LADDER, a sketching language for user interface developers

Summary:

Hammond and Davis created a sketch language, LADDER, that allows developers to easily design a geometric shape recognition system. The language handles shape drawing, displaying, and editing, and shapes are defined using primitives, constraints, and other shapes. Many primitives and constraints are already defined for the user, with primitives such as Lines, Arcs, and Spirals. Constraints are for individual shapes, such as if the shape has a positive slope, and constraints can also describe shape interactions, such as if two shapes are intersecting.

Shape definitions are hierarchical and incorporate geometric information about primitives and their relationship to each other. For instance, an arrow is defined as a shaft and a head, where the shaft is defined by a line and the head is defined as two lines meeting at an acute angle. One of the shaft points must also touch the head where the two head lines meet. Shape groups are also formed from shapes that are often drawn together. One example the authors give involves arrows touching polygons, which are defined as forces within a mechanical engineering domain. Shapes can also have editing constraints. A shape can be allowed to be "rubber-banded", or stretched, on a fixed point, rotated around a center, or forced to remain static if the system designer chooses.

Shapes, when displayed to the user, are displayed by their ideal strokes. If only lines are allowed in a domain, then only shapes made from lines can be drawn. Creating these ideal shapes happens through a series of mathematical equations. The features of a shape are quantified, and then the constraints are translated into equations that the features are plugged into.


Discussion:

The LADDER system provides a great foundation for geometric recognition systems. It seems as if entire applications can be built relatively quickly, as long as the domains involved do not have complex shapes that cannot be broken down into primitives.

Since I haven't used the system too much, I'm not sure how tough it is to build complex applications. I also do not know how easy it would be to integrate the language into an existing application, or if LADDER is only designed to build complete systems. Having the ability to pick and choose what LADDER offers might also be beneficial; for instance, I might not want the system to clean the strokes as they are drawn. The paper did not provide many details about customizablity in this regard.