Re:Coding Challenge for MI5
by maffydub on Monday September 12, 2016 @04:55AM
(#12478135)
I unpacked the chunks and there doesn't look to be much more (or it's subtle).
The IDAT chunk (which makes up most of the file) is DEFLATE-d but uses full color (rather than palettized color, which would have saved lots of space, and might be how you'd get 40% compression). It also doesn't use any row filtering.
I checked incase something was hidden in the DEFLATE encoding itself (e.g. using sub-optimal block encoding selection) but this round-trips exactly, so I don't think it can be.
The IDAT chunk (which makes up most of the file) is DEFLATE-d but uses full color (rather than palettized color, which would have saved lots of space, and might be how you'd get 40% compression). It also doesn't use any row filtering.
I checked incase something was hidden in the DEFLATE encoding itself (e.g. using sub-optimal block encoding selection) but this round-trips exactly, so I don't think it can be.