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If you render out (output to a file) a DWG file from the hidden line renderer,
import (insert) it into AutoCAD 2000 or AutoCAD LT, and want to do a hatch on
it, you will find out that it now has x,y, and z coordinates, which makes it
impossible to do a hatch on it. If you've got the big AutoCAD, you can always
use flatten.lsp to push all lines down to the same level, but I have found a much
easier way that works in LT too.
1. Render out the hidden line rendering to a DWG
2. Open it in AutoCAD.
3. Select all and then open the Properties dialog.
You should see in the properties dialog LINE (xxx) (x depends on how many lines are selected)
4. For "Start Z" enter 0, then go to "End Z" and enter 0.
5. Click anywhere in your drawing, and all the lines will have the same Z coordinate