LADDER, a sketching language for user interface developers

Authors: Tracy Hammond, Randall Davis

Comments:

Discussion:
This paper describes LADDER - a language to develop sketch recognition system.

This language allows users to specify the shapes to be recognized including the ways they can be edited and displayed.

Description limitations: LADDER can describe only shapes that have fixed graphical grammar. It cannot describe free hand diagrams. The shapes should have primitive components and should not have lot of irregularities.

Shape definition: This includes definition of
* list of components - primitives that make the shape
* geometric constraints - like angles between lines...
* aliases
* editing behavior - Editing operations(trigger) that can be performed on the shape and the corresponding effect (action)
* display methods - 4 ways of display can be set for a shape - original strokes, cleaned up (beautified) stroke, ideal shape and the alternate custom shape.

LADDER also allows to define hierarchical shapes, abstract shapes and shape groups.
It contains predefined components, constraints , editing behavior and predefined display to describe a shape. Vector (keyword) is used to define the minimum and maximum number of components for primitive shapes which can have variable number of components.

Multi domain recognition system: the system uses a bottom - up approach in recognition.
Low Level recognizer - recognizes primitive shape / combination of them from the stroke drawn and passes the result to high level domain shape recognizer. The limitation is when the low level recognizer fails, the domain shape recognizer would also fail.

Domain shape recognizer - this recognizer is based on Jesse rule. It searches for all combination of shapes that can satisfy the rule.A greedy algorithm is used to make the system faster. The down fall is in cases of ambiguity, the system may select wrong shape. The system was slower still and it was fixed by pruning the unrecognized strokes from the recognition tree.

The system also contains methods to find editing triggers and methods to solve constraints in the shape (ideal strokes).

Discussion:
As the author highlights, there is inherent problem in the bottom - up approach, the failure in low - level recognizer can cause failure in domain shape recognizer.
The ability of LADDER to define editing and display methods makes it powerful.