Numerical artefacts?

[Dim3nsioneer 557]

Sometimes Cura 13.06.4 makes strange things when dealing with a horizontal surface created in Blender:

The layer height was set to 0.1mm.

To me it looks as if Cura interprets a z-position of e.g. 3.00001mm or similar as >3mm and adds a new layer. Depending on the program used to design a 3D model such artefacts may appear due to the numerical precision.

To check this hypotesis, I moved the lowest z positions up by 0.05mm in Blender. This is what I got:

Yes, this is what it should look like...

To the Cura designers: Is there a possibility to make Cura a bit less sensitive to such minor precision fluctuations? At least when not sclicing with a layer thickness of 20um....

 

[gr5 2,184]

Daid wrote cura - he will probably reply to you but...

You have to slice somewhere. If you always raise the slices by .000001 then this problem will just happen somewhere else. The root problem is that blender doesn't have a flat bottom.

The way Cura works is it performs a step where it slices the model and ends up with many random line segments in no particular order. This is because the stl format doesn't have the polygons in any particular order.

Then Cura has to take these line segments and figure out which ones are connected into loops.

Anyway it's that first step where it does the slice -- are you saying when cura goes to slice at 3mm it should slice at 2.99mm, 2.98mm and 3.01mm and pick whichever is most similar? So if it looks almost the same at 2.98,2.99,3.0 but completely different at 3.01 and different again at 3.02 it should pick the 2.99 slice because it is more consistent?

Really blender should make flat bottoms flat and not with bumps in them.

Also notice that this layer would have printed fine anyway as Cura is adding some solid support for what is about to happen in layers above.

 

[Dim3nsioneer 557]

I agree on Blender's (non-)precision...

And Cura is doing a fine job usually (no offense...), but I'm just thinking of those people who now buy the Ultimaker2 as they are told it's plug-n-play. I think the software is a major piece in this 'one-button-3D-printer' puzzle. And it has to be almost fail-save for those people...independet of the model's quality as non-experts will certainly deal with non-perfect models.

It may be a very simpel idea, but what about giving the user a choice? Cura could for instance slice the STL three times, once the 'normal' way, once with an offset of +something and the third time with an offset of -something. Then the three results could be compared. Identical layers are then identified as 'stable'. For the 'unstable' layers, Cura could ask the user which version he wants. As any file-sync-software is doing it when both file locations have been changed... maybe there are even some criteria to decide which of the three versions is the best and Cura could do the job itself...

 

[Daid 306]

The user has a choice already, you can simply adjust the "cut of bottom" value a bit to lower/raise the model a bit. This will adjust the slicing position.

Comparing slices is a difficult and expensive (in time) processes, which isn't as easy as comparing files for changes.