Lightmap bleed problem

Hi, I am having a wierd bleeding problem when i generate my lighmaps. I am using a stationary skylight and a directional light. I have the “Cast shadows” option on the skylight checked because I want interiors to be darker than exteriors. The roof and the walls are seperate objects and the last image shows the used UV layout of the roof object. Thanks for the help!

Hey Kurtaa1,

If there is any gap between your roof and your wall then light will bleed through. Try moving the wall or roof to overlap and then bake again.

Also if you don’t want any light from the outside to bleed in there is an option in your light under lighting to turn off the indirect lighting. It is a check box that will effectively use the geometry and not allow for any bleed around that object. That is with our without the wall. You will have to illuminate your scene with a light inside of your room with another light but I believe you had that intention anyway.


I can’t turn off the indirect lighting because then it will become pitch black and thats not what I want either. There are no gaps between the meshes


If you don’t have any gaps, then this appears to a be a light map issue. I see how your UV’s are layed out but not how your light map uv’s are set up.

Some steps I would like for you to take

1.) Make sure your light map uv’s are set up correctly.
2.) Make sure the light map is set up for the proper channel. If you create a light map channel inside of a 3D modeling software after your base UV channel that call channel will be 1. UE4 uses a numeral system starting with 0 as it’s first channel.
3.) You can try incrementally adding resolution to your light map channel. In your screenshot it is set to 64 you may want to try 256 and increase from there.
4.) Set your world settings for light/material/shadows etc. to Epic
5.) Set your light map gathering to high or production
6.) See if you are able to create a different project and import these assets and create the same result.
7.) Make a list of all of the steps you took if you are able to recreate this issue and let me know that way I can test this and narrow down the possibilities.

Here is some documentation in case you have any questions on light map unwrapping and importing into UE4:


I start a blank new project without starter content and with maximum quality.
I create a box and edit it
Generate lighmap UVs = checked
min lightmap res = 512
source lightmap index = 2
destination lightmap index = 2
and then I apply the changes
back in the viewport I scale it a bit (make it look more like a wall) and build a little house of duplicates of the box.
I set my directional lightsource to movable and add a stationary skyligt.
I give the skylights indirect lighting intesity value to about 2.6.
To let in som light into the “house” I make a little gap on one of the walls like so

I still get the unwanted bleed effect as before, this time not AS noticable though.

Hey Kurtaa1,

I am going to link you to the lighting troubleshoot guide available on the wiki page. This will literally go through step by step many lighting issues and you can create the effect that you desire based off of this material.


I have checked that already, they dont seem to be discussing the problem that I have wich is a bright line on the edge of the object.

Hey Kurtaa1,

I am sending you a literal translation of what it is you are experiencing. Have a look at that section.

Things you need to check

1.) Change light index and channel from 2 to 1. I say again 1 in a modeling program equals channel 0 in UE4
2.) Keep tweaking the settings to improve your resolution
3.) Read through the guide.


Hi Kurtaa,

Eric and I have plans to re-visit the guide and add additional topics at some point in the future, unfortunately, with other priorities we’ve not been able to get this done just yet.

If you want to see some of the additional topics that are covered you can see our list in my Forums post here: [TUTORIALS] Photons Be Free: Mini-tutorials and other curiosities - Community & Industry Discussion - Epic Developer Community Forums

Definitely feel free to post a comment for any that you would like to see if they are not listed. I cannot guarantee a timeline, but I will definitely work on them at some point when I get time. :slight_smile:

Now to discuss the problem you’re more than likely seeing with this.

You mention that you’ve used BSP to generate a box and I’ll assume a subtractive brush to make the door way. The problem with this will be directly related to the way BSP will generate the cut out geometry with the way the mesh is broken up. This will need to be adjusted in a 3d modeling application for the best results, as BSP is honestly just a quick way to prototype a level for simple shapes and blocking out something. I’ve had issues with Movable/Stationary/Static lighting all not giving the proper results because of the way the object was cut when these were combined.

Some other quick notes on topics I briefly glanced at above.

  • Movable lighting does not need lightmaps setup. These will not help in any way since the light is not static and will not bake lighting to the texture.
  • Shadow bleed with dynamic lighting can be handled by adjusting the Cascaded Shadow Map settings and the Shadow Bias/Filter sharpen settings (detailed on the wiki) that can be found in the details panel of the light source.

I hope this helps clear up some things that you may be seeing that are wrong here.


I did not use a BSP I used a Basic cube, I am getting a better result with a whole connected mesh rather than putting together several simple ones