In 3D flow visualization, we often encounter occlusion problems when displaying dense sets of lines or multiple surfaces. A vital rendering aspect is the careful selection of primitives that best communicate the relevant features in a data set. We start with an optimization-based approach for steady flows that adjusts the opacities of lines striving for a balance between the presentation of information and occlusion avoidance. After extension to animated lines in unsteady flow, we consider the problem for surfaces. Afterwards, we focus on inertial particles, which are relevant in many engineering problems, such as dust particles interacting with aircraft. The governing ODE of even simple inertial flow models is up to seven dimensional, which makes feature extraction a challenging task. In particular, we study the mass-induced separation behavior of nearby inertial particles and extract the cores of their vortical motion.