Strange artifacts when rendering

[sir_clive]

Dear all,

this is my first post and I am a Blender newbie, so I hope that I am not asking something stupid here.

I have modeled a plane consisting of parts which I will cut out to make a “real” RC plane out of it (yes, I’m misusing the Blender for engineering purposes ). Actually, I don’t really need the plane rendered, but I tought I’d give it a try anyway. The result was not what I expected, as you can see in the attached results.

On the left side, you see the plane’s inner living, just a screenshot showing nothing suspicious. On the right side is the result of the rendering (F12)

The wings are modeled by extruding a polyline imported from a 3DS file, scaled, “knifed”, and “bisected” where necessary. I removed the duplicates and made the normals consistent, but that didn’t help.

I also gave it a try with a fuselage set to visible (I know some parts are protruding, the fuselage was only used to trim the parts down to the size I’ll need later ). The fuselage was drawn by taking a cube, stretching it cutting, extruding and generally modifying it.

On the screen (left side), it looks just fine, but the rendering is pretty strange:

However, as I made a minor change to the “subsurf” modifier of the fuselage (actually, I pulled the value under “render” down to 0), the rendering of the fuselage started to work fine! Setting the value back to 2, it still rendered nicely:

I’m confused about the problem with the fuselage, but at least it’s working now. The wings, however, are still not.

Oh, I almost forgot: this is the version I am using:

alex@charlie:~$ blender --version
Blender 2.70 (sub 0)
build date: 2014-03-19
build time: 05:42:45
build commit date: 2014-03-19
build commit time: 05:02
build hash: 19e627c
build platform: Linux:64bit
build type: Release

I hope somebody can help me out with this one!

[sir_clive]

Oh. I just realized that in the area of the horizontal stabilizer, I had the “helper” objects turned on while rendering! So please ignore the horiz stab and how it looks. Had I turned off that helper object (the boxy-style stabilizer), you would see that the horizontal stabilizer looks pretty much the same as the vertical one, as far as rendering artifacts are concerned.

[IkariShinji (Thies Schulz-Holland)]

From the looks of it I would say you have overlapping geometry here: Identical geometry that occupies exactly the same space. This confuses the render engine and it renders surface A for some pixels and surface B for others, leading to the artefacts you can see (this effect is also known as “Z-fighting”).

As seemingly only the symmetrical parts of that mesh are concerned, I would take a wild guess and say: Mirror modifier. Perhaps a mirror modifier on an already complete mesh. Or two mirror modifiers on the same mesh as a left-over from joining two meshes.

[sir_clive]

Hello Ikari,

thank you for your answer! Actually, the wings should only consist of one mesh, so I suppose Z-fighting should not be the problem. I’ll try to reproduce it by starting out with a fresh polyline and extract it just the same way as I extracted the wings to see if it happens again. Maybe I just use the extraction function in a wrong way, always producing two surfaces instead of just one.

Mirror modifier is a good hint too, but unfortunately, the rudder and the vertical stabilizers are also affected, and these parts do not have a mirror modifier attached to them.

[IkariShinji (Thies Schulz-Holland)]

Maybe it’s easier if you strip down your scene to the simplest version that still shows the issue and then upload the file to pasteall. This way you can provide us with a download link and we (as in: the community) can examine the file directly.

Might speed up the process of solving this problem?

[sir_clive]

That’s very nice of you - I just reduced the scene to just one wing half, one light, camera, a few materials and the polyline I used to extrude my wing.

When extruding the wing again using that polyline, it seems I get the same problem:

I have uploaded the .blend file to http://pasteall.org/blend/28633. I extrude by selecting the line in object mode, TAB to EDIT mode, press E, press ESC, grab and pull the X axis.

ACTUALLY, as I re-read this, I tried to extrude my wing in a little bit different way: in EDIT mode, select all vertices of the line, press F to make a face out of the line, then while still in EDIT mode, grab the X axis and move it. Voila, it worked - a perfectly nice wing which renders just at it should!

Now, the problem is solved (kind of at least), but I’d still be thankful for a tip - what did I do wrong in my previous attempt? Everything looked fine, the wire frame view also showed nothing suspicious, and yet the rendered image was still all wrong.

[IkariShinji (Thies Schulz-Holland)]

The problem is that polyline from 3ds max. It’s not just a single sequence of vertices connected by lines, but actually a double row of vertices connected by polygons (I pulled one vertex a little bit down here to illustrate):

Those extra vertices are the tiniest bit apart, so that “Remove Doubles” seemingly can not spot them. This also explains the weird extrude “constraint”, as Blender wants to follow the normals of those invisible polygons.

Once the extra vertices are deleted, a new extrude works as expected and renders fine:

So the issue indeed was Z-fighting, as the extrusion of that double line of vertices created two co-planar surfaces.

[sir_clive]

Wow, thank you for solving this mistery!

I’ll try to find out where the error actually happened. This is a line I actually drawn myself (manually) in an older version of TurboCAD, as a polyline, so it could be that either the export from TurboCAD had a problem, or that import into Blender didn’t work correctly.

Again, thank you!