Degenerating and Regenerating
As you zoom and pan around your drawing, you may wonder how the image that you see on-screen is related to the DWG file that AutoCAD saves on the hard disk. Well, maybe you don't wonder about that, but I'm going to tell you anyway!
When you draw and edit objects, AutoCAD stores all their geometrical properties (that is, location and size) in a highly precise form — technically, double floating-point precision. The program always maintains that precision when you save the DWG file. For computer performance reasons, however, AutoCAD does not use that high-precision form of the data to display your drawing on-screen. Instead, AutoCAD converts the highly precise numbers in the DWG file into slightly less precise integers in order to create the view that you see on-screen.
The happy consequence of this conversion is that zooming, panning, and other display changes are a lot faster than they would be otherwise. The unhappy consequence is that the conversion, which is a regeneration (or regen for short), occasionally leaves you with some on-screen debris to deal with.
In most cases, AutoCAD performs regenerations automatically when it needs to. You will sometimes see command-line messages like Regenerating model or Regenerating layout, which indicate that AutoCAD is taking care of regens for you.
If, on the other hand, you see the command-line message Regen queued, AutoCAD is warning you that it's not performing a regeneration, even though one might be advisable now. In addition, you might see a warning dialog box with the message About to regen -- proceed? These messages are AutoCAD's way of saying, “What your drawing looks like on the screen at the moment may not exactly match the real version of the drawing database that gets stored when you save the drawing. I'll update the display version at the next regeneration.” You can control whether regenerations happen automatically with the REGENMODE system variable — see the online help for more information on this variable and refer to Chapter 26 for more information on system variables in general.
The REGENALL (REA) command (once again, only available from the command line) regenerates all viewports in a paper space layout. If you run the REGENALL command in model space, it has the same effect as the ordinary REGEN command.