zopflipng resets the resolution / image DPI about zopfli HOT 2 OPEN

You're correct that this should be fixed.

The problem is that ZopfliPng strips the pHYs chunk. You can avoid that 
behavior by adding the command line argument 

   --keepchunks=pHYs

By default zpfliPNG only keeps IHDR, PLTE, tRNS, IDAT and IEND.

Original comment by [email protected] on 30 Oct 2014 at 8:48

from zopfli.

Hi,

--keepchunks=pHYS is indeed the way to keep the DPI.

By default zopflipng removes optional chunks (such as pHYS starting with a small letter), and only keeps the critical ones containing the pixel data (afaik web browsers don't use DPI, please correct me if this is wrong).

In case the information is needed, it can be kept with keepchunks, at the cost of some bytes. For example the pHYS chunk is 21 bytes and uncompressible (section 4.2.5 at https://www.w3.org/TR/PNG-Chunks.html: 9 bytes of data, plus the standard 12 bytes of PNG chunk overhead).

So this is by design. If anything should be fixed, it's the documentation, I'll look into it :)

from zopfli.