Thumbnail Rendering
From MatterControl Manual
The Thumbnail Rendering section controls how model thumbnails are displayed in the Queue and Library.
Flat
Flat mode renders the thumbnails as a two-dimensional picture of the model taken from the top down.
3D
3D mode renders the thumbnails as a three-dimensional picture of the model taken from the front, off center.
Switching
Switching between modes will show a window warning of a temporary increase in processing requirements while the new thumbnails are generated.