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Activity 12: Basic Video Processing

Hello! In this activity we will try to process a video of a kinematic event in order to extract information such as constants, frequencies, etc. For our group, we took a video of a 3D spring pendulum which we observed in one plane. We would like to trace its path and then try to determine its phase-space plot. The mass was covered in masking tape with the bottom colored red to facilitate easier segmentation.

The video was taken using a Canon D10 camera at frame rate of 30fps. 

Media 1. Video of the spring pendulum (first 50 frames only)

The frames of the video were then extracted using Avidemux 2.5. The mass was then segmented from each frame using parametric segmentation. The patch of the region of interest (ROI) used for color segmentation is shown in Figure 1.
Figure 1. Patch used to segment ROI 

Using morphological operations, particularly Open and Close operations, the segmented images were cleaned. The extracted frames for different observation time t and the corresponding segmented images are shown in Figure 2.

In order to ensure that there is only one blob and there are no more "dots" in the segmented  image, we filter all the images by size. From the area histogram (Figure 3) and by observation, the area of the blob of the mass is between 250 to 1000. Hence, we consider only those blobs that fall within this range.

Figure 3. Area histogram used to get size threshold for the blob of the mass
From the filtered images, we trace the position of the mass at different times by computing for the centroid. The plot in Figure 4 shows the track of the mass. The values for the y-component were negated so that it becomes easier to compare the track computed track with the images. The green and red dots represent the starting and end points, respectively. The mass follows an elliptical path which shifts over time.
 
Figure 4. Trajectory of the mass 

Now, we get the phase-space plot for both the x and y axes by taking the displacement for each axis and dividing it by the time interval dt (dt = 1f /30 fps). The phase space plots are for both axes are elliptic spirals as shown in Figure 5. However, the phase plot for the x component is noisier than that of the y component. 
(a) Phase space plot for motion along x
(b) Phase space plot for motion along y
Figure 5. Phase plots of motion along x and y
I give myself a grade of 9/10 for this activity because I am not very satisfied with what I have shown. I would like to thank my groupmates in doing the experiment, Ms. Aimee Rarugal and Mr. Benjur Emmanuel Borja.

References:
[1] M. Soriano, "Basic Video Processing", AP 186 Manual, 2012

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