Visual Exploration of Simulated and Measured Blood Flow - Scientific Visualization: Uncertainty, Multifield, Biomedical, and Scalable Visualization - page 295

Fig. 25.4 Comparison between a semi-transparent visualization ( a ) and a ghosted view technique
( b , c ) applied to the enclosed vessel surface to show the internal flow. In b the hidden streamlines
are depicted in grey and omitted in c to reduce visual clutter
The flow strongly depends on local variations of the enclosing vascular structures.
Large changes in flow speed occur at stenotic regions, and turbulent flow occurs
primarily at bifurcations or strongly curved areas. Thus, it is important to investigate
the morphology of anatomical structures and the internal flow simultaneously. In
case of simulated flow, such an integrated analysis may reveal that a significant flow
feature is due to a small variation of the surface, which may result from an inaccuracy
in the segmentation. The simplest idea to display flow and vascular anatomy at the
same time is to render the vascular surface transparently. However, depending on the
transparency level, either the vascular anatomy is hardly recognizable, or the internal
flow is strongly obscured by the vessel wall.
As a remedy, smart visibility techniques [ 50 ], such as ghosted views, may be
employed. The flow may be considered as an important object and the vessel walls
transparency is modified to reveal flow lines. This idea has been realized by Gasteiger
et al. [ 12 ]. The specific solution to provide ghosted view visualizations is based on
a Fresnel reflection model [ 39 ], where the reflection term is replaced by opacity. In
Fig. 25.4 a comparison of that technique with conventional semi-transparent render-
ing is presented. Gasteiger et al. refined their technique by an integration of landmarks
described in the next section, and the ability to remove all hidden flow lines to further
reduce visual clutter (see Fig. 25.4 ).
VanPeltetal.[ 46 ] presented an anatomical context based on methods inspired by
medical illustrations, where the detail is removed while the morphological informa-
tion is preserved (see Fig. 25.5 ). To this end, they used cel-shaded silhouettes, com-
bined with superimposed occluding contours. Hidden contours were visible during
viewpoint interactions in order to resolve occlusion problems and to clarify spatial
relations. Their user evaluation showed that these methods had a positive impact for
the purpose of anatomical context representation of the flow.