unroll help please rhino

[Willallison]

Thanks Gary - at least it looks like the original surface now!
Interesting to to note that your unrolled surface looks quite different to mine.... mine bears a closer resemblance to the original surface - close enough that I alsmost missed the difference - which makes me nervous about some of my other panels...
Is there any way of checking it's accuracy to the original surface? (Other than building it in very expensive cnc cut frp/foam panels that might not fit!)
I've also sent you a PM

Thanks again...

[duluthboats]

Surface area is one way that might help check. I don't have much time now but will look at again in the AM.
Gary

Originally Posted by duluthboats

Surface area is one way that might help check. I don't have much time now but will look at again in the AM.
Gary

you could check in workshop maxsurf and compare,

[Willallison]

Thanks LJ - I have considered doing that, but I know that transfering between programs can introduce errors ... still... worth looking at.

I compared surface area's, as Gary suggested:
Original surface was 1955058.61 sq.mm
Gary's squished surface 1955134.96 sq.mm
Which at 1st glance appears to be quite a differenec, but is in fact only about 0.2%

This got me interested, so I compared a number of those that I've unrolled to their originals. In all cases the unrolled surfaces are marginally larger, but all by less than 0.3%. This would suggest that, so long as the basic geometry is the same, the unrolled surfaces could at least be trimmed if necessary to fit.

Comparing the perimeter length showed the squished and original surfaces to be within 1mm of each other, however, so perhaps it is indeed the geometry that is slightly different. I wonder though if it's any less accurate than one would achieve by doing it the old fashioned way - by hand - or indeed if it's more accurate...

I also compared the areas of unrolled and original surfaces that were already horizontal (and flat). In these instances the differences between original and unrolled surfaces were negligable...

[dougfrolich]

I think that the issue with unrolling your surface is that it is not a fair, and developable surface. Use th surface analysis tool to see what I mean, set it to auto range and Gussian.
Doug

[duluthboats]

I played with it some more. Here is your original surface unrolled no other changes. And here is that surface rebuilt and squished. I overlaid them and there is very little difference, just less than 3mm from one end to the other. I think they are very close to what they should be. I would go with the original and unroll.
Gary

[Willallison]

Thanks Doug. The polysurface is in fact made up of two smaller surfaces. I had checked the fairness of the larger and more irregular of the two (which is ok) I didn't bother checking the smaller one as it (should!) simply be a flat plane derived from the 1st one. Lesson learned....
However, there was still an obvious difference between the original and unrolled versions of the larger of the two - though Gary's efforts seem to have been more succesful than mine....

[Willallison]

Ok - now this is just getting silly!
Due to the unfairness, I re-constructed the surface and tried unrolling once more. As you can clearly see in the attached file, there's a considerable difference between Gary's and my unrolled surfaces. Where Gary's accurately reproduces the curvature of the sheer (the top edge of the surface) mine straightens it out. And edges that were (and should be) parallel are now not....
I'm sure this is more a shortcoming in my Rhino prowess than a failing of the program - but it sure is bloody frustrating!!

[Willallison]

By Jove, I think I've done it!
After some very patient (and much appreciated) help from Gary, I think I've had at least partial success. For those interested, and in the hope that it may help others (and I guess so one of you can tell me what a twit I am if it's wrong! ) I'll describe what I did.
Essentially, this surface is a vertical plane - albeit a rather irregularly shaped one. I reconstructed the surface by extruding vertically down from the top edge (the sheer) and then trimmed to match my orginal surface.
As I described above, this still failed to unroll successfully.
So, per some of Gary's instructions (for reasons I shan't go into here, I couldn't carry them out to the letter) I duplicated the edges and constructed a new surface by using the Patch command. I then rebuilt this surface with only a few points. Unrolling this surface gave (what at least appears to be) the correct surface - and one that matches very close to the one Gary kindly made for me.
Quite why one needs to construct a new surface and rebuild, rather than simply rebuilding the original (tried that...) I can't figure, but hey, success is its own reward as they say, so if you'll excuse me I'm going to go do a little jig around the room

As always, thanks to all...

Will I am cracking up I have been strugglin with Rhino for a year, but my tutor is very patient I could have lofted a squillion boats longhand whilst I have bin piddlin' around avec Rhino

[Willallison]

Glad my trials and tribulations are of at least some use - you will be further amused then to know that I've moved on to another surface...

It's worse!!!!

measuring lengths and stuff) I know I can only get within 4mm, how can I reset, looked most places