3D trees should have solid surfaces for the leaves, trunk and branches.
You can't use the display order to manipulate 3D objects display position in relation to other 3D objects but you certainly can with 2D elements such as lines and fills.
When you refer to a "zone" in the background do you mean just a 2D fill?
I just placed a 3D tree in front of a wall (with a window) and it automatically masked the wall.
I could add a solid fill and I could manipulate that with the display order of the fill in relation to the other objects.
How are you greying out the background of your elevation.
Barry.
One of the forum moderators.
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