Bezier Curve problem inside for loop

Question

Michael Jacobson 2018-3-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/387550-bezier-curve-problem-inside-for-loop

评论： Arthur Vidal 2018-3-11

Hi, I have a problem when generating multiple points from two x & y functions inside a for loop. If I use the plot function to plot x and y, then the curve will not appear. If I use the scatter function it will plot each point though, but I want a smooth curve.

figure; hold on
for u = 0:.01:10
    x = (1-u)^3*1+3*(1-u)^2*u*3+3*(1-u)*u^2*5+u^3*8;
    y = (1-u)^3*1+3*(1-u)^2*u*0.5+3*(1-u)*u^2*2.5+u^3*2;
    plot(x,y)
    scatter(A(1,:),A(2,:))
    xlim([0 10])
    ylim([0 4])
end
hold off

Also, is there already a built-in function which will achieve what I'm trying to do? If not what would be the best way to create a function to use for future reference?

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Answer 1

Arthur Vidal 2018-3-11

1
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https://ww2.mathworks.cn/matlabcentral/answers/387550-bezier-curve-problem-inside-for-loop#answer_309452

Hi Michael,

For your case, for is not recommendded. Instead, use vectors and dot operations to get your code working well. The following code should work.

figure; hold on u = 0:.01:10; x = (1-u).^3*1 + 3*(1-u).^2.*u*3 + 3*(1-u).*u.^2*5 + u.^3*8; y = (1-u).^3*1 + 3*(1-u).^2.*u*0.5 + 3*(1-u).*u.^2*2.5 + u.^3*2; plot(x,y) xlim([0 10]) ylim([0 4]) hold off

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Michael Jacobson 2018-3-11

Thank you, the code works perfectly. Can you explain the logic a bit more though? you say use vectors and dot operations. When I create a range of numbers for variable u, does that mean it is now a size(1,i) vector where i is determined by the increment of the range? If u is a vector does that mean when I input it into the function for x & y, then x & y are now vectors with the same size of u? I don't understand where the dot operation is being conducted. Also, is the for loop incorrect because x & y act as constants with individual u values when implemented in a for loop? and the plot function is meant to pass a vector if I want to generate a curve?

Arthur Vidal 2018-3-11

The way your did first, only one value was being calculated per iteration and you were plotting a single point, not a set of points. Plot needs to have two vectors and it connects all the points with a curve. The dot operation is necessary to work with vectors. Dot operation means that you are multiplying or powering element by element, not the whole vector or matrix.

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