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I have the windows on the house mesh shown in this image set up to emit indirect light (material emit: 1.00, World Indirect Lighting Factor 5.00, Bounces 4) and it's almost fine, except the fact that every other window frame on the bottom ground floor is significantly darker than the one next to it, and other windowframes in the render, for that matter:

Given it occurs on every duplicate of the window-having mesh (two storeys of windows on one protrusion is a mesh, so there are four visible in the scene) I can only presume it represents some error in the construction of the mesh (a similar problem occurred earlier when a stray face on the attic floor covered part of the windows there) but I have been unable to find either redundant faces or incorrect normals in those places. Can anybody tell what's going wrong in this render that would cause this problem?

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  • $\begingroup$ Is it possible the normals on those faces are inverted? If that's not it, is it possible you could upload your .blend for closer inspection? $\endgroup$ – gandalf3 May 10 '14 at 21:01
  • $\begingroup$ They're not, I went in and checked in great detail multiple times and if you look closely, the texture is actually rendering correctly under the "shadow". I could upload the blend, but I don't know how to do that while keeping the texture file structure intact, so it would probably be untextured. $\endgroup$ – CleverCrumbish May 10 '14 at 21:04
  • $\begingroup$ You can press Pack all into .blend in File > External data to quickly embed the textures into the .blend file. $\endgroup$ – gandalf3 May 10 '14 at 21:08
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    $\begingroup$ Well, I have a partial answer. You can change where the dark spots show up by moving the UV's for the windows. You can also clear it up entirely by making the window texture completely yellow. I have no solution to this which allows you to keep the crosshatching as is. Maybe duplicate the window polygons and offset them a bit and make this layer the crosshatching with a stencil to make the yellow invisible for this layer. Also, now make the bottom layer entirely yellow. So you need 2 textures and two layers for the windows. I tried adding yellow borders to the UV islands it did not work $\endgroup$ – MarcClintDion May 10 '14 at 23:11
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    $\begingroup$ Reasonable, and technically doable. Any idea why it only turns up on the bottom floor? EDIT: Got it! The UVs for every other window are upside down! I can rotate them and it makes the problem go away. Will leave the question open in case someone wants to add a reason why this happens $\endgroup$ – CleverCrumbish May 10 '14 at 23:41
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This issue arose because the UV for every other windowpane was upside down, correcting this resolved the lighting issue, although it is unclear why.

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