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Implicit surface plot of a finite cylinder

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Matt J
Matt J 14 Feb 2021
답변: Matt J 14 Feb 2021
Using fimplicit3, I can plot an infinite cylinder, like so:
args={'EdgeColor','none','FaceColor','c','FaceAlpha',0.3};
fimplicit3(@(x,y,z) x.^2+y.^2-1, args{:} ); axis([-3,+3,-3,+3,-3,+3])
However, what I would really like is for the cylinder to be of finite length, say 4. My first idea for how to do this was the following. This does produce a finite cylinder of the correct length, but it is closed at both ends, which I do not want.
fimplicit3(@(x,y,z) x.^2+y.^2-1 + (abs(z)>2), args{:} ); axis([-3,+3,-3,+3,-3,+3])
I believe I know why this is happening. There is a discontinuous jump from negative values to 1 at the end caps, and so the fimplicit3 algorithm assumes there are roots there. However, I wonder if there is a way to trick fimplicit3 somehow so that I get a cylinder with open ends. Any ideas?

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Matt J
Matt J 14 Feb 2021
The solution, as it turns out, was fairly simple:
args={'EdgeColor','none','FaceColor','c','FaceAlpha',0.3};
fimplicit3(@fun, args{:} ); axis([-3,+3,-3,+3,-3,+3])
function val=fun(x,y,z)
val=x.^2+y.^2-1;
val(abs(z)>2)=nan;
end

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