  3D animations are an effective method to learn about complex dynamic
phenomena, such as mesoscale biological processes. The animators' goals are to
convey a sense of the scene's overall complexity while, at the same time,
visually guiding the user through a story of subsequent events embedded in the
chaotic environment. Animators use a variety of visual emphasis techniques to
guide the observers' attention through the story, such as highlighting, halos
-- or by manipulating motion parameters of the scene. In this paper, we
investigate the effect of smoothing the motion of contextual scene elements to
attract attention to focus elements of the story exhibiting high-frequency
motion. We conducted a crowdsourced study with 108 participants observing short
animations with two illustrative motion smoothing strategies: geometric
smoothing through noise reduction of contextual motion trajectories and visual
smoothing through motion blur of \rev{context} items. We investigated the
observers' ability to follow the story as well as the effect of the techniques
on speed perception in a molecular scene. Our results show that moderate motion
blur significantly improves users' ability to follow the story. Geometric
motion smoothing is less effective but increases the visual appeal of the
animation. However, both techniques also slow down the perceived speed of the
animation. We discuss the implications of these results and derive design
guidelines for animators of complex dynamic visualizations.
