Problem with Lightmap

[Captain Sparrow]

hi,
I've got a problem with my Lightmap. When I build lightning with Lightmass I get these dark spots on my mesh. But as you can see in the images, these dark spots only appear on the shadow side of the building. When the mesh is directly lit by a directional light, the shadows are fine. I also don't understand why those windows are full of these black spots at the edges, because in my lightmap the windows are those 4 rectangles in the upper left corner. I think there is enough space between the different parts in the lightmap.
Can anyone help me to solve this problem?

http://imageshack.us/f/850/detaillightninglit.jpg/
http://imageshack.us/f/214/lightmap.jpg/
http://imageshack.us/f/827/detaillightningshadows.jpg/

[Tearl]

Turn off scene ambient occlusion and see what happens.

[darthviper107]

Your lightmap resolution might be too low, though I would think 128 would be fine for a window section like that

It doesn't actually look that bad though and if you have darker textures it won't be visible really.

[Captain Sparrow]

hey, thank's for the replies.
@Tearl: Yes, I already tried it without ambient occlusion, but it didn't really improve the result.

@darthviper107: You are right, a darker texture could help here. In case I can't find a better solution, I will darken the texture a little bit. But even when I increase the Lightmap resolution to 256x256, there is no visible improvement. I mean. it doesn't look that bad, you're right, but in the shadow parts it looks somehow dirty and at the sun side it looks clean. Here is an other image, in lit mode and with the lightmap res. set to 256, which shows these irregular dark spots.
http://imageshack.us/f/823/litmode.jpg/

could this also be a problem with my normal map?

[Ryan]

I am curious if you rebuild using production settings or not.

The normal map probably is not a factor but it's easy to test for. Two ways really:

1) just place a flat normal material on it and rebuild
2) you could also use the LightmassReplace node and put in a constant3 (0,0,1) into the lightmass input and the existing normal into "realtime".
That would let you keep the material intact but guarentee lightmass does't even see normals.

It does look a bit like ambient occlusion to me but you said it was disabled.

Then again sometimes DXT compression just does this.

[Captain Sparrow]

I tried what you suggested, Ryan, but unfortunately it didn't change the appearance of the mesh, so my normal map seems to be ok. I also checked that the settings are set to production and AO is set to "off" and I did another test build, without any normal map applied and with a Lightmap resolution of 512. You can see the result here. What I don't understand: why are those dark spots so irregular? I mean, if it was the Lightmap which caused these dark spots, wouldn't then all the (same) meshes look the same? Because as you can see in the picture, the upper two meshes seem to be more messed up than the lowermost mesh.

and Ryan, could you please explain this DXT compression? Can you avoid it?

http://imageshack.us/f/526/withoutnormalmap.jpg/

and thank's again for your comments and help.