*2.1. Experiment System*

The experimental system used in this study consisted of a welding system, high-speed photographic system and an image processing system, as shown in Figure 2. An 850 nm laser source and some 850 nm near-infrared filters equipped on camera were used in a high-speed photographic system to eliminate the light of strong arc. A high-speed camera was located in different positions to obtain weld pool images with various visual angles. The side-view of the weld pool during welding was recorded in position 1, as shown at the position of high-speed camera 1 in Figure 2. The top-view of the weld pool was recorded in position 2, as shown at the position of highspeed camera 2 in Figure 2. Otto Arc MIG-500DP (OTTO Arc, Shanghai, China) was selected as a welding power source. Welding position was PA (Flat position, as per ISO 6947). The frequency of an electric signal acquisition system was 5 × 105 Hz.

**Figure 2.** Experiment system and image processing steps: (**a**) original image; (**b**) binarization and median filtering; (**c**) image contour coordinates.

An image processing system based on LabView (LabVIEW 2017, National Instruments, Austin, TX, USA) was developed to capture the outline of droplet and pool surface from the side-view picture. The image processing flow is shown in Figure 2a–c.
