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.
Figure 1. Selected matched (full lines) and unmatched features (dashed lines).
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.
Figure 2. Typical triangulation of tracked features.
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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(a) Measurement of edge length variations. |
(b) Illustration of edge length variations. |
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Figure 3. Skin strain measurement by edge length variations. |
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(a) Measurement of triangle area variations. |
(b) Illustration of triangle area variations. |
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Figure 4. Skin strain measurement by triangle area variations. |
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