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rho_err = abs(line.rho())-img.width()/2 说line.rho是原点到直线的距离，那rho_err代表什么
应该是距离中心的偏移量，感觉算的有问题。
abs(line.rho()) * cos(line.theta()) - img.width()/2 可能会更好一些，我没测试过，只是建议。