Biomedical Engineering Reference
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models based on existing knowledge of fluid sound rendering. Both types of ground
materials exhibit very interesting high frequency features adequate for their restitu-
tion through an actuated vibrotactile floor: as opposed to rigid surfaces, the overall
signal is not reduced to transients at the moment of impact, but can produce a sig-
nal during the entire foot-floor contact duration. Although mainly focused on the
vibrotactile modality, the approaches described here are multimodal. The synthesis
models are also capable of generating acoustic feedback, due to common generation
mechanisms and physical sources. The visual modality is an absolute requirement
on its own, since interacting with virtual environments without visual feedback is of
little interest, except in very specific cases.
12.3.5.2 Frozen Pond and Snow Field
In a multimodal scenario, Law et al. [ 57 ] designed a virtual frozen pond demonstra-
tion in which users may walk on the frozen surface, producing patterns of surface
cracks that are rendered and displayed via audio, visual and vibrotactile channels.
Audio and vibrotactile feedback accompany the fracture of the virtual ice sheet under-
foot. The two are derived from a simplified mechanical model analogous to that used
for rendering basic footstep sensations (see Sect. 12.3.2.2 ).
Based on the floor tile interface described in Sect. 12.3.4.1 , the authors designed
a virtual frozen pond demonstration that users may walk on, producing patterns of
surface cracks that are rendered and displayed via audio, visual, and vibrotactile
channels. The advantage of this scenario is that plausibly realistic visual feedback
could be rendered without detailed knowledge of foot-floor contact conditions, which
would require a more complex sensing configuration.
Vibrotactile and acoustic feedback are generated through the simplified fracture
model described in Sect. 12.3 . The visual rendering of crack surfaces on the ice is
generated with sequences of line primitives on the ice texture. Cracks originate at
seed locations determined by foot-floor contact, as illustrated in Fig. 12.10 . In another
application [ 57 ], using the same interface, the authors simulated a snow field, as also
showninFig. 12.10 . Users were enabled to leave footsteps onto virtual snow, with
acoustic and vibrotactile similar to the feeling of stepping onto real snow.
12.3.5.3 Walking on Fluids
Cirio et al. [ 13 ] proposed a physically-based vibrotactile fluid rendering model
for solid-fluid interaction, allowing “splashing on the beach” scenarios. Since fluid
sound is generated mainly through bubble and air cavity resonance, they developed a
physically-based simulator generating real-time bubble creation and solid-fluid inter-
action and synthesizing vibrotactile feedback from interaction and simulation events.
The vibrotactile model proposed by Cirio et al. is divided in three components, fol-
lowing the physical processes that generate sound during solid-fluid interaction [ 12 ]:
(1) an initial high frequency impact, (2) small bubble harmonics and (2) a main cavity
 
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