This project is called "FixPaint". It's an example of how to use the new SetHiddenPoints command that was introduced in DesignCAD 23.
As some of you old-timers know, the DOS version of DesignCAD didn't really have a "solid fill" hatch pattern -- instead it had the Paint command. A paint entity was essentially a vector that included a number of horizontal line segments, spaced one pixel apart at whatever zoom factor was active at the time, connected by hidden segments that diagonally connected the horizontal ones.
When DesignCAD teleported to Windows, it suddenly had the Win_Solid hatch pattern available so Paint wasn't necessary any more. The Paint command was dropped, and Paint entities simply loaded in as vectors. Unfortunately, the hidden diagonals were no longer hidden, so all Paint from older drawings looked like an angry scribble. The Harley motorcycle drawing in the Extended Sample Drawings/2D Samples folder is a perfect example.
This program will hide non-horizontal lines on any selected vector entities. Let me repeat that last part -- ANY selected entities. Unfortunately, there's no reliable way to detect if a vector used to be paint or not, so you'll have to pick them manually before running this command.
THIS ONLY WORKS FOR DESIGNCAD 23 OR LATER. You have been warned.
*Addendum* I have also attached the compiled exe as FixPaint.zip. It may not work on all machines -- if you get Side By Side error messages, sorry.