Unreal Engine　コツコツ自習_DDX,DDY

The documentation on DDX and DDY is currently seriously lacking. From what I understand from looking it up with google it somehow takes the neighbouring pixels. But it hard to imagine the use of it and how to use them. My guess is it is somehow possible to use them for cheap blur by stacking them or to generate outlines/lines by color difference? Most helpful would be if there are any examples using those nodes, anything maybe in one of the many UE4 example projects?