*2.2. Algorithm to Extract Characteristics of Pool Oscillation, Droplet Transfer and Arc Profile*

The outlines of droplet and pool surface captured by the image processing system are shown in Figure 2c. The contour coordinates were deformed with the droplet transfer and the pool oscillation. The dynamic information of the droplet and weld pool can be obtained by tracing the contour coordinates as a function of time, as shown in Figure 3.

**Figure 3.** Contour extraction of droplets and weld pool.

A reference point (A) was defined on the weld pool surface to trace the pool surface. The x coordinate of the reference point is constant; the fluctuation of the y coordinate of the reference point is the direct information about the weld pool oscillation. To avoid the hindrance of droplet transition to the surface contour extraction of the weld pool, the reference point (A) was located on the weld pool surface 1.8 mm from the center of welding wire, as shown in Figure 4.

**Figure 4.** Position of the reference point: A.

The center of gravity of the droplet as approximated to the average value of its contour coordinate, as calculated by Equation (1) [1].

$$(x\_{\text{Gt}}, y\_{\text{Gt}}) = (\frac{\sum\_{1}^{n} x\_{nt}}{n}, \frac{\sum\_{1}^{n} y\_{nt}}{n}) \tag{1}$$

(*xGt*, *yGt*) is the coordinate of the center of droplet at t, (*xn*, *yn*) are the coordinates of the droplet contour, n is the number of contour pixels. The droplet diameter can be calculated by Equation (2) [1].

$$D\_{droplet} = \sqrt{\left(4 \times S\_{droplet}\right) / \pi} \tag{2}$$

*Ddroplet* is the equivalent diameter of droplets, *Sdroplet* is the area of droplet profile calculated by the number of pixels surrounded by the droplet contour line. Droplet velocity can be calculated by measuring the center coordinate of droplet in continuous photographs, as shown in Equation (3).

$$V\_{droplet} = \frac{\sqrt{\left(\mathbf{x}\_{Gt1} - \mathbf{x}\_{Gt2}\right)^2 + \left(y\_{Gt1} - y\_{Gt2}\right)^2}}{|t\_2 - t\_1|}\tag{3}$$

(*xGt*1, *yGt*1) and (*xGt*2, *yGt*2) are the coordinates of the center of droplet at *t*<sup>1</sup> and *t*2.

Changing the exposure time and the number of filters of high-speed camera can obtain different shooting effects. Increasing the exposure time and the number of filters equipped on the camera can cause the background light stronger than the arc light, which can filter the arc light, as shown in Figure 5a. Reducing exposure time and the number of filters leads to a higher arc light intensity than the background light intensity, and a precise arc contour can be obtained, as shown in Figure 5b. The arc characteristics were defined by its root diameter (DR) and projected diameter (DP) during pulse on the period, as schematically shown in Figure 5b.

**Figure 5.** (**a**) Complete filtering out of the arc; (**b**) typical nature of arc profile.
