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Lightmaps artifacts with objects behind a surface

I'm experiencing strange artifacts trying to build the lightmaps.

The scene is part of an archviz project, walls and ceilings are built with 2d poligons with a 2 sided diffuse material. The interior is lit by indirect lighting only. In this scenario lights are mapped more ore less correctly:

alt text

In the exactly same context, introducing objects behind these meshes (e.g. another 2d exterior shell behind the wall with the same 2 sided material) changes completely the luminosity inside the environment (even if it does not occlude it in any way) and causes strange artifacts as in the picture below:

alt text

The artifacts are completely independent from the lighmap size of the visible objects (I don't think they are bleedings from other objects), and seem due to some interaction with the objects behind the wall.

What may these artifacts be due to? any suggestion?

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asked May 28 '14 at 10:30 AM in Rendering

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avatar image Lovecraft_K ♦♦ STAFF May 28 '14 at 05:39 PM

HI devel.bmad -

Just to clarify your setup, we are looking at the interior ceiling which is constructed with planes on which you have applied a two-sided material. Lighting works fine until you add another set of planes on top (what is the distance?) for the exterior which I am assuming is the roof. The second set of planes also has a two-sided material.

Just from first glance, why do your planes need a two-sided material? This setup, again if I am understanding you correctly, may be causing the errors in the lighting you are seeing. As the plane is trying to deal with rendering two unique lighting situations one in the interior and one between interior and exterior planes.

Let me know -

Eric Ketchum

avatar image devel.bmad May 29 '14 at 09:59 AM

The second set of planes (the roof, but it is useful just to prevent light leaking inside in other parts of the model) is about 40cm (unreal units) away. The planes are two sided because without the external layer the walls otherwise would not stop the sun to enter directly in the room, which would mess up the lighting.

Anyway, in case of external layer present, using 1 side material seems to lighten a little bit the artifacts but they are still in the same places.

Curiously, changing the num of indirect lighting bounces in word settings from 3 to 4 seems to lessen a little bit these artifacts, while changing from 4 to 5 worsen them again

avatar image Lovecraft_K ♦♦ STAFF May 29 '14 at 03:38 PM

Hi devel.bmad -

Can you post a screenshot of that particular mesh'es UV layout from the Static Mesh editor and one from the same location as above, but with Ligthmap density turned on (Viewmode >> Lightmap density) and Wireframe? I believe I now know what the problem is, but I want to confirm before I tell you something incorrect.

Thank You

Eric Ketchum

avatar image devel.bmad May 29 '14 at 04:44 PM

Here you go, static mesh (vertical part of the wall, in yellow): alt text static mesh (ceiling, in yellow): alt text

avatar image devel.bmad May 29 '14 at 04:45 PM

Lightmap density: alt text Wireframe (disabled sky sphere): alt text

Thanks!! :)

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Hi devel.bmad -

The Lightmap density for your ceiling piece is extremely high which leads me to believe that you may have some compressed UVs for that piece which can cause the sort of distortion you are seeing. Look closely at that unwrapped pieces 2nd UV channel.

Thank You

Eric Ketchum

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answered May 29 '14 at 05:18 PM

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Lovecraft_K ♦♦ STAFF
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avatar image devel.bmad May 29 '14 at 09:45 PM

Hi Eric, what do you mean with compressed UVs? is there a way to check the generated map directly over the unwrapped pieces in the 2nd UV channel? Thank you anyway for your help, I will further investigate different UV layouts for this mesh.

avatar image Lovecraft_K ♦♦ STAFF Jun 01 '14 at 02:30 PM

Hi devel.bmad -

Not directly over the UVs no, but that is what the lightmap density visualization is sort of letting you see. With a red area you are getting a very high render push from the lights and this is usually a result of squashed or stretched UV islands which it looks like based on the density of squares that I am seeing in your visualization is that the UV island for that part of your mesh is stretched out particularly when compared to the piece of geometry immediately beside it.

If you would like to post a link to the mesh's FBX or original model file, I would be happy to take a closer look at it to try to determine exactly where the issue lies.

-Eric Ketchum

avatar image devel.bmad Jul 01 '14 at 10:18 AM

Hi, I've haven't figured out why these sort of artifacts appear and why one object behind another creates shadows in the foreground one, but very likely the artifacts in my case were much worsened by the fact that I had very strong indirect lighting in my scene (around 75). With the same amount of indirect lighting, though, using a lightimportancevolume and one sided materials greatly improved the situation and made the artifacs nearly not visible.

avatar image Lovecraft_K ♦♦ STAFF Jul 01 '14 at 01:17 PM

Hi devel.bmad -

Go into the details panel of your mesh and turn on the following settings (click the check marks): Under Lighting >> Lightmass Settings >> Use Two Sided Lighting AND Cast Shadow as Two Sided. See if this clears up your artifacts issues.

Thank You

Eric Ketchum

avatar image devel.bmad Jul 01 '14 at 03:34 PM

Hi Eric, no, sorry, the artifacts get much worse with two sided and cast shadow as 2 sided. A single sided material still looks like the best option.

avatar image cadviz Jul 16 '14 at 03:53 AM


I may guess it is a << Static mesh FBX export / import >> issue

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