This page is to show the result of capturing what is written on a whiteboard by taking two pictures.  The first picture is of the whiteboard with all the stuff on it you want to save.  The second picture is the same whiteboard under the same lighting conditions after it is erased.  The basic concept is that the information you really want is the stuff that was erased, which can be found by subtracting the two pictures. 

This method was first brought to my attention by Randy Glenn in a message to the PIC email list.  It sounded like such a good idea that I had to try it myself.  As you can see below, it works superbly well. 

The first step is to take a picture of the whiteboard with all the writing on it that you want to save (image A):

Then erase the whiteboard and take a picture of that (image B):

Then subtract A - B.  All the background that was not effected by the erasing ends up black, and anything that was erased ends up in negative (image N):

This image contains all the information you want, but it is negative of what the ideal whiteboard with the writing on it would look like.  It also doesn't contain brightness values over the full black to white range.  By examining the image with an image editor, I found that the black writing (white in the image) only got to a brightness of about .6 (on a 0 to 1 scale).  To make the final image, I mapped the .6 to .01 range of image N to the full 0 to 1 range of the final output image (image D):

Wow, that really worked well!  Note that the variations in the whiteboard background all got nicely cancelled out.  This includes reflections from the room lights and some background markings that were stuck on the whiteboard, like under the "A" in "BLACK TEST".  The background is also a nice full white instead of a varying muddy gray.  This image is now quite suitable for printing on white paper.