=== Convert pdf pages to a picture ===

   $ gs -sDEVICE=pngalpha -o <output-%04d.png> -r1200 -dFirstPage=<start page idx> -dLastPage=<end page idx> <input>

n.b., the resolution of 1200 pixels for inch is quite large. Probably one wants to convert the result in smaller files.


=== Remove pictures from a pdf file ===

   $ gs  -o noimage.pdf  -sDEVICE=pdfwrite -dFILTERIMAGE <input>


=== Convert an image to black and white ===

    $ convert <input> -colorspace HSL -channel L -separate <output>