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Lightmap UV wrapping error

I recently got this error when working on a mesh, I've checked my UV unwrap both in 3ds max and in the engine for anything out of the 0-1 bounds but everything looks fine, I suspect it may be a mesh problem since other things I have unwrapped do not cause this error, although I have no idea what the cause, I've already looked over it for any obvious problems.

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I've uploaded the mesh here:



The problem occurred not long after making modifications to the front of the house, (the area with the overhang) if anyone can take a look that would be great.

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asked Mar 31 '14 at 05:17 PM in Rendering

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2 answers: sort voted first

I was experiencing the same problem and I even didn't think about the lightmap index because in UDK it was always channel 1 by default. Why does U4 apply channel 0 from time to time? Is it just a bug or it depends on something when export? Thanks.

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answered Jun 05 '14 at 01:06 AM

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avatar image Tim Hobson ♦♦ STAFF Jun 06 '14 at 09:52 PM

Hi Rambooze,

It's not a bug, When there is no second uv channel the engine will select UV channel 1 (Channel 0 in the engine) as the default.

When you auto generate a second uv within the editor you will have to manually change the setting from coordinate 0 to 1 for the lightmap to work.

Thank you!


avatar image rambooze Jun 08 '14 at 12:34 PM

Thank you Tim. So it means that initially I exported my objects just with one UV channel and later re-exported with two. Now it's clear to me.

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I am not sure what problem you are referring to, but just an advice - pack your lightmap UVs tighter. You are wasting a lot of space there. What you can to is:

1) Select your house and apply Unwrap modifier 2) switch to channel 2, and answer "move" when asked 3) Go to face subobject and Ctrl+A to select all 4) Go to Mapping > Flatten. Use value of around 0.02-0.1 for padding, click ok. 5) Now your uvs should be neatly packed within 0-1 area with some padding 6) Collapse back to Editable mesh and reexport your object

Hope this helps.

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answered Mar 31 '14 at 05:24 PM

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avatar image Chryseus Mar 31 '14 at 05:45 PM

This is exactly what I did with a spacing of 0.05

In UE4 the lightmap looks all stretched which doesn't make any sense.

avatar image Alexander Paschall ♦♦ EPIC Mar 31 '14 at 06:15 PM


Thank you for your report. I am investigating FBX issues, but I was unable to find a problem with the provided files. At what point do you get the message? Could you please provide a screenshot of the error occurring?

Thank you,


avatar image Chryseus Mar 31 '14 at 06:43 PM


I get the error when building the lighting, here is what it looks like: http://u.cubeupload.com/Chryseus/JFJ9R6.png


Nevermind I think I found the problem, I didn't realise I had to also unwrap the first UV channel as well.

avatar image Jacky Mar 31 '14 at 06:48 PM

Are you sure you have the right UV channel set for lightmaps?(you can check it in static mesh editor > Details panel > Static Mesh Settings > Lightmap Coordinate Index)

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