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[IMAGING] PNG Component


Hello,

Pull Request: https://github.com/apache/commons-imaging/pull/38

I have requested to merge the pull request before. This email is not 
about that. I am happy that someone responded to my prior email 
regarding how to get my PR merged. Anyways, I have added a commit again 
to my fork (should I call it branch?). Here, I am going to discuss about 
my new contribution on Dec. 1, 2018 (IST).

The PNG component used the ScanExpediter object to unfilter, and 
un-interlace PNG image data and convert it into BufferedImage data. The 
filtering logic was embedded in it, which is incorrect. Hence, I created 
an object - AdaptiveFilter - which encapsulates the filtering logic. All 
you do is feed it each scan-line serially, and it will filter/unfilter 
the image data.

In addition, the PngWriter object now uses AdaptiveFilter to add 
filtering functionality while writing PNG image data (before it only 
used the NONE filter type, which was very inefficient). Now, the output 
of PngMultipleRoundTest has 40-50% less size than the input PNG files in 
data/images/png/3. I hope this will be appreciated.

Regards,

Shukant Pal


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