Dear Ryo,
I replied to fast.
With the latest version of PyFerret, when I use FRAME with the -nodisplay -scrip options, I end up having white horizontal lines over the coast topography (go fland 5 black)
That's a long-standing known problem, not only with Ferret but also with all sorts of graphical packages.
Search Google with "white thin lines PDF" without the quotation marks. Also search the email archive of Ferret.
But the workaround is
1. Create a PDF file instead of PNG.
2. Rasterize it with antialiasing switched off, if the PDF file shows the thin white lines.
On the Net, You'll find a lot of discussion about the relation between antialiasing and the white lines. Google again for "rasterization".
I guess the graphical engine for "-nodisplay" uses antialiasing and as a result, bakes white lines in the PNG image. I hope PyFerret will be modified so that the white lines don't happen in the PNG images it produces.
Cheers,
Ryo