8.2.6 Rulers
 |
|
Figure 8. 11 An example of a ruler
showing the distance between the tar-get
object and the nearest neighboring
object |
A ruler provides quantitative spatial measurement,
such as a graduated scale or a
numerical readout. The most common
form of ruler is a linear dimension such as
in a mechanical drawing, with end marks
and arrows showing the extent of the measurement,
and a numerical callout showing
the value. Other variations include dimensioning
for angle, area, and radius of curvature.
Rulers can measure the host object
itself, such as the dimensions of its bounding
box; or they can measure the spatial
relationship between the host object and its
surrounding objects, as a quantitative
feeler. For example, while an object is being
dragged, the distance between the host
object and its neighboring objects might
appear, as shown in figure 8. 11.
Rulers can also be used as tools without a predefined host object. The user selects
a particular tool from a toolbox depending on the kind of measurement needed, and
snaps the ends of it to the objects to be measured. A familiar metaphor is a tape measure,
where the user snaps one end to an object and moves the other end around in
the world to measure distance. A similar scheme can be used for tools that measure
angle, area, and volume.
Often rulers are dynamic, stretching as the host object is manipulated, and with
the numerical callout updating continuously. They can also be passive, like a traditional
ruler with tic marks. For example, static rulers might be placed alongside the
edges of the floor in the office layout example to assist the user in placing furniture.
Doing the real ruler one better, live tic marks on the ruler might indicate the position
and extent of the currently selected target object.
8.2.7 Snaps
In design applications some assembly is often required, but arbitrary attachment of
objects is often not allowed. In these cases, feedback is needed on an object to tell the
user that other objects can be attached to it, and to indicate where and how the
attachment can be made. Feedback elements that provide this hybrid of relational
and control feedback are called snaps. The most common form of snap element is the
snap point, which is a point position on or near the host object that is often represented
by a small graphic symbol, such as a dot or ball. Figure 8.12 shows a desk with
several snap points. Notice that the "hidden" one in the middle appears smaller than
Figure 8. 12 Snap points indicate points on an object where object
snapping and attachment can occur. (Courtesy of Tech-niCon
Corporation)
Different forms of attachment require different forms of snap feedback. Typed
attachments are selective about with whom they will partner, and oriented snaps
impose a particular geometry on the connected objects. Each requires a different representation
in the form of visual and audio snap elements so the user can easily tell
them apart and get a sense for what will happen as a result of the attachment. For
example, typed snaps may take male and female forms to indicate which attachments
go together, and oriented snaps may use indicators to show the orientation that the
attachment will impose.
Two important concepts in manipulation, attachment and snapping, were
touched on here. These will be discussed at greater length and from other perspectives
in later sections. Because of its common use in other 2D and 3D applications, the term
"snaps" was chosen to name this form of feedback element although it is needed for
all forms of attachment, not just snapping.
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