# Deformed surf using griddata ?

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Faez Alkadi on 24 Aug 2017
Commented: Faez Alkadi on 25 Aug 2017
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])
Walter Roberson on 24 Aug 2017
I notice that only 554 of the 604 points are unique.

John D'Errico on 24 Aug 2017
Edited: John D'Errico on 24 Aug 2017
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) Faez Alkadi on 25 Aug 2017
Edited: Faez Alkadi on 25 Aug 2017
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.

Walter Roberson on 24 Aug 2017
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.
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Faez Alkadi on 25 Aug 2017
Walter,
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.
Thank you