Fsurf plot wrong when x coefficient is 0

Question

Xinhua 2019-4-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/457671-fsurf-plot-wrong-when-x-coefficient-is-0

评论： Star Strider 2019-4-22

The script will produce graph like this

syms x y z u v
fsurf(214.90609811873839701574097069388 - 2.032409294075229574399388894873*v,v,u,[-200,+200],'FaceColor','red');
hold on
fsurf(2.5564704196832668259276033495553*y-14.255253156645956380543793784454+0.00000000000*x,[-200,+200],'FaceColor','green')
fsurf(256.06484646683490299896860383123 - 1.2578521595702952545440650237956*x,[-200,+200],'FaceColor','blue')
xlabel('X')
ylabel('Y')
zlabel('Z')
hold off

As you can see the green plane is wrong MatLab is treating y as x, despite the x coefficient is 0

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Answer 1

Star Strider 2019-4-21

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https://ww2.mathworks.cn/matlabcentral/answers/457671-fsurf-plot-wrong-when-x-coefficient-is-0#answer_371564

在 MATLAB Online 中打开

The fsurf function cannot guess what you are thinking.

Try this version of your code:

syms x y z u v
f1(v) = 214.90609811873839701574097069388 - 2.032409294075229574399388894873*v;
f2(x,y) = 2.5564704196832668259276033495553*y-14.255253156645956380543793784454+0.00000000000*x;
f3(x) = 256.06484646683490299896860383123 - 1.2578521595702952545440650237956*x;
figure
fsurf(f1,v,u,[-200,+200],'FaceColor','red');
hold on
fsurf(f2,[-200,+200],'FaceColor','green')
fsurf(f3,[-200,+200],'FaceColor','blue')
hold off
xlabel('X')
ylabel('Y')
zlabel('Z')
view(-50,15)

Experiment to get the result you want.

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Xinhua 2019-4-22

Yep that works,never know we have to address the y variable this way.Strange behavior though.

Thank you.

Star Strider 2019-4-22

As always, my pleasure.

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Answer 2

Walter Roberson 2019-4-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/457671-fsurf-plot-wrong-when-x-coefficient-is-0#answer_371566

在 MATLAB Online 中打开

When you construct an expression A*x + B*y + C with symbolic x and Y, and A is 0, then the symbolic engine does not deliberately hold on to the 0*x "just in case" you wanted to know later that X had been part of the equation at some point. Instead it simplifies it to B*y + C -- an expression with a single symbolic variable.

When you fsurf() an expression with a single symbolic variable, then fsurf() assumes that that symbolic variable is the independent variable, and it always puts the independent variable along the x axes. It does not look at the name of the symbolic variable. There is no difference between

syms x y u
fsurf(5*x + 2)
fsurf(5*y + 2)
fsurf(5*u + 2)

The only context in which it looks at the actual names of the variables is that it uses symvar() on the expression to order the variables, and symvar sorts the variables by name.

syms a b c
fsurf(5*a - 3 * b + 2)   %a sorts first, it is the independent variable
fsurf(5*c - 3 * b + 2)   %b sorts first, it is the independent variable