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I have'nt tested it but I think it will work, instead of circle draw ellipse() but need to adjust the parameters, like

center=your half circle centre
axes = Size(radius ,radius);
angle=0;
startAngle=0;
endAngle=180;

Now draw ellipse like,

ellipse(img,Point(centX,centY), Size(radius,radiua),0,0,180, Scalar(0,0,255), 2, 8,0);

You can change start angle and stop angle and draw different area you need.