When you work on a
drawing in AutoCAD it may look like you are creating graphics. It is true
that what you see in the AutoCAD editor is graphical. However what is really
happening is that a database of AutoCAD objects are being created and these
entities are being rendered on the screen. The database contains and
organizes the entities in a hierarchical fashion. When you save the drawing,
the database is in the dwg file. When the drawing is opened, the database is
placed in memory. All the objects in the database have an ObjectId that is
unique in the AutoCAD session. This ObjectId is similar to a Primary key in a
relational database. Also many objects refer to other objects and may require
them to exist such as a line needing to have the layer that it has. |