3.3 Creating Edges

   The set of tiled buttons named Edges contains radio buttons indicating the current type of edge that can be drawn. Furthermore there is a so-called check button labeled Curve. If the curve button is on, each new edge that is created, having more than one segment, is drawn as a Bezier curve. If the curve button is off, a multi-segment line is drawn as a set of connected straight line segments. To create a new edge, first choose the desired edge type in the edge tiles. There are three sorts of edges that you can create:

• Straight edges.   Drag with button-2 from a node to a node. This means you push and hold down button-2 somewhere inside the source node and move the mouse with button-2 still pushed down. During dragging the mouse pointer turns into a  and a line from the source node follows the movement of the mouse.  You can always abort creating an edge while dragging by clicking button-1. When you are arrived in the target node and release button-2, then a straight edge is created. The two points of the edge at the borders of the connected nodes are called the end points  of the edge.
• Segmented edges.   These edges consist of more than one segment. The points where the segments are connected are called intermediate points.   To create such a segmented edge, release button-2 in the background while you are dragging. Then the first intermediate point is created. After that, the line follows the mouse all by itself. Then you can add another intermediate point by clicking button-2 somewhere else in the background. You complete the edge by moving into the target node and clicking button-2.
• Curved edges.     You can also create an edge drawn as a Bezier curve . For this, turn on the Curve toggle in the tiled menu. The Bezier curve requires exactly two intermediate points, which makes four edge points in total. These intermediate points are added in the same way as creating normal multiple line segments as described for segmented edges. When the four points are determined, a smooth curve is drawn between the end points and the two intermediate points act as controlling points.

  

Figure 3.1: Three forms of edges.

After creating the edge, the edge is selected which is visible by a selection handle on each line point. You can adjust the line points by moving the handles; see section 3.6.

When an edge is created and it is the only straight edge connecting this particular couple of nodes, it will go (virtually) through the center points of the nodes. When there are multiple straight edges connecting the same pair of nodes, they will be equally distributed by default over the opposing sides. Segmented and curved edges are not equally distributed when they connect the same pair of nodes. TCM tries to intersect the first and last segment of a segmented line orthogonally with the sides of the connected nodes. If this is geometrically impossible, the segment will be directed to the center of the node like with straight edges .

Tip: When you want to enforce that a straight edge is always orthogonal to a node, then you can draw a segmented line of three points that looks like a straight line. You enforce then that the line is always connected orthogonally with both of the nodes (providing that this is geometrically possible). This gives often a more pleasant optical result when you want to connect two nodes that differ a lot in size.

It is possible to move the end points of an edge as well, in the same way as you move the intermediate points. When you drag the handle of an end point to another position then the new end point will be the position at the border of the node shape that is the closest to where you released the handle. However, when you drag the handle of an end point into a different node shape then the edge will be redirected to that node.