Polygons that share the same position, but flipped normal on one have artifacts.

I made some airvents… i duplicate them and flip normals on duplicate so they share the same position but now have an inside to them… Problem is that in the scene i get strange flickering black artifacts as i move the camera around… i already disabled shadows on lights just for testing purposes… flickering remains… any ideas? This does not happen in unlit mode.

It looks like a typical z-fighting issue. Most likely you have a duplicate faces there somehow, or your normals are not really flipped.

Thats not what is happening… already checked… it does this same thign with lamps from the unreal market htat i purchased. There is no black geometry to z fight.

In a case like this I would recommend checking two sided on your material rather than duplicating and inverting your mesh. This will allow the material and textures to display on the inside as well. If that doesn’t suit your needs let me know why, and we can work out a solution for this situation. :slight_smile:


Well it would make all the geometry 2 sided and i only need certain peices of it to be. So is this just a flaw we have to live with and design around? Because then i need to go mess with some of the assets i purchased on the unreal market and adjust their geometry/materials as well…

A good way to work around this would be duplicate the vent material (or make an instance) and have a one sided and a two sided version.

Apply the two sided material to sections of the mesh that you need as a separate element (you will need to assign another material ID to those parts of the mesh in modeling software).

If for some reason Enabling Two sided, as suggested, does not suit you, could you post a screenshot of the mesh in question, taken from static mesh viewer with vertex normals display enabled?

No point… the mesh is simple… flat plane for wall… copy paste invert normal.

Ed will i have to go through all meshes that have this issue and reimport them to use 2 sided materials? its going to waste a lot of man hours for me… any plans on fixing this?

Hey Ed, just responding with a question… will be a major pain to reimport every mesh and modify it so they use double sided materials… since i have a lot of meshes like this. Dont really want to waste man hours. Are there any plans to fix this issue?

This also begs the question… if i use single sided polygons and just use a double sided material… will collision work on both sides? Or just the original side thats visible… because i need both sides to have collision.

Hey Kurylo3d,

This doesn’t seem to be an issue with the engine. I ran a test using a workflow similar to the one described initially and didn’t experience any flickering. Make sure that when you flipped the normals on your mesh in your 3d application that you did so correctly.

Regarding your question about a one sided polygon. You will have to check “double sided geometry” in the static mesh settings or else it will only collide on the original side. The double sided material is visual and doesn’t influence collision.



The geometry and normals are correct Ed… it even does this on unreal market lamps from one of the model packs… If it was double sided z fight issue u wouldnt see blackness… u would see more of the same vent and probably wouldnt even be able to realize there was any z fighting… No amount of lighting can light up those black artifact spots. just solid black… u can put 8 point lights in 8 different angles all around the object… so obviously something very wrong is happening. No idea why u dont experience it but i experience it on a gtx 980 and a gtx 590.

On a side note… any reason it would turn black completely after a lighting build? Does double sided materials lighting not work well with built lights?

I’m not sure why you mentioned z-fighting or double sided geometry in regards to the issue. Let me restate my post to clear up any misunderstanding.

It has been determined that this isn’t a bug after following the workflow of taking a static mesh, duplicating it, and flipping the normals.

I posted a screenshot showing that I experienced no issues with the workflow you described. I then go on to answer a question you asked about if a one sided plane would have collision on both sides if the material is set to “2-sided”

The answer is no, if you want a one sided plane to have collision on both sided you need to enable “double sided geometry” in the static mesh settings with per poly collision.

If you are sure that you have found a bug you will need to reproduce the issue with minimal assets needed and a clear set of steps that will reproduce with either Epic content or Engine content otherwise this investigation cannot continue.

So basically its a bug that will not be fixed because u have no default content that mimics this type of geometry?

I mean … u have the screenshot… dont know how much more proof of bug i can give u.

Like i said… in unlit mode u do not encounter the bug… but in lit mode u do… u shouldnt… If it was z fighting ud still see the same black spots in unlit mode … or at the very least be able to light up those spots in light mode with 8 point lights.

Here is another screen shot of another asset on the unreal market that you could use to test with.

Fun fact… the artifact doesnt happen in any mesh in the asset browser as u can see in the screenshot… just in the world.

A screenshot of visual artifacts doesn’t necessarily mean that the issue lies with the engine. There are many cases when the issue lies with the content. This is why usually more information is needed and it is recommended that tests be run in a blank project with engine content to prove that the bug lies in the engine and isn’t user error/bad content.

In the bug reports section our main priority is to determine if the post is a bug with the engine that needs to be reported.

Usually we will answer questions or offer additional assistance as a courtesy, but that is not the purpose of this section.

If you are unable to reproduce an issue at all with the assets provided in the engine content folder, most of the time that means the issue is with the user’s content.

If you would like, it is also acceptable to provide a small sample project that reproduces the issue with your own content. This can be sent to me in a PM over the forums. Please only include the assets needed to reproduce the issue.

Also, we are not directly responsible for marketplace content. If you are seeing visual artifacts with marketplace assets you need to contact the seller that you purchased the assets from.

As a final note, I’m not stating that this issue has anything to do with z-fighting. There isn’t enough information provided to determine a cause.

I hope that clears everything up!

Hey Ed, I know its been a while since i responded to this thread, but the issue may have to do with both sides sharing the same vert… like the same polygon pointed out… then a duplicate with flipped normal, except instead of it being seperate perhaps the verts for both objects are merged… Thats the only idea i have on why it happens. Certain assets on the unreal market have this issue like the lamp i showed u in the previous screenshot… Is there anyway u could of think of fixing this without a complete overall on geometry?

This issue looks like a bug that we logged not too long ago here: Unreal Engine Issues and Bug Tracker (UE-41922)

The user that reported the issue noted that adding a constant pixel depth offset (0.5 for example) to the applied material fixed the issue. Also, unchecking “us as occluder” gets rid of the artifacts as well.

Try that and see if that fixes the issue if not your problem is something else.

Just found if i merge all verts in a mesh by .0001 … it fixes the problem… no idea why.

well, I’m glad you resolved your issue. I’ll mark this as answered for now. Feel free to leave a comment if anything else comes up.