How to brush data automatically from by using meshgrid?

I already read the question of igor ( http://www.mathworks.com/matlabcentral/answers/8706-bug-a-programming-emulation-of-brush-tool-in-figure-editor ) and also the answer, but i want to brush data automatically of this function:

[us,P] = meshgrid(0:0.05:2, -2:0.05:2);
Q = sqrt(us.^2*i_max^2-P.^2);
surf(us,P,real(Q),imag(Q), 'Edgecolor','blue');

Can somebody tell me how that works? I just what to "cut out" special parts of that figure. i.e. brush them white, so that they become invisible.

Thank you for your help.