Unroll developable cylindrical Srf generates Area deviation

Hey friends,

I am currently designing a rooftop geometry, based on combined cylindrical surfaces.
When I unroll these surfaces, although they are mathematically developable, I noticed, that there are area deviation from original to unrolled surfaces… Is it a bug? One surface generates a deviation of 0.05% multiplied by 80 times gives me a deviation of ca. 4%… To much…

Why is there anyhow a deviation although mathematically they should exaclty match…

thanks for explantions. Matthias.

Hi Matthias - testing this with a simple cylinder, the deviation is much smaller than that. Could you post a 3dm model that shows this issue?

If multiple surfaces each have a deviation of 0.05%, then the total deviation will be 0.05%. Percent deviation does not simply add for multiple surfaces.

Total percent deviation of multiple surfaces = sum of the area changes / sum of the original areas * 100

Percent deviation = Change in area / Original area x 100.

Change in area = Original area * Percent deviation / 100

Example: 80 surfaces, original area of each surface is 50, percent deviation of each is 0.05%

Change in area of each = 50 * 0.05 / 100 = 0.025

Total change in area is 80 * 0.025 = 2

Total area = 80 * 50 = 4000

Total percent deviation = 2 / 4000 * 100 = 0.05%

Most likely your surfaces as modeled are not exactly developable. Post a .3dm file with an example of a surface which has the deviation. You can use the vertical arrow above where you type your post.

190704_Test Cylinder Unroll.3dm (2.8 MB)

Dear David,

Got your calculation.

Attached you get the 3dm file. All surface are based on ExtrudeCrv. So should be 100% cylindrical.
I tested: If i cut the cylindrical surface in a diagonal angle, than it generates the deviation in area calculation between original and unrolled srf.

By the way: Rhino on Mac 6 Beta…



Anyways, what is your precision set to?
And the size of your mesh?

Hi Matthias - try setting the file tolerance to .001 before unrolling.