Sometimes I really don't want to spend time on drawing nice graphical
diagrams and would rather stick with a very quick hand drawn diagram.
Such a hand drawn diagram can then be photographed and converted to
something... well... presentable ? Below I describe how to get rid of
the background, which such a photo undoubtedbly will produce.
Original Photo
Value Propagation
The first step we perform is to propagate dark values. This can be
found in the Gimp menu Filters | Distorts | Value Propagate. The
settings are shown below
The total image after this step has a more consistent value
distribution throughhout the writing. This is better visible if we look
in detail at the text.
Without dark
value propagation
with dark value propagation
Edge detection
The next step is an edge detection step based on the difference of
gaussians. SThis edge detection step will effectively make the
background (which is currenltky still a bit grey) white. This is done
with Gimp in Filters | Edge Detect | Difference of Gaussians. The
settings are shown below
The effect on our example is shown below
An obvious effect of this operation is that the image becomes lighter
in general
Light Intensitity
To solve this problem we apply an histogram normalization. In gimp this
is in Colors | Auto | Normalize. The result:
Removing the speckle
The remaining speckle can be removed with a selective gaussian blur.
Filters | Blur | Selective Gaussian Blur.
Beware that this operation can take quite some time and might only be
necessary when there is some anyoying speckle present.
Crop, rotate and perspective
The last step that can be performed is a rotation / cropping to align
the image properly. The results are shown below, comparing the original
image against the transformed image and then the pre-scaled image for
your browser.
