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Filtering a noisy scanned hand written
image
The original image
The original image is a scanned, hand written image with a strong
additive noise due to the scanning process and the poor quality of
the original paper. Images such as the one used in this example can
be filtered in order to remove the noise from it.
It is
important to note that simple filtering alone cannot create wonders,
and it’s also important to bear in mind that none of the filtering
methods are able to make a new image containing more information
than the original. After filtering we always lose some information,
but if the filtering is chosen well then the information that we
need stays on the image, and we lose the noise. A filtering method
is only able to create a new image where the information important
to us appears stronger than before the filtering.
One of the best filters (in these
special cases like this example) for removing additive, salt and
pepper noise is a very simple filtering method called the Median
filter.
Median filtered image with 2x2 sized matrix
Using the Median filter, the additive
salt and pepper noise is removed, but we also lost some information
from the handwriting itself. In this case a 2x2 sized matrix was
used for the Median filtering due to the small size of the noise
regions. The larger the matrix size, the more noise is removed, but
also more information is lost from the important parts of the image.
So the best solution is to find a middle ground that removes the
most noise while preserving the most image integrity.
In the next step, the errors in the hand
writing generated by the Median filter should be fixed. This image
contains only black and white pixels. There are many excelent
filtering methods for these binary images as they are called, but
unfortunately the original images are not always binary. So the
image should be binarized now.
Binarized image with Floyd-Steinberg error diffusion dithering
For binarization, the Floyd-Steinberg error diffusion dithering
method was used. On the already binarized image the holes in the
hand writing due to the Median filter could be filled up with a
binary Dilation filter. Dilation filter makes the lines stronger,
fills up the small holes and make the lines continuous. In this
example a 3x3 sized full one matrix was used for dilation.
The final filtered image after dilation
(above) and the original image (below)
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