Scientific rotoscoping: a morphology‐based method of 3‐D motion analysis and visualization

Abstract

Three-dimensional skeletal movement is often impossible to accurately quantify from external markers. X-ray imaging more directly visualizes moving bones, but extracting 3-D kinematic data is notoriously difficult from a single perspective. Stereophotogrammetry is extremely powerful if bi-planar fluoroscopy is available, yet implantation of three radio-opaque markers in each segment of interest may be impractical. Herein we introduce scientific rotoscoping (SR), a new method of motion analysis that uses articulated bone models to simultaneously animate and quantify moving skeletons without markers. The three-step process is described using examples from our work on pigeon flight and alligator walking. First, the experimental scene is reconstructed in 3-D using commercial animation software so that frames of undistorted fluoroscopic and standard video can be viewed in their correct spatial context through calibrated virtual cameras. Second, polygonal models of relevant bones are created from CT or laser scans and rearticulated into a hierarchical marionette controlled by virtual joints. Third, the marionette is registered to video images by adjusting each of its degrees of freedom over a sequence of frames. SR outputs high-resolution 3-D kinematic data for multiple, unmarked bones and anatomically accurate animations that can be rendered from any perspective. Rather than generating moving stick figures abstracted from the coordinates of independent surface points, SR is a morphology-based method of motion analysis deeply rooted in osteological and arthrological data.

Spring or string: does tendon elastic action influence wing muscle mechanics in bat flight?

“…One scan came from the American Museum of Natural History (29 mm isovolumetric voxels, GE phoenix vjtomejx s240) while the other three bats were scanned at Stony Brook University (36 mm isovolumetric voxels, Scanco mCT40; n ¼ 3). The scans were segmented in OsiriX and registered to the digitized three-dimensional marker constellations using a combination of marker-based XROMM [22] and scientific rotoscoping workflows [23].…”

A biplanar X-ray method for three-dimensional analysis of track formation

“…By viewing undistorted pairs of video frames from these perspectives, we reconstructed each bead's three-dimensional position by rotoscoping. Rotoscoping entails simultaneously aligning or registering a digital sphere model to its two X-ray shadows (Figure 2.2, 2.3; as described in Gatesy et al, 2010). We saved or "keyed" each model's XYZ coordinates for multiple frames.…”

“…In conjunction with existing methods for reconstructing skeletal movement (Brainerd et al, 2010;Gatesy et al, 2010), particle trajectories will document the link between foot kinematics and real track morphology. Live animals cannot be expected to make reproducible steps, so bead number will likely be limited.…”

“…Hardware and software tools originally created for capturing three-dimensional skeletal motion (Brainerd et al, 2010;Gatesy et al, 2010) are applied to reconstruct both foot movement and particle trajectories during track development. We present results from two case studies using artificial mud laced with metal beads, which serve as markers.…”

The birth of a dinosaur footprint: Subsurface 3D motion reconstruction and discrete element simulation reveal track ontogeny

“…Individual phalanges were rigged to the TMT using a series of joints placed at the centroid of the preceding condyles, and the tibia was rigged so as to be rotated from the ankle joint. The bones were then rotoscoped to the X-ray videos in Maya through scientific rotoscoping (33)(34)(35) to produce a digital reconstruction of the 3D kinematics of the limb above and below the surface of the substrate throughout the duration of the step cycle.…”