Not the parent so I can’t tell you exactly what he did, but some suggestions:

- The PNG specification contains plenty of information including width and length that allow you to use 2d arrays rather than ArrayLists

- The RGBA 32bit integer doesn’t need to be unpacked.. you can just perform shifts and transformations of each of the 8bit channels.

- If you do unpack, you should use native 8bit unsigned ints

- You don’t need an unmodified copy.. that’s your file

- Undo stack should contain deltas, not the whole image over and over again.

Edit: formatting

The obvious thing to do would be to store the image in memory as just an array of 8-bit of native unsigned integers (RGBARGBARGBARGBA...) and see where that got you. I assume C# has the equivalent of a Java byte[], except that fortunately it is unsigned in C# instead of signed in Java.

I expect that you would get quite a performance boost with just that.