pamdeinterlace

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