FBB 2: The real test

Since the FBB code seemed to be working, it is time for a real test. To use it under a field condition, where the amount of remaining foam is usually larger. We used our full frame video data (though monochrome) from the Duck Mini experiment, in particular our pet run #2701215.

Here is the timex of that stack (note that the coordinates are pixels and not FRF distances).



The boxes correspond to the two test areas for the code. The magenta box (north of the pier), shows a greater contrast between the breaking areas and the offshore region, whereas for the south box (yellow), the contrast is less. Possible causes are a difference in gain or sun glare. In this case, it seems that it corresponds to a dark colored water (sediment in suspension?) as seen here



North Box Results
For the North Box, it seems that the code was doing a decent job, although it seems that we are missing some events. A sample movie of the approaching waves with the detected events marked as blue dots can be seen here:



However, if we count the total number of events, the spatial structure of the bar seems to be well recovered, as shown in the following figure. The left panel is the number of breaking events, the middle panel is the time exposure and the right panel is the overlay between the two other panels. In this kind of figure, we are,looking for areas with a good agreement which will show up with a white color. Red (magenta) areas correspond to areas where the time exposure signal is brighter than the FBB one. This suggests that tracking the FBB will "move the bar" further onshore, as opposed to the expected behavior of moving it offshore (since the effect of persistent foam would be reduced). Is worth noticing though, that we are comparing time exposures (ie brightness) against number of events. Perhaps a better comparison would be to identify the location of the events, and extract its brightness information.



Another possible way to look the results is using a timestack. I randomly selected the pixel 120 (alongshore) and here is the result of the detection process:



It can be seen that we are doing a pretty good job in the surf, but not so good in the swash. However, our area of interest is the surf. A few number of events were missed near t=600 samples, probably due to the area requirements imposed in the code. Additionally, a very bright patch of foam is responsible for the no detection of events near t=1150 and t=1700 (t is the abcissa).


South Box Results
Here we found more problems. It seems that the persistent foam was very bright, therefore identification of the breaking events was much more difficult. The time exposure give us a hint, because it can be seen that the bright area is very broad (cross-shore wise), whereas for the north area is relatively narrow.

Here is the movie, where we can see that several events are missed.


And here is the FBB, timex and overlay.


To do

Check the overall effect of area requirements in the code, when we are working with lof sampling rates and coarse resolution. It seems that for the lab experiments, where we had Delta x=25 cm and Delta t=0.1 s, that wasn't a problem. Here, the sampling rate was 2 Hz and the resolution was 3 m.