier splines. From the start point of an edge it can go straight to the end line or have multiple splines in between. Every cubic bezier spline needs three points to be drawn, so we can check if the number of points in a edge path is a multiple of three to determine if we have splines in between. I havent tested if the splines are connected seamless or if they can have straight sections in between.// there is also a forall_edges(edge, g) macro for(ogdf::edge edge = g.firstEdge(); edge; edge = edge->succ()){ auto polyLine = ga.bends(edge); QPainterPath path; // here we should check if the line really has at least one point auto pointIterator = polyLine.begin(); path.moveTo((*pointIterator).m_x, (*pointIterator).m_y); // move the line to the starting point if(polyLine.size() % 3 != 0) // straight line to either the starting point of the spline or the end point { ++pointIterator; } for(uint i = 0; i < polyLine.size() / 3; ++i) // iterate over the splines. Every cubic bezier spline needs 3 points { /// get the three points and increase the iterator afterwards /// maybe we need a path.lineTo(point1) here as well, to connect multiple splines auto point1 = *(pointIterator++); auto point2 = *(pointIterator++); auto point3 = *(pointIterator++); path.cubicTo(point1.m_x, point1.m_y, point2.m_x, point2.m_y, point3.m_x, point3.m_y ); } if(polyLine.size() % 3 == 2) // Straight line to the end point { path.lineTo((*pointIterator).m_x, (*pointIterator).m_y); ++pointIterator; } graphicsScene->addPath(path); // Add the edge to the scene}
And finally we show our widgetcontentWidget->setScene(graphicsScene); // Setting the scene to the graphicsViewcontentWidget->show();

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