grayscale and curve
I scanned a document at 600x600 on grayscale and tried to change it by using curves. (I needed to remove grey areas in otherwise text picture). Nothing seems to be visible on the screen. I then saved the resulting picture in a different name) I found that the file remained unchanged.
However if I create a grayscale picture by changing the mode of a file obtained by scanning the same document in color and then changing mode to grayscale, then the curve operation works.
Is there some switch in Gimp that I have missed.