Spacing points on a 2D plot evenly

Question

francis steyn 2016-7-30

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评论： francis steyn 2016-7-31

Hi

Im plotting a 2D closed boundary but the points going around a curve of the boundary are spaced closely to each other. The points on a straight section are spaced further away from each other. The boundary is smooth.

Is there any way to make the distance between each point the same, without changing the structure of the shape?

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Answer 1

Walter Roberson 2016-7-31

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Calculate the distance between adjacent points. Create a cumulative sum of that. Divide the total distance up into as many equal segments as you desire. Use the cumulative sum to locate the adjacent points that each of the equal-length segments would fall between. The difference between the required distance and the point before, divided by the length of the segment, gives you a proportion. The proportion times the delta x and the delta y between the point and the next point gives you the delta x and delta y relative to the first point at which to place your equal-distance point.

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francis steyn 2016-7-31

编辑：Walter Roberson 2016-7-31

在 MATLAB Online 中打开

Thanks, I tried this but corrections are not working.

% cumalative sum of distances between points
    % save_MFX1 and save_MFY1 is the old x and y points
    c_sum = [];
    deltax = [];
    deltay = [];
    % doing all points
    for i = 1 : length(save_MFX1)
        % treating boundaries
        if i == length(save_MFX1)
            c_sum = [c_sum sqrt( (save_MFX1(end) - save_MFX1(1))^2 + (save_MFY1(end) - save_MFY1(1))^2 )];
            deltax = [deltax save_MFX1(end)-save_MFX1(1)];
            deltay = [deltay save_MFY1(end)-save_MFY1(1)];
            break
        end
        c_sum = [c_sum sqrt( (save_MFX1(i) - save_MFX1(i+1))^2 + (save_MFY1(i) - save_MFY1(i+1))^2 )];
        deltax = [deltax save_MFX1(i)-save_MFX1(i+1)];
        deltay = [deltay save_MFY1(i)-save_MFY1(i+1)];
    end
    % divide into equal spaced points
    correct_distance = sum(c_sum)/length(save_MFX1);
    % getting proportion
    prop = [];
    new_points_x = [];
    new_points_y = [];
    % new x = old x + (proportion * deltax)
    for i = 1 : length(save_MFX1)
        prop = [prop (correct_distance-c_sum(i))/c_sum(i)];
        new_points_x = [new_points_x save_MFX1(i)+(prop(i)*deltax(i))];
        new_points_y = [new_points_y save_MFY1(i)+(prop(i)*deltay(i))];
    end
    % checking to see if new_sum is same as c_sum
    new_sum =[];
    for i = 1 : length(save_MFX1)
        if i == length(save_MFX1)
            new_sum = [new_sum sqrt( (new_points_x(end) - new_points_x(1))^2 + (new_points_y(end) - new_points_y(1))^2 )];
            break
        end
        new_sum = [new_sum sqrt( (new_points_x(i) - new_points_x(i+1))^2 + (new_points_y(i) - new_points_y(i+1))^2 )];
    end

Walter Roberson 2016-7-31

cumsum()

francis steyn 2016-7-31

Found my mistake, the cases where correct distance is longer than the interval aren't treated.

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Answer 2

Image Analyst 2016-7-31

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https://ww2.mathworks.cn/matlabcentral/answers/297758-spacing-points-on-a-2d-plot-evenly#answer_230357

编辑：Image Analyst 2016-7-31

Yes, John D'Errico has uploaded a function that will be perfect for you. It's called interparc

http://www.mathworks.com/matlabcentral/fileexchange/34874-interparc

Be sure to look over all of his other amazing submissions - you'll undoubtedly find other things there that you can use.

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Image Analyst 2016-7-31

Oh, I thought others were allowed to help so that's why you asked here in Answers. But since now you say " I have to write this" (alone), then, good luck with it. Perhaps you're still allowed to follow the general guidelines laid out by Walter.

francis steyn 2016-7-31

My mistake, should have specified at the start, sorry. Yes Walter is on to something.

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Spacing points on a 2D plot evenly

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Spacing points on a 2D plot evenly

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