I'm using LibreCad to make the drawing, exporting it as a graphic, importing that graphic into Impress, adding text etc, and then exporting that as a pdf.  The resulting file looks great but when it's printed only the text prints well.  The drawings are horrid. I've tried printing the file from linux, pc, and mac to different printers and nothing helped so it's something inside the file.

I went online and found lots of complaints about this but none of the proffered solutions worked.  Print as image makes it worse. I use to work for HP's printer division and chasing down print defects like this is what I used to do so I think I know what's going on.  I can't explain how I knew this would work, but I started testing outputting the cad file as different graphics format at various resolutions and finally got it to work by exporting from LibreCad as a png with a rez less than 150 and then using the lossless compression option when exporting the Impress file to pdf. Only low rez works, Jpegs don't work, Tif's don't work, svg's sorta work.  EPS works but the file is much larger so I stuck with the png solution.

You don't have to print to test this, print preview from the pdf reader will show this.

I no longer have the tools to delve very deep so I can only guess that there's something in the compression algorithm that your code is using for processing the image file that is reacting to the fine line hash marks in the cad drawings. That compression algorithm doesn't deal well with all of the parallel lines so it creates something like a moire pattern when it comes time to translate the file into raster data for printing.  Changing image type changes how the compression is done and the lower rez image decreases the frequency of the interference pattern.

hope that helps, thanks for the good work.