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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?

  • If you did the pencil and red on different layers you could remove the whole layer at once – Zach Saucier Jan 31 '15 at 21:36
  • Sorry, I should clarify: I'm working with a scan of my actual paper notebook. – David Millar Jan 31 '15 at 21:43
  • Can you attach a sample image? – Michael Schumacher Feb 1 '15 at 11:40
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For Gimp

To remove all content of a scan except red drawings we may do as follows:

  1. Scan image in desired resolution

    enter image description here

    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.

  2. Isolate red color with the Select by Color tool enter image description here with an appropriate threshold to make sure only pixels on the red lines are going to be selected (here this was 10):

    enter image description here

  3. Select > Grow the selection by an appropriate value (5 pixels in my case):

    enter image description here

  4. Select > Feather the selection for a smoother transition (here by 5 px)

  5. Select > Invert the selection

  6. Press Del or Edit > Clear to remove everything in the background:

    enter image description here

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a much easier solution is to use a piece of vellum or tracing paper when you are working on the draft. Simply lay the vellum over your red areas and write on it. When you're done the vellum is not actually part of the artwork and the red can be scanned without any need for alteration.

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