# using pcfitcylinder - unexpected results?

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Dan Howe on 16 Apr 2020
Commented: Dan Howe on 28 Apr 2020
Hi, I'm looking to use the pcfitcylinder function and to check how it works, was trying to construct a simple test case.
I've got the code below to generate some point cloud data to fit a cylinder to. I think the point cloud data is correct but the fitted cylinder is nowhere near close. Could anyone advise what the issue is (I'm sure it's what I'm passing into the function)?
Thanks
length=10;
ntheta=100; % number of points to sample around circumference
nz=4; %number of layers to slice along cylinder
A=rand(ntheta,nz); %generate random points
A=A*2*pi; %convert to angle
B=0; % eventually want to try B=rand(ntheta,nz);
X=(r+5*B).*cos(A); %X co-ordinate
Y=(r+5*B).*sin(A);
z=[0:length/(nz-1):length];
Z=repmat(z,ntheta,1);
mypointcloud=pointCloud([X(:),Y(:),Z(:)]);
maxDistance=1;
cyl=pcfitcylinder(mypointcloud,maxDistance)
scatter3(X(:),Y(:),Z(:));
hold on
pcshow(pcloud);
plot(cyl);

Prabhan Purwar on 21 Apr 2020
Hi,
Please refer to the following code:
length=1;
ntheta=100; % number of points to sample around circumference
nz=20; %number of layers to slice along cylinder
A=rand(ntheta,nz); %generate random points
A=A*2*pi; %convert to angle
B=0; % eventually want to try B=rand(ntheta,nz);
X=(r+5*B).*cos(A); %X co-ordinate
Y=(r+5*B).*sin(A);
z=[0:length/(nz-1):length];
Z=repmat(z,ntheta,1);
mypointcloud=pointCloud([X(:),Y(:),Z(:)]);
maxDistance=0.005;
cyl=pcfitcylinder(mypointcloud,maxDistance)
scatter3(X(:),Y(:),Z(:));
hold on
pcshow(mypointcloud);
plot(cyl);
Output:
Kindly refer to the following link:
Tip:
The following parameters plays a major role, as the algorithm to find a cylinder is based upon MSAC which relies upon the density of point clouds.
ntheta=100; % number of points to sample around circumference
nz=20; %number of layers to slice along cylinder
Dan Howe on 28 Apr 2020
Thanks for the response but I only have a few 'slices' of point cloud data along the cylinder axis. I was surprised that the fitted cylinder could be so wildly out.
Given that my data slices are more or less planar I could just fit circles along each slice and best fit a line through circle centers. but pcfitcylinder seemed a more elegant approach if it gave a reasonable fit.

R2016a

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