Constraining a layout

Constraints are requirements given to a layout algorithm by a user to maintain some level of higher user control over the resulting layout. Such constraints might be that all directed edges of a graph have to point in the same direction. A layered layout is an example of a layout algorithm that has one constraint placed upon the graph, in regards to its directed edges. Research has been made into this area, looking at how to incorporate the addition of constraints placed upon portions of the graph into the resulting layout. However the addition of constraints to a layout algorithm, should still take into account a user's wishes, and update the layout in response to a users actions.

The addition of constraints in an interactive system is explored by Ryall et al. in [9]. They developed an interactive graphing tool called Glide that allowed the user to draw graphs and place constraints or what Ryall et al. called Visual Organisational Features or VOFs. The VOFs were passed to the underlying force-directed layout algorithm in the form of spring like forces that affected the placement of the vertices or objects in the final layout. However because the VOFs were an approximation of constraints, the layout algorithm can never be guaranteed to enforce or satisfy the constraints. The examples used in their work, show the value of having the ability to place constraints onto portions of a graph, in order that the user maintains some higher level of control in regards to the final layout, and can interact with a tool to guide it as the tool performs graph layout.