1. Here's how

The guidelines below assume that you have got access to Adobe Photoshop or a similar image editor.

1.1: Render an uncompressed image in .TGA, .PNG or .BMP format:

1.2: Skip this step if you are using Lumion 9.0 or newer.

If you are using Lumion 8.5 and older, please disable any effects that change the contrast, saturation etc if you want the cleanest possible Custom Output Maps:

1.3: In Photo Mode or Movie Mode, render a Sky Alpha mask in .BMP format:

Photo Mode:

Movie Mode:

Sky Alpha map example:

1.4: Add a duplicate of the Sky Alpha map to a new layer in the still image in Adobe Photoshop:

1.5: Select the Sky Alpha mask layer and in the top menu, select:

• Select  -> Select Color Range -> Fuzziness = 200 -> Click on the black part - > OK

1.6: Hide the Sky Alpha mask layer.

1.7: In the top menu, select:

• Edit -> Copy Merge

1.8: In the top menu, select:

• Edit -> Paste

1.9: Hide the Background layer:

1.10: In a new layer below your cut-out building layer, paste a sky photo of your choice and adjust it as desired: