I have a line-drawing that I would like to trace into a vector graphic, but cannot find documentation on how to perform some of the basic graphic functions necessary to do this. Here is an example screenshot:
Here I have pasted a line drawing of a submarine into a cell. I want to trace some of the lines from this drawing, add some new lines, then make some measurements using the scale in the drawing (20m).
To do this I tried first to lighten the image so it would be a very light gray--that way when I draw my vector line polygon I can clearly see it. Even better would be to change the color of black in the drawing to a pale yellow. Then it would be easy to see my black trace line. How can I lighten the drawing and/or change the color of the drawing from black to pale yellow?
For example, I tried using the "lighter" control shown in the screen shot, but it did not appear to do anything. I also tried using the "fill opacity" in the graphics toolbox (not shown), but that did not work either. (I think that must only apply to objects that I draw, not to pasted images.)
If I have a line I have drawn on top of the image, how can measure its length according to the scale in the drawing?
When I try to Binarize the image (according the answer below) it does not work for me, but produces this: