Inverse Halftoning – contd

This update comes a little late as I was busy with working on the jpeg2000 plug-in and forgot to update on my final status on inverse halftoning. I decided on the algorithm to use, implemented it in C, and finally wrote the plug-in which seems to be working well with exception on images of the order 1024 and higher. My guess is that I am not doing memory management well and hopefully I can resolve this issue soon. I started working on jpeg2000 plugin in the meanwhile as I was getting bored of working on halftoning for a while!

Here are some scanned newspaper images on which I tried the algorithm using the plug-in.

news_1 halftoned newspaper image

news_1_ihalf inverse halftoned image

news_2   halftoned newspaper image

news_2_ihalf  inverse halftoned image

The basic algorithm goes as follows:

If we take a one step wavelet transform of a halftoned image, the low pass frequency part will be less noisy whereas the high frequency part will be extremely noisy as it detects many edges in the halftoned images.  The basic idea is to remove the noise in these images and preserve the edges.  In order to preserve the edges, I get the high frequency content from blurred versions of the image.  This way the noise gets reduced and I get  the useful edge information.  I get the blurred image by using 3×3 kernel.  If we increase the radius of the kernel the final image become very blurry.  It can be argued that is process is almost similar to simply blurring the image, but in actuallity there is a slight difference.  I am using the high frequency content from blurred images and low frequency from the original halftoned image.  Further research can be done to improve the sharpness of the images, but for now this method works and I will concentrate on making it memory efficient.  Once I get that working, I will work on further increasing the quality of inverse halftoned images.  Another update on my jpeg2000 work will soon folllow…

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