  Background: Because of the difficulties involved in learning and using 3D
modeling and rendering software, many scientists hire programmers or animators
to create models and animations. This both slows the discovery process and
provides opportunities for miscommunication. Working with multiple
collaborators, we developed a set of design goals for a tool that would enable
them to directly construct models and animations. Results: We present
SketchBio, a tool that incorporates state-of-the-art bimanual interaction and
drop shadows to enable rapid construction of molecular structures and
animations. It includes three novel features: crystal by example, pose-mode
physics, and spring-based layout that accelerate operations common in the
formation of molecular models. We present design decisions and their
consequences, including cases where iterative design was required to produce
effective approaches. Conclusions: The design decisions, novel features, and
inclusion of state-of-the-art techniques enabled SketchBio to meet all of its
design goals. These features and decisions can be incorporated into existing
and new tools to improve their effectiveness
