Process of extracting the edges and tilt directions of EPBs, using the airglow image captured at 17:26 UT on April 3, 2022 (01:01 LT on April 4) as an example. (a) Airglow image mapped to geographical coordinates. The gray line is the coastline. The black star marks the location of the all-sky imager at Zhuhai. (b) Binarized airglow image. (c) Binarized image after erosion and dilation. (d) Image after removing small connected regions. (e) Image after filling holes in connected regions. (f) Extracted edges of connected regions. (g) Principal Component Analysis (PCA) is performed on the edges of EPBs. The red arrow indicates the first principal component (PC1) direction, and the blue arrow indicates the second principal component (PC2) direction. The green lines represent the tilt directions of EPBs following the PC1 directions. The percentages at the top of the image represent the variance contributions of the first and second principal components of the two EPBs. (h) The edges and tilt directions of the EPBs are represented by the black dots and green lines on the airglow image.