Preview Display Glitch

Recently I was working on a project and my Preview display started to glitch. This was in Rhino 5.0 64bit and both GH and Rhino are fully updated. I did a internet search and found another thread which did not seem to get resolved with what appeared to be a similar sort of glitched view-port.

(red stuff should be the exact same geo as the the grey poly srfs)

I tracked the error I think to the objects existing very very far from the origin.

In this case I was working with inches and the original data set came from surveyor lines, at some point I scaled that data into 24 inches(yes going from absurdly large numbers to 24) and just didn't bother to move it back to 0,0 until I was almost done with the project. I believe the glitch only appeared after I scaled the data but am not certain on that. so originally I was thinking the issue was related to geometry size vs scene scale, but my scene scale was in inches so I just ignored the error and continued working. I just now moved everything to 0,0 to organize the scene a bit. The display instantly started working again in GH. I think it is probably safe to assume that my glitch was caused from working so far away from origin. I thought I would share in case others had a similar issue.

(the closest to the same chunk of surfaces I have left no longer glitching)

Replies to This Discussion

Permalink Reply by David Rutten on September 12, 2013 at 8:10am

Hi Donald,

yes, this sort of artifact is typical when dealing with very large numbers. Either because your model is enormous or because it is far away from the origin. The bigger the number, the less room there is for accuracy. Digital numbers are stored a bit like scientific notation:

0.643 × 10³

Depending on the type of number, there are actually 8 or 16 digits rather than above three. However you can see that when the exponent is zero, you have 3 decimals to work with. If the exponent becomes one, then the number on the left (known as the mantissa in programming) is multiplied by ten, leaving you only 2 decimals. If the power is three, then multiple the mantissa by a thousand, leaving you no decimal places.

In other words; the larger the power, the bigger the numbers and the bigger the gaps between two adjacent numbers. When something is really far away from the origin, the gaps become so big as to not allow for smooth geometry.

Meshes in Rhino use a lower accuracy than Nurbs geometry, which is why they suffer first. In Rhino5 high precision meshes are optional, in Rhino6 they will become standard.

There is at present no fix for this problem other than moving the geometry closer to the origin and modeling in sensible units, which is what you've already done.

David Rutten

david@mcneel.com

Tirol, Austria

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