Sets the thickness of the planar solid entity (DXF 39) in drawing units. Thickness is the extrusion length along the normal. A positive value extrudes along the normal. A negative value extrudes opposite from the normal. A zero value defines a planar solid without thickness. The initial value is zero by default. 

Example of Working with the Planar Solid Object 

Specific Planar Solid Properties 

OdDbSolid::thickness() method