Re: Write pre filtered and compressed data

Incidentally, if you cannot do it the way I described and have to
rewrite all the image data the IDAT chunks will change and then you
cannot copy unknown chunks unless they are marked safe-to-copy.  This
is because they may depend on the precise form of the IDAT chunks.

Also, when you mark IDAT as "unknown" make sure to specify the default
for unknown chunks as "save".  This will allow you to copy any PNG
(even if it has an unknown critical chunk.)

John Bowler

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