# Detection of 3D blobs with flat surfaces

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Roohollah Milimonfared on 25 Aug 2019
Answered: darova on 26 Aug 2019
I have a 3D gray-scale array that represents a bag with several objects inside. I need to find (segment) any object with flat surface/s in the bag. Knowing the approximate intensity range of these objects, I binarized the volume, and removed objects with volumes below a threshold.
The result was getting some of those objects as individual blobs and some attached to nearby objects.
Now, I would like to detect blobs with flat surfaces. Any suggestion?
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Roohollah Milimonfared on 26 Aug 2019
That's right. Only expand it to 3D. An example of a 3D blob with flat surfaces is a cylinder (flat surfaces at its two ends).
In my problem, the blobs have more complicated shapes, but some have flat surfaces that I would like to use as a characteristic to detect them.

Image Analyst on 26 Aug 2019
Edited: Image Analyst on 26 Aug 2019
Try convhulln() and extract any blob that has points on the convex hull.
Roohollah Milimonfared on 26 Aug 2019
I have come up with this trivial solution.
In this problem, the direction of the flat surface is fixed (y-axis is always the axis normal to the flat surface). Sort the y-coordinates (the first column in ConvexHull) in descending order:
ConvexHull = stats.ConvexHull{1,1};
ConvexHull = sortrows(ConvexHull,1,'descend');
Find the Standard Deviation of the first 10% of the ConvexHull coordinates.
threshold = size(ConvexHull,1) * 0.1;
y_coor = ConvexHull(1:threshold,1);
x_coor = ConvexHull(1:threshold,2);
z_coor = ConvexHull(1:threshold,3);
y_std = std(y_coor) % 0.948
x_std = std(x_coor) % 23.676
z_std = std(z_coor) % 49.474
The value of y-std found to be significantly lower than its counterparts (x_std & z_std).
The positions of the 10% of the ConvexHull points in the blob indicates there should be a lower STD for y-coordinates:
Still, need to verify this for my other random-shaped blobs with flat surfaces.
Thoughts and opinions on this is much appreciated.

### More Answers (1)

darova on 26 Aug 2019
Once you have boundary faces: find all neighbour faces for each node
If angles between surfaces is about zero then we have a flat face

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