Unfold tower point cloud "data voids"

[CMacM]

Hi,
I have a project where we have carried out a laser scan survey of two heritage structures, both old somewhat circular fort flankers for conservation work. We have been asked to supply "unwrapped" external and internal elevations of the structures which I hope to provide by way of useing the unfold function in CC.
I've imported the registered point clouds for each of the towers which includes all of the structure inside and out. Created a polyline path and run the unfold command, with a thickness of 2m to make sure I get both the inside and outside elevations. I'm getting mixed results - the outside elevation looks good but the internal elevation contains a series of striped data voids. The only difference between the two is that there outside elevation is relatively flat while internally there is a butress/step along the lower portion of the elevation.
I've tried increasing the number of nodes in the polyline path and also the search radius to increase the thickness of the slice but this doesnt fill in all the gaps. The walls are relatively straight and I've used the UCS Z value native to the registered point cloud.

Any suggestions?
Ciara

[daniel]

For the internal wall, are you defining a different polyline? Because the only reason why these stripes would appear is that the polyline is too far.

[CMacM]

I'm using the same polyline for the internal and external which unwraps both sides, then I clip out the relevant sides for export to CAD.
The polyline is placed within the centre/core of the wall and then I define a distance of say 2m for the thickness. I've tried setting the polyline at the base of the wall but it doesnt seem to produce as good results.
Is the thickness defined as being either side of the polyline or is it external/internal to the polyline?
Ciara

[daniel]

Sorry for the delay, I was travelling last week.

Would you be able to share the cloud with me? (to admin[at]cloudcompare.org)

[CMacM]

Hi Daniel,
thanks for getting back. Would be great if you were able to take a look. I've emailed you a link to the point cloud.

thanks
Ciara

[daniel]

Thanks for the data.

So I confirm that you really need to define 2 polylines, one for each face of the wall, otherwise you'll get 'empty stripes' as the farther the points are from the segments of the polyline, the more space there will be between them (it's like the paper box patterns with a polygonal or round shape):

paper_box.JPG (12.24 KiB) Viewed 2705 times

So either you can manually draw the 2 contours, or you can take more time and 'extract' the contours from the cloud directly. Of course this requires quite some tinkering... I used the Cross Section tool first to get slices of the cloud with clean wall contours:

cross_section.JPG (72.46 KiB) Viewed 2705 times

Then I cleaned the slices, subsampled them, and extracted the enveloppe of each (with the Cross Section tool again):

contours.JPG (16.88 KiB) Viewed 2705 times

Eventually I was able to use these polyline in the Section extraction/Unfold tool to get nice 'developpée':

inner_wall.JPG (151.57 KiB) Viewed 2705 times

outer_wall.JPG (200.6 KiB) Viewed 2705 times

I'll share with you the files in case it's useful to you.