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Figure 10.3.
Edge detection of a droplet. The edge can be displayed using the ‘Show Edges’ button in the main viewer.
Canny edge detection is the process by which the software determines the edge pixels of a droplet.
Figure
10.3
shows the same droplet as
Figure 10.1
, however the ‘Show Edges’ option has been toggled in the main
viewer, and so the edge detection is now visible.
The edge detection algorithm looks for large changes in pixel lightness that would signify the edge of a region
of darkness (a droplet). When an area of the image changes from light to dark over a small number of pixels,
there is likely to be an edge.
Figure 10.4
shows a change in lightness between pixels in a straight line. The
pixel on the left has a lightness value of 50, which increases to 240 over the 6 pixels in the line. The change
in lightness between the two central pixels is the highest, shown by the steep gradient of the line between
these points.
10.3 Canny Edge Detection
Figure 10.4.
Lightness value change from 50 to 240, with a steep gradient between the central two pixels.
There are two gradient thresholds involved with the edge detection, and they are shown in
Figure 10.5.
Threshold 1 determines the minimum change in lightness required for an edge to be detected. To detect
and edge in
Figure 10.4
, threshold 1 would need to be lower than the difference between the central two
pixels. This corresponds to point a) in
Figure 10.5.
If the angle is below 10° then the software uses a circle fit to determine the contact angles, and this is not
displayed on the image.
Summary of Contents for L2004A1
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