Brian Gregory wrote:
On 17/02/2020 21:23, Big Al wrote:
optipng doesn't change the image quality any? I've never used it,
just installed it on my system. I ran a test and some images were cut
in half. My eye doesn't see any quality diff.

Further to my first reply. "Cut in half" is rare and suggests that the
original program that created them didn't choose the compression
parameters at all well, or maybe chose them aiming for highest possible
execution speed, or tested only on a photo where the more intensive
compression often makes only a little difference.

Cutting a file size in half, is a typical number for a
lossless compression method. There's really nothing out
of the ordinary about it.

And it's not really a surprise, when the same method is
not able to improve on that number.

Switching the compressor on (compression setting 0 versus 1),
gives you the 2:1 improvement. The values 2 through 9 take
a lot more time, and don't seem to measurably improve things.
The 70MB PNG file created with compression=0, became 35MB
when the compression was set to 1 instead (you can do that in
GIMP).

One thing I can't figure out, is why a compression setting of
0, takes so long to write the file. That's a puzzle.

Something like LZ77 might do a bit better than that, and the
same 0..9 control values might make a bit of difference.

Whereas JPG or MPG2 might achieve 100:1 lossy compression and
then you're impressed.

At a compression setting of 0, DEFLATE should be turned off
entirely. A compression setting of 1, turns on DEFLATE.
I don't really see what DEFLATE can do, when the control
is set at 2 through 9. If the DEFLATE method had a "better
dictionary" it planned to use, it would probably be using
it in every case, instead of waiting until the control is
set to 2 or 9.

Paul