Skin Strain Measurement Technique
Step 3: Skin Strain Measurement
Once the features have been extracted from each image in a sequence of images, an attempt is made to match each feature to a feature from the next frame. A simple matching algorithm is applied to match as many features as possible. Figure 1 illustrates some matching successes and failures from one image to the next. This process, although imperfect, results in a large number of features being tracked for varying periods of time.
Changes in local skin strain are estimated by observing changes in a triangulation of tracked features. The subset of features of a frame that are tracked in the subsequent frame is used to construct a Delaunay triangulation such as shown in Figure 2.
The change in local skin strain is evaluated by measuring variations in the triangulation. A first method, illustrated in Figure 3, consists of measuring the relative increase or decrease in triangulation edge lengths. A second method, illustrated in Figure 4, relies on the relative increase or decrease in triangle areas. Each pair of successive images is analyzed, yielding a map of relative changes in skin strain over time. Skin strain measurements are illustrated with color coded triangulations. In both cases, the range of relative variations is given by a bar at the bottom of the image. A maximal increase in edge length or area will be shown in red while a maximal decrease will be shown in blue. Measurements can also be made over a span of more than one frame.
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