I am new to processing, I've read the manual and done some experiments to familiarize with the code but I wondered if there was an easy way to retrieve the coordinates of a curve that you have drawn for example and that you want to be reproduced in processing. For example, I have a photo of a painting and I want to extract the coordinates of a line or curve in that painting and draw it in my processing window, what is the easiest way? because for the moment I am doing trial and error because I have no coordinates grid and i find hard to compute the position of correct tangent point to get the curve I need. 

thanks a lot in advance!