Finally, you may be wondering about effects (and yes, scanlines are
an "effect"). Effects are no longer hard-coded into the system.
Rather, you can provide a PNG file in the artwork directory that will
be loaded and overlaid on top of screen bitmaps. See the description
of -effect in windows.txt for more details.

-effect <filename>

Specifies a single PNG file that is used as an overlay over any game
screens in the video display. This PNG file is assumed to live in the
root of one of the artpath directories. The pattern in the PNG file is
repeated both horizontally and vertically to cover the entire game
screen areas (but not any external artwork), and is rendered at
the target resolution of the game image. For -video gdi and -video d3d
modes, this means that one pixel in the PNG will map to one pixel on
your output display. For -video ddraw, this means that one pixel in the
PNG will map to one pixel in the prescaled game screen. If you wish to
use an effect that requires mapping n PNG pixels to each game screen
pixel with -video ddraw, you need to specify a -prescale factor of n as
well. The RGB values of each pixel in the PNG are multiplied against the
RGB values of the target screen. The default is 'none', meaning no
effect.