Question: plot3d error message

Hi experts!

Suppose we want to plot a function depending on integral evaluation, like

> S := Int(sin(sin(x+u)+sin(y-u)),u=0..Pi);
              |    sin(sin(x + u) - sin(-y + u)) du
> plot(subs(x=0,S),y=0..1);

The same with plot3d gives and error message:

> plot3d(S,x=0..1,y=0..1);
Warning, expecting only range variables [x, y] in expression Int(sin(sin(x+u)-sin(-y+u)),u = 0 .. Pi) to be plotted but found name u

Apparently the behaviors of plot and plot3d differ.  Solutions?




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