Why is there a jagged line when I plot this function?
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Hi guys,
I am trying to plot an implicit function and am getting strange results. I understand the smooth part of the function, but not the jagged part. Why is the jagged part there at all? Also, what can I do to eliminate it?
Here's the code:
f1=(@(x,y) (5.*y.^(1./2) -3)./(2.5.*y.^(1./2)+2.5.*x.^(1./2)-3)-y);
fimplicit(f1,[0 4 0 4],'black')
回答（2 个）
tmarske
2020-3-16
It's because fimplicit is using too few evaluation points, resulting in too coarse an interpolation (by default it uses 151). You can override this using the 'MeshDensity' argument:
f1=(@(x,y) (5.*y.^(1./2) -3)./(2.5.*y.^(1./2)+2.5.*x.^(1./2)-3)-y);
fimplicit(f1,[0 4 0 4],'black', 'MeshDensity', 5000)
6 个评论
econogist
2020-3-16
Walter Roberson
2020-3-16
What would you expect the result to look like? There are two lower-branch solutions for each x, and at some point they become equal and cross; what would you discard?
econogist
2020-3-16
Walter Roberson
2020-3-17
Ah, what is happening is that your function has a discontinuity in it, and what fimplicit is detecting is not a zero but rather a sign change.
econogist
2020-3-17
Walter Roberson
2020-3-17
编辑：Walter Roberson
2020-3-17
If you have the symbolic toolbox, you can solve f1 for x in term of y to get a single x for each y. you can then linspace() for y and put those through the equation to derive x, and then plot(x,y)
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