Inaccuracy on Circles and Arcs

Tygan

Hello,

I am not sure if this problem is related to this:https://forum.onshape.com/discussion/comment/4133#Comment_4133
When I draw a circle they are slightly faceted and a projected line does not align properly with the element it is based on.


This results in some weird glitches like a coincident constrained point which is not on the corresponding line:


Is this only a visual problem, what points are used if i export a stl for example?

Here is the Document:
https://cad.onshape.com/documents/369e63f73b7555945f0bf77e/w/e3791927b25ecb2534a0eb4e/e/12d45d9ece01aa78014522aa

The screenshots are from Skecth 3 and 4.

tim_hess427

I believe this is the same issue as in the thread you linked. As I understand it, its purely a visual issue - the curves are approximated for the purpose of drawing on the screen, but the underlying geometry is still correct.

STL exports have their own mesh settings that you can change while you're exporting the file.

S1mon

All CAD systems I've used have this issue to varying degrees. Even with the fastest graphics cards, there's always a tradeoff of how fine to do the on-screen tessellation vs the performance. Each time the geometry changes, those triangles (3D) or line segments (2D) need to be updated. Every time you spin things around the graphics engine needs to figure out which triangles are being displayed, what color are they, where are they, etc... It's a lot of math. GPUs accelerate the math, but they don't magically make it go away. You can increase the tessellation quality (right-click on the part and select "Edit Appearance...") which may help, but depending on the curvature and the zoom you may still see things which don't quite make sense.
 

Tygan

I am fine with the tessellation for better performance. But the corresponding Lines should fit, especially in sketches.



The outline of the grey plane is the same as the orange line, first I thought I made some misktakes and projected the wrong edge.
It looks like the origin for the tessellation is different and thats why they are not aligned.

S1mon

Those kinds of artifacts are definitely the ones that drive me crazy, but it's difficult to have all the different curves, edges, surfaces, etc all have the same tessellation parameters. You'd certainly like the end point of that line to be "on" the curve that its aligned to, but you can also see that the chord height of the curve is making it look that far off. I haven't dug into this enough in Onshape to see what it's doing right and wrong, but typical tessellation algorithms have a lot of controls. Onshape only exposes an overall quality value, but some other tools give more fine-grained controls (this is from Rhino):
 
In this case what you're fighting with is the "maximum distance, edge to surface". Other tools would call it chord height. You'd like it to be a small enough value that at a "reasonable" zoom level that end point looks like it's on the curve. I can't remember, but I think the "jagged seams" option being off would fix the mismatch between the shaded area and the curve.

Here's another example of similar tessellation controls:
https://docs.unrealengine.com/en-US/Engine/Content/Importing/Datasmith/SoftwareInteropGuides/CAD/index.html