and vice versa.
Meaning, when the Normals are correctly oriented in Houdini, but exported as stl (through quick&dirty RMB on Node and save) the Normals have to be flipped in other apps. If I add a reverse node and they are oriented towards the inside, they are oriented correctly in other apps.
Why is this?

Some programs are right-handed and others left handed. Houdini is left handed, so geometry exported from Houdini in some formats will appear incorrectly oriented in other apps. Some formats have an implied orientation such as fbx/alembic and will be properly oriented on export.

Thanks for your nice explanation. Didn't know that.
Seems as if I will need to add an intermediate app in between then, as the stl is flipped in some slicers and in others not. An obj is imported in some and in other can't be loaded or displays with a closed top if it should be open.
In Blender the STL is imported correctly, mostly, as some parts, in this case the bottom of a sort-of-tube-object, had the normals inverted.

So the quickest and safest way seems to be to import the file in the Windows 10 3D-Builder App, do a quick auto-repair and resave.

And yes, the Houdini object is 100% correct (double checked the orientation of the normals), I even added a clean node to it, with manifold and others parameter checked. No points or anything was touched with that.

Hm, IMHO Houdini shouldn't struggle at all with this most basic of operation. Imho the file export (especially stl and obj) should have a re-look-into.

STL files are imported and exported as-is. Append a reverse node before saving the output as an stl to flip the handedness if your application requires backwards faces.

The same should be done on import of an STL file, append a reverse node after the file node reading the stl that was saved as right-handed.

jsmack
STL files are imported and exported as-is.

I think the point of this thread is that perhaps they shouldn't be

Since STL definition clearly states that the orientation in .STL file is defined by right handed rule, it'd make sense if Houdini respected that by default

tamte

jsmack
STL files are imported and exported as-is.

I think the point of this thread is that perhaps they shouldn't be

Since STL definition clearly states that the orientation in .STL file is defined by right handed rule, it'd make sense if Houdini respected that by default

Exactly
Imho stl and obj hasn't be touched for a long long time.

tamte

jsmack
STL files are imported and exported as-is.

I think the point of this thread is that perhaps they shouldn't be

Since STL definition clearly states that the orientation in .STL file is defined by right handed rule, it'd make sense if Houdini respected that by default

Noted same thing here.
Is anyone made a request for this ?

Not a critical thing but also certainly not hard to solve too.