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Help me making my plane print friendly ?

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Message 1 of 8
Saygus
814 Views, 7 Replies

Help me making my plane print friendly ?

Hello, i exported a plane from gta v, then i cut the wings etc on fusion 360 and i filled the big holes still on fusion 360

Here the result when i import the .obj in MeshMixer:

jet.PNG

 Then i tried the inspector and i have a lot of errors

inspector.PNG

If i click auto repair all I only have a few polygons left 😞

I can reduce the amount of errors with "Close cracks" but i still have critical errors and it reduces quality, i also tried the "Make solid" function but i can't get a correct result, always loose quality a lot.

My goal is to add pins to plug the wings, but when i try a boolean difference i have an error

I'm very new in the domain, do you have a solution so i can do i want to do ? Or at least a lead that could help me ? Thx !

7 REPLIES 7
Message 2 of 8
MagWeb
in reply to: Saygus

As this is a common issue allow some explaining:

 

Basically pink markers in Inspector mean small not connected surfaces. This "error" is meant to remove outliers as one might get in a scanning result. Inspector will remove these surfaces.

If you are dealing with a mesh coming from some CAD package: Most likely you do not want to remove tiny elements. So pull the SmallThresh down to zero. This parameter defines what size ratio you consider as being small.

 

Now: Most probably dimming down SmallThresh some markers will turn to red. Red means a non manifold issue. There may be non manifold faces (more than two triangles share the same edge) or non manifold vertices (some triangle "spins off" a closed surface but is connected via a single vertex). Now: It is hard to repair such an issue. The problem is to decide which faces are right, which are wrong. Inspector deletes all triangles involved in such an issue and fills the resulting hole (better or worse). Now on a low density mesh it might happen that two manifold issues are next to each other. Deleting and filling might give garbage on such meshes.

 

So you need to learn how to avoid such issues on a handshake between a CAD BRep representation and a mesh. The problem is the commonly used exchange format STL.

 

A STL (here I use human readable ASCII) isn't literally a mesh but a bunch of loose triangles.

This is a simple 3 sided pyramid made of 4 triangles only:

solid OBJECT
  facet normal -0.77459667480253958 0.44721359068394201 0.44721359068394201
    outer loop
      vertex 0 0 1
      vertex 0 1 0
      vertex -0.86602538824081421 -0.5 0
    endloop
  endfacet
  facet normal 0 -0.89442719099991586 0.44721359549995793
    outer loop
      vertex 0 0 1
      vertex -0.86602538824081421 -0.5 0
      vertex 0.86602538824081421 -0.5 0
    endloop
  endfacet
  facet normal 0.77459667480253958 0.44721359068394201 0.44721359068394201
    outer loop
      vertex 0 0 1
      vertex 0.86602538824081421 -0.5 0
      vertex 0 1 0
    endloop
  endfacet
  facet normal -0 0 -1
    outer loop
      vertex -0.86602538824081421 -0.5 0
      vertex 0 1 0
      vertex 0.86602538824081421 -0.5 0
    endloop
  endfacet
endsolid OBJECT

There simply isn't some information about which triangle loop should be welded to another. This welding magic happens in the software importing a STL using a simple rule: Weld faces sharing a common edge. 

The direction the front of a triangle points to is based on the order the vertex lines:

Lets say there are 3 points with XYZ coordinates. A = (0,0,0) B = (1,0,0) and C = (1,0,1)

> The triangle ABC looks to the opposite of ACB

 

Now it might happen that there are two or more faces with exactly same vertex XYZ coordinates (possible reason below). To avoid such redundant faces MM ignores further appearance of a face if it already exists in the loaded ones. That's no problem as long as the second appearance owns the same ABC (or ACB) order than its first. But for the list of triangles might be arbitrary and if there are different direction-versions of a redundant face the result in the scene might show flipped regions because of that. In MM the backfire of a triangle is shown with a pink stripe pattern.

 

Handshake from CAD via STL:

If you did a pyramid in CAD and mirror it at the ground plane you have two bodies like this:

Ohne Titel4.jpeg

You have three basic options:

  1. Export each body as a separate mesh
  2. Export both bodies to a single STL
  3. Do a boolean union first and export this result.

Option two might be fatal exporting to STL if both bodies share a same common surface (as the pyramides' bases here).

Here are the resulting files (lefthand the result of a single export):

Ohne Titel.jpeg 

The orange marked faces are redundant faces resulting of the common base triangle(option2) while there aren't such duplicates in option1 and option3

 

What happens on import to MM?

For option2 MM will ignore the lower orange region and will try to weld green, blue and the remaining orange region. But welding the orange to blue and green rises a non manifold issue: More than two faces share the same edge.

MM highlights such edges with a red line:

Ohne Titel1.jpeg

Two sides of the blue object set to transparent to show the bad triangle:

Ohne Titel2.jpeg

 

End of the Story:

You can avoid such problems

either:

Export a scene using another (indexed) format (as OBJ) which ships the information of how to weld faces.

or:

Export bodies with common surfaces as separate files

or:

Join different bodies to a single one (IMO the best way)

 

 

 

 

 



Gunter Weber
Triangle Artisan

Message 3 of 8
hfcandrew
in reply to: MagWeb

That was the best explanation on this topic I have read to date! You should write a textbook.

Message 4 of 8
Saygus
in reply to: MagWeb

Wow thank's a lot for these info about how a stl file work ^^

Actually i was working with .obj files

 

Now i can add pins and create holes with boolean diff (no error anymore), but the hole appear only behind the striped parts like that:

Capture.PNG

 

 

 

Message 5 of 8
MagWeb
in reply to: Saygus

I'm pretty sure that somewhere in the life of that model (might be before it was added to gta v) such a STL conflict was involved or it is simply bad meshing.

 

Where you see striped surfaces you are watching flipped faces. You need to invert their front face direction. There's a blue line around that region on your image. This line means an open boundary.

If this line is a closed loop there's no connection to the white region. In this case the workflow is easy:

  • Go to SELECT. Enable AllowBackFaces and double click that region. The pink stripes should turn to black (= selected faces watched from the rear). Run Edit/FlipNormals on that selection. Now the region should be white as the rest.
  • That done you need to weld the boundaries. Zoom in near to a region of the boundary.

    Go to SELECT and set a minimal brush Size. Double click on a triangle at the boundary. Only a loop of faces around the boundary should be orange now. Do the same on the other side of the boundary. Now if you have both "banks" selected run Edit/WeldBoundaries > Accept.

    If there are still cracks or holes remaining: Use Inspector to close them clicking the blue spheres.

If there's a connection between flipped and right faces:

  • Run EDIT/GenerateFaceGroups. Set Mode to NormalAngle, AngleThresh to max and Accept
  • In SELECT (AllowBackFaces enabled) Double click the striped region to turn black and do Edit/FlipNormals
  • Use Inspector to get rid of the open boundary.

 

 



Gunter Weber
Triangle Artisan

Message 6 of 8
Saygus
in reply to: MagWeb

Ok look like it works now

 

Capture.PNG

 

 

 

 

 

 

 

 

 

 This said i still have billions of blue lines in the cockpit

cockpit.PNG

 

 

 

 

 

You think i can export and pray for my slicer to deal with it ? ^^

 

Many thank's anyway !!

Message 7 of 8
MagWeb
in reply to: Saygus

Hard to say. Can't judge if the open bondaries are near enough to a closed surface. Also might depend on the scale you want to print and the layers' height...

 

I would do this to give the isolated shells some thickness:

  • Go to SELECT and select some area on the main body of the plane. Hit E on your keyboard to expand the selection to the connected surface. Hit Y to separate it.
  • Hide the separated body via eye icon in the object browser.
  • Switch to the object with the tiny elements now.
  • Check if there are elements without a blue boundary.
  • If there are any without boundaries: In SELECT double click them until all such elements are selected and hit Y again.
  • Switch back to the object with pieces all owning an open boundary.
  • Hit ctrl+A (cmd+A on macOS) to SelectAll
  • Run Edit/Extrude on that selection and set a sufficient negative offset to get a bit of intersection between the parts.
  • That done show and activate all objects to EDIT/Combine them again.

Start of the art slicers should handle intersecting elements just fine. 



Gunter Weber
Triangle Artisan

Message 8 of 8
Saygus
in reply to: MagWeb

Very nice !

 

I'll print the cockpit alone to see if it works good, but i'm confidant 😉

 

Thx again for your help, MechMixer is a really impressive tool when we know how to use it !

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