As a puzzle author, I often write puzzles with red ink and then test-solve them with pencil. What would be the most efficient way to remove the pencil and blue gridlines from the paper to leave only the red puzzle clues behind?
To remove all content of a scan except red drawings we may do as follows:
Scan image in desired resolution
This image deliberately was a poor scan using a black and a red pencil to make it harder for us. Using a bright red felt pen will definitely make isolation much easier, but this probably is not what we have in reality.
Isolate red color with the Select by Color tool with an appropriate threshold to make sure only pixels on the red lines are going to be selected (here this was
Select > Grow the selection by an appropriate value (
5pixels in my case):
Select > Feather the selection for a smoother transition (here by
Select > Invert the selection
Press Del or Edit > Clear to remove everything in the background: