6 (Invert) and 13 (Copy Pen) Didn't work
Well, I tried what you did.
First I drew my curve in a picture box (black foreground color and gray background color), then as the mouse moved, first I set the DrawMode = 6 (Invert) and then drew the same line at the same coordinates after that I set the DrawMode = 13 (Copy Pen) and drew the new line...
What happened?
I got white vertical lines all over my curve... The vertical line moved but left a white trace (white vertical line) in the picture box.
I simply can't understand what I'm doing wrong.
(I didn't play with the default color values which means I simply gave the right coordinates to the Line method and no color value...).
Can you help me?