Remove every other row in a Netpbm image. See also:
pammixinterlace
. More information: https://netpbm.sourceforge.net/doc/pamdeinterlace.html.
pamdeinterlace path/to/image.ppm > path/to/output.ppm
pamdeinterlace -takeodd path/to/image.ppm > path/to/output.ppm