ftp.nice.ch/pub/next/graphics/bitmap/dips.1.00.s.tar.gz#/GUI_DIPS/_gui_dips/masks

homo_3x3.mask
 
hsobel_3x3.mask
 
laplace_3x3.mask
 
vsobel_3x3.mask
 

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.