PNG file is built from chunks which are either important data (like image data) or some meta data which is not that important. Chunks can be of arbitrary size (there is a limit in the spec of course) and we can use that fact to insert an extra chunk with meta data to the input PNG file to increase its size.
You can read about the PNG header and its chunks:
- http://www.libpng.org/pub/png/spec/1.1/PNG-Rationale.html#R.PNG-file-signature
- http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html
Running the code:
cabal sandbox init
cabal install --dependencies-only
cabal run -- -i input.png -s 100 -o output.png
As a result of the executed command your output.png will have an extra of 100 bytes in its body but the viewer will still show you the same picture as if you opened input.png
There are some shortcuts taken in the implementation and there might be some cases where the program misbehaves. But it works in most situations so it is fine for now.