Algorithm A1 Preprocessing procedure for hydrofoil-region suppression

Input: Averaged grayscale image I Output: Residual image D 1.   Store the original image: $$I_0\leftarrow I$$ 2.   Determine the maximum image intensity: $$I_{max}\leftarrow max\left(I_0\right)$$ 3.   Extract high-intensity pixels: $$I_{\mathrm{th}}(x,y)\leftarrow I_0(x,y),I_0(x,y)\ge 0.85I_{max}$$   otherwise $$I_{\mathrm{th}}(x,y)\leftarrow 0$$ 4.   Detect image edges using the Sobel operator: $$E\leftarrow \mathrm{Sobel}\left(I_0\right)$$ 5.   Suppress the hydrofoil region and its edges: $$I_w\leftarrow I_0-I_{\mathrm{th}}-E$$ 6.   Binarize the resulting image: $$B\leftarrow \mathrm{Binarize}\left(I_w\right)$$ 7.   Retain the two largest connected components: $$B_2\leftarrow \mathrm{AreaFilter}(B,2)$$ 8.   Identify the removed small components: $$R\leftarrow B-B_2$$ 9.  Set pixels corresponding to R to zero in $I_w$. 10.Apply median filtering to $I_w$ using a $6\times 6$ window. 11.Apply median filtering to $I_0$ using a $6\times 6$ window. 12. Compute the residual image: $$D\leftarrow \mathrm{MedianFilter}\left(I_0\right)-\mathrm{MedianFilter}\left(I_w\right)$$ 13.Return D.