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Thread: Whiteboarding example
face BY : Philip Pemberton email (remove spam text)

Vitaliy wrote:
> We start by taking a picture of the whiteboard with a cheap digital camera:
(snip explanation)
> Voila, we now have a faximile of the board -- in under three minutes:

I never did find the little Delphi app I wrote to do whiteboard image
enhancement... so I've been rewriting it in C++ using the CImg image
processing library.

At the moment, it does basic contrast enhancement, based on a partial
implementation of an algorithm I found in a research paper (Zhang, Z. and He,
L. "Whiteboard scanning and image enhancement," _Digital_Signal_Processing_ 17
(2007) pp.414-432). I've also got a partial implementation of a Hough
transform edge detector, which attempts to detect the corners of the
whiteboard, and then applies a perspective correction to straighten it.

The image enhancement algorithm splits the photo into squares of 15x15 pixels
in size, then takes all the pixels in that area and sorts them by luminance.
The top 25% of those values are averaged together to produce an "average
colour" value, which is then applied to the image (division operator on each
colour channel) to remove the background.

Catch is, it falls over when a square is completely filled with ink.
"Something divided by zero, AARGH, fail!"

I think I need to rig up a plane-fitting algorithm to convert the reference
image into a set of values to plug into the equation:
  z = A*x + B*y + C

Unfortunately, I've yet to find a decently well-written explanation of how one
goes about doing such things on computers... :(

> P.S. "Whiteboarding" -- sounds like something the CIA would do to the
> terrorists, doesn't it? :)

Don't encourage them :)

<49AD30A3.400@philpem.me.uk> 7bit

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