There is this strange issue with shadows that appears across various windows from the base game and DLC. This isn't apparent on every window, however, but it is visible on enough of them that it's quite bothersome as a player once you've noticed it.
From what I can tell, the problem lies in the way that the outdoor shadows are cast on the glass.
If you look and compare these windows to the various doors with glass, this same issue is not present on any of them.
I've noticed this in my game as well.
I play with all of the graphics options on maximum.
I usually play with walls in cut-away mode so I don't usually see windows from the outside of a building I'm playing in. I noticed this when I was looking at some windows from a new pack I purchased. I thought it was just those that were bugged but then when I looked at other windows to compare I saw that a lot of others do it as well.
It's really annoying once you notice it.
The Sims Team
