Scratches on the surface from the edges (black lines)

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tvvladimir
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Joined: Tue Aug 21, 2018 12:31 pm

Scratches on the surface from the edges (black lines)

Post by tvvladimir » Wed Dec 05, 2018 10:58 am

Hi guys,

I get some sort of scratches on the renderings. They do not always appear, only sometimes.
Please see the picture, it is all covered by these scratches. It looks as if the edges of faces were tiny visible as black lines (shadow lines).
It seems that this only happens where there are a lot of very tiny surfaces as if they are collapsing on each other. But they do not, the surface is absolutely flat.
Scratches.png
I have tried Properties >> Normal >> Auto Smooth.
I have tried to rotate lights and camera.
Nothing helps.
I sometimes had the same problem in LuxRender. I have read somewhere that this issue has been resolved in LuxCore, but it seems that it is still present.

Please advise, what should I do?

Regards guys,
Vladimir

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B.Y.O.B.
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Re: Scratches on the surface from the edges (black lines)

Post by B.Y.O.B. » Wed Dec 05, 2018 11:05 am

I recently added an entry about this to the FAQ:
https://wiki.luxcorerender.org/BlendLux ... cate_faces
(See the entry "There are dark triangular artifacts on my model, and I already checked for duplicate faces")
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lacilaci
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Re: Scratches on the surface from the edges (black lines)

Post by lacilaci » Thu Dec 06, 2018 5:35 pm

B.Y.O.B. wrote:
Wed Dec 05, 2018 11:05 am
I recently added an entry about this to the FAQ:
https://wiki.luxcorerender.org/BlendLux ... cate_faces
(See the entry "There are dark triangular artifacts on my model, and I already checked for duplicate faces")
I would also try switching to cpu! This does look like the issue I keep mentioning on cpu/gpu rendering discrepancy. I had few models that shown this problem on gpu only.

Super annoying if you test in viewport on cpu and then hit render to discover these artifacts.

But the best and safest way is to make sure your cad models don't have elongated triangles. Usually there is an option on polygon ratio(to keep squares) or max edge length. Low poly triangulated models are always a problem in offline rendering. I've seen similar, although less prominent issues in corona too..

Fox
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Re: Scratches on the surface from the edges (black lines)

Post by Fox » Fri Dec 07, 2018 3:06 am

If you did boolean modifier in Blender to cut holes in mesh, there is this issue with unmanifolded edges and triangles that look like edges because they have 0 diameter and surfece normal is in unknow direction (backwards or in any other wrong direction).
Its better to have fairly dense mesh where you cut smaller hole, blender can't build faces between very long and short edges.
ctrl alt shift m cant find that bad geometry, see if the the face normals are all in same direction.
Bad Normal Sample.png

tvvladimir
Posts: 4
Joined: Tue Aug 21, 2018 12:31 pm

Re: Scratches on the surface from the edges (black lines)

Post by tvvladimir » Fri Dec 07, 2018 9:55 am

Hi everyone,

Thanks for the answers! The model is exported from CAD Solidworks. After I exported it to Blender I scaled it down to about 100 times. I have tried to scale the model up but instead I received even more scratches (black lines). And because the camera settings were changing because of the scaling, I decided to try change the minimum epsilon from 0.00001 to 0.0001 (multiply by 10). It worked like a charm! No problems with black lines. All fixed! Thank You so much for the help B.Y.O.B.!

"lacilaci" I think You are correct too! I have given the whole blender file to another person to try and he had no black lines whatsoever! But I need to use the GPU for faster render results and also not to overwhelm the PC that I'm working on.

"Fox" I don't do any edits of geometries inside Blender, I import all of them from Solidworks. But working with meshes, edges is too hard in Blender in anyway. Much faster to edit anything in CAD software, although You are right and I get so many uncomfortable faces.

Thanks again guys!

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