I work with answer bubble sheets that contain long lists of bubbles representing letters and numbers. Printing the specific letter or number inside each bubble as a prompt reduces frustration for the test taker and helps prevent human errors.
The problem is, having the pre-printed prompt characters in the bubbles increases recognition errors.The ideal solution would be a two-color answer sheet with dropout characters, but that's not practical in our case.
My solution is to use a dotted font for the prompt characters.
The dotted font renders each letter as a series of separate small dots rather than solid lines. Strong despeckling of the fields then removes the letters completely.
I've had good success with this approach so far, and thought it might be helpful to others.
Here's one possible source for dotted fonts: https://www.fontsquirrel.com/fonts/list/tag/dots