Regen command in CAD is the command to regenerate the object on screen Regen (RE). The way to use the command:
Menu: View–> Regen
Command: Regen or Regenall
Regen command is to calculate and regenerate all objects on the current viewport. Similarly, Regenall is to calculate and regenerate all objects on whole drawing.
When do we use Regen command? For exemple, when we adjust resolution of a cirle from display resolution 2000 instead of 100 before, or change the establishment about linetype,… To update those changes, we use Regen command.

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