problem creating a mask

Good morning,

I need some help creating a mask. I have a tool (such a pencil) and I can draw lines, once I have my line y want to obtain a masc with all black and in white the line I have drawn (I have the coordinates of my line). The problem is that I used roipoly and this is Ok when I draw a close object (such a square) but when I have a simple line I don't know how to do it.

Thank you