UnrollSrf on symmetrical object creates asymmetry

I have studied this object, its symmetrical, it has to be as I used mirror to make one side from the other ! using 0,0.
the only difference (deliberate) is the feet holes.

I select the faces that are non stretch on the bends for extractSrf, ignoring the far rear bend, else it wont unroll, I flip result to get unrollSrf to give the right way round for my needs. To get the rear face’s other bend I do this again selecting the unselected bend this time. I join the two up and in the process of all this I spot that if I mirror the right across to the left using centre of top hole the webs are mismatched. They are the same widths, but out when mirrored. the top hole is bang on centre line.
There is also a sliver missing, yet as said one half is mirror of the other.

symmetrical item sees UnrollSrf create asymmetry.3dm (1.3 MB)

something wrong but what ?

file attached has the original solid on one layer, and left and right halves for comparison on two more, the extractSrf on the ‘on’ layer.

…and now I see asymmetry on upper web, yet again made with mirror, if one mirrors the result of unrollSrf on hole centre (which I have also moved to axis line , its a tad out. two minute kinks in the sweep, one I now fix with blend, mirror it to the other and they dont match.

Top web symmetrical with mirror sees asymmetry with unrollsrf.3dm (289.6 KB)

I do an experiment as I find UnrollSrf had items not same size or proportion, eg a simple rectangle becomes non parallel sides. I have now drawn what dhould have resulted as an accurate rectangle. Then to lay the sides up to it correctly.
I flatten manually this shape, and do maths on the bend circumference using extract isocurve and analyze length, I get 0.064inch, lay the upright down 0.064 out from start of bend. UnrollSrf the bend and it doesnt reach the surface, close but not there. Thus UnrollSrf is nor recreating the surfaces.
is this due to the micro sized edge found in this thread ?

UnrollSrf versus maths.3dm (97.0 KB)



I have now noticed that in this, a simple rectangle loses its 90deg aspect in unrollSrf, i.e no longer a rectangle. (I have drawn a red line snap to same grid point to show the grid as grey wasnt so good for the printscreen)

Can one afford to ignore this difference ?

Would the micro edge of the surface attached to it be responsible ?

Rectangle loses exact shape in UnrollSrf.3dm (140.4 KB)


I don’t know how you’re determining this, both the original and the unrolled part in your file have the same dimensions to 3 decimal places and 90.000 degree corners.

I simply zoomed in, same amount on both. The graphic was zoom in on the offender, hit print screen, paste into photoshop, then shove hand to right to see the original surface., printscreen again.

I always zoom in to check all is good as such micro differences can catch one out if left unattended, and certain things such as a rectangle should be spot on,. Of course with Rhino we can zoom forever, but whilst looking at the curved surface edge to see if it started at the rectangle corner, I saw a slight thickness there so had to investigate.


i am not sure i get what your problem is. just to be sure that you know, rhino uses mesh to display surfaces which will lead to strange visual glitches at some points. you can avoid that by setting a tighter display mesh but it really only is for your consciences it will not change the actual accuracy of your model. also when you zoom in way beyond tolerance you will see artefacts. even two copied lines over each other will divert at these extreme points. that is no indication for the accuracy just a display “effect”.

1 Like

So as the surface after unrollSrf is a surface, the line in wireframe I see being a tad out compared to grid, and compared to the original surface also given snap to grid, comparing wireframes and not shaded mode which can vary depending on mesh settings, the difference in wireframes whilst being visible can be ignored.
I was trying to see why one half of this shape didnt match the other half when mirrored, and wondered if the skew was starting at the rectangle itself.

needing to know at what point one can ignore differences, what setting to have on dims and angles , how many decimal places etc, to see a match and move on.

I have been caught out so many times in the past with things that appear to join up, but later on show naked edges, and those pesky naked holes, all because I thought things aligned and matched, so now zoom more to check. having progressed for days into a project then had to redo curves and throw away days of work due to not having checked deep enough, I am now wary and zoom to check wireframes, I also use mesh max angle 0.35 to get a better visual for shaded view and surfaces , though the save goes from Mb to gigabyte.


yes, drawing accurately is the basis for it to work. rhino is very precise, it requires to work the same. main pitfall nr 1 is i would say having too many snaps turned on, mostly snaps which are useful but should never stay on all the time like “near” for instance. many things can happen when you start snapping just a bit aside, trying to find mistakes later is always a tedious workflow.

this is very general of course, i honestly did not check your model, i assume mitch checked and thought its good, so i assumed its the general visual glitch stuff.

Hi, having fixed the macro edge, both the asymmetry of two halves, and the asymmetry of the rectangle , are now solved, i.e. dont happen.

one microscopic line, and it meant zooming in a lot, as done on the rectangle edge, was the cause of all this !..so such is necessary it seems to spot for nasties.

SelSmall or SelShortCrv didnt find anything , so without such doing so, only a lot of zoom found it.

Good point on snaps, I have now turned off all snaps but Int as I want intersections of curves, in other words T joints, yet its snapping merrily along an S curve.