think-cell technical report TC2003/01 A GUI-based Interaction ...

4.2 Interaction Toolbox INTERACTION CONCEPT
part, but snaps to certain snapping objects when the mouse pointer is sufficiently
close. Snapping objects may be virtual guidelines that serve the purpose of ruler
and compass, or the mouse may also snap to drawing objects that are part of
the document. A gravity function determines what “sufficiently close” means and
resolves conflicts between two or more snapping objects which are about equally
close to the mouse pointer location. There may be gravity functions of different
strengths, relating to the significance of different snapping objects.
A lot of research has been conducted on snapping (see Sect. 3.2.1) and it turns
out that the distinction between mouse pointer and drawing caret helps to un-
derstand and efficiently use snapping mechanisms. In this distinction, the mouse
pointer always moves smoothly in the continuum, exactly reflecting the movements
of the mouse, while the drawing caret indicates which position in the document will
be referred to when a mouse button is pressed. It is the caret that is snapped to
discrete positions while the pointer continues to move freely. Some recent software,
including Microsoft Visio [Micc] and Autodesk AutoCAD [Aut], adheres precisely
to this definition, while other programs like Microsoft PowerPoint do not explicitly
display the caret but implicitly apply mouse interactions to the closest snapping
position. In the latter case, it may be a good idea to highlight the snapping object
when snapping occurs, so that the user has a chance to move the mouse further
away if snapping is not intended.
As for zoomed views, the velocity of gravity functions is usually based on screen
coordinates, not document coordinates. As a consequence, in an enlarged view
drawing objects appear more detailed and the sphere of influence of a snapping
object becomes smaller in relation to the drawing. Thus, in the case of many
snapping positions being close to each other, using a highly zoomed view makes it
easy to snap to the desired position.
4.2.4 Mode Switch
In the present scenario of one application being built on top of another, there is a
radical method to avoid interference of distinct applications’ user interfaces: I could
implement some sort of escape key that switches between two completely indepen-
dent user interfaces, one for each application. This is in fact how the Microsoft
implementation of OLE (Object Linking and Embedding) appears to the user, a
technique that is used to embed charts, diagrams, images, movies, formulae and
other complex objects in documents created with PowerPoint, Excel, or Word.
Double-clicking an OLE object switches to the UI context of some other applica-
tion. The advantage is obvious: Menus, toolbars and shortcut keys are completely
changed by the mode switch and can all be used without any restrictions that
emerge from another application using the same UI widgets.
But this advantage is only of theoretical relevance. If some well-known Power-
Point shortcut key takes on a completely different meaning in “think-cell mode”,
then mistakes in the user input and user dissatisfaction are almost guaranteed.
Another problem of this approach is the technical and cognitive overhead that is
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