Deformed surf using griddata ?

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Faez Alkadi 2017-8-24

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https://ww2.mathworks.cn/matlabcentral/answers/353948-deformed-surf-using-griddata

评论： Faez Alkadi 2017-8-25

采纳的回答： John D'Errico

xyz_board_ordered_dvd.mat

在 MATLAB Online 中打开

I have a set of data as attached. but when I surf them after using griddata. the result is deformed surf AS SHOWN IN THE PICTURE with red squares. Z values on the surf were supposed to follow the edges of the shape. I think its because there is something wrong with (z_surf) matrix in my code. But I don't know how to solve it.

Hope you can help me to solve this problem.

Thank you.

 x_limit=xyz_board_ordered_dvd(:,1);
 y_limit=xyz_board_ordered_dvd(:,2);
 z_limit=xyz_board_ordered_dvd(:,3);
 dx=1;
 dy=1;
x_edge=floor(min(x_limit)):dx:ceil(max(x_limit));
y_edge=floor(min(y_limit)):dy:ceil(max(y_limit));
[x_surf,y_surf]=meshgrid(x_edge,y_edge);
z_surf=griddata(x_limit,y_limit,z_limit,x_surf,y_surf);
surf(x_surf,y_surf,z_surf)
xlabel('X')
ylabel('Y')
xlim([0 120])
ylim([0 120])

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Walter Roberson 2017-8-24

I notice that only 554 of the 604 points are unique.

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John D'Errico 2017-8-24

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编辑：John D'Errico 2017-8-24

在 MATLAB Online 中打开

Easy, peasy.

Griddata will fail here. In fact, my own gridfit will produce some artifacts.

But essentially, you have a simple ribbon of the form z(y), which seems to be independent of x.

x = xyzSTL_board_ordered_dvd(:,1);
y = xyzSTL_board_ordered_dvd(:,2);
z = xyzSTL_board_ordered_dvd(:,3);
plot(y,z,'o')

There is NO x in that relationship. So all you need to do is fit the curve z(y). Use a spline. Start out by removing any replicates. Using my consolidator tool, as found on the file exchange :

[yhat,zhat] = consolidator(y,z,@mean);
plot(yhat,zhat,'-')
spl = pchip(yhat,zhat);

Use pchip here. This is fairly important.

[xgrid,ygrid] = ndgrid(linspace(min(x),max(x),10),linspace(min(y),max(y),100));
zgrid = repmat(ppval(spl,linspace(min(y),max(y),100)),10,1);
surf(xgrid,ygrid,zgrid)

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Stephen23 2017-8-25

Another option would be to fill "missing data" areas with NaNs, which will not be plotted.

Faez Alkadi 2017-8-25

编辑：Faez Alkadi 2017-8-25

John and Stephen,

Thank you so much for the explanation,

The data I got is from an .stl surface file. I'm wondering if there is any easier way to meshgrid and surf the a face of an .stl file.

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Walter Roberson 2017-8-24

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You can get closer to what you want if you add the 'nearest' option to the griddata() call.

You will get an odd artifact, which I figure is due to peculiarities about which is the "closest" point in 3-space.

There is no reason for the surface to follow the edge using griddata(). Gridding takes place in 3-space, so when you get sufficiently far from the edges, the closest points might be from a different z than you were picturing.

If you have a surface that you know should be a ribbon, then there are ways to decompose that.