Algorithm 1 Adaptive threshold algorithm
Input: Image$I$, $(x,y)$ is used to represent the coordinates of the pixel. $w_{t_0}$ and $w_{t_1}$ represent the number of pixels in the COVID-19 region and the normal region, respectively. $u_{t_0}$ and $u_{t_1}$ represent the average gray levels of the two regions, respectively.
Output: Threshold $T$
Initialization: $t$=0
1 while $t$<=255
2 if $\max \{w_{t_0}\times w_{t_1}\times {(u_{t_0}-u_{t_1})}^2\}$
3 $T$=$t$;
4 bleak;
5 else $t$=$t$ + 1;
6 end