Structure-Based Brushing

Brushing, in the context of multivariate visualization, refers to an interactive process for localization a subspace within an N-dimensional space [18,26,23]. Many useful operations, such as highlighting, deleting, masking, or aggregation, may then be performed on the isolated subset of elements that lie within the brushed subspace.

Brushing is a direct and data-driven metaphor. The operation may be performed in screen space, for example, via methods such as rubber-banding rectangles, or various mouse lassoing operations. Brushing may also be performed in data space; for instance, it has been applied to various multivariate display techniques such as parallel coordinates, scatterplots, and dimension stacking. Details of brushing in data space can be found in  http://www.cs.wpi.edu/~matt/courses/cs563/talks/brush.html

We introduce a new variant of brushing that is performed on hierarchical space. We term this operation structure-based brushing. Figure 6 shows our structured brushing interface. The triangular frame depicts the hierarchical tree. The contour near the bottom of the frame delineates the approximate shape formed by chaining the leaf nodes. The colored bold contour across the middle of the tree delineates the tree cut S(w) that represents the cluster partition corresponding to a level-of-detail w (ref. section 5.1). The colors on the cut corresponds to the colors used for drawing the nodes on the main parallel coordinates display (ref. section 5.2). The two movable handles on the base of the triangle, together with the apex of the triangle, form a wedge in the hierarchical space.

Brushing Interaction
The user localizes a subspace within the hierarchical space by positioning the two handles at the base of the triangle. The embedded wedge forms a brushed subspace within the hierarchical space.
 
 

Figure 6:Structure brushing tool.

Advantages
Since there is a color correspondence between the data display and the structure display. Sets of elements may be selected by positioning the wedge handles so as to bound the range of colors spanned by the elements. Moreover, similar elements are selected as a group, since by our coloring criteria, similar elements are drawn in similar shades.