View Single Post
  #98  
Old February 19th 20, 12:33 AM posted to alt.windows7.general,alt.comp.os.windows-10,microsoft.public.windowsxp.general
Brian Gregory[_2_]
external usenet poster
 
Posts: 166
Default Image formats

On 18/02/2020 23:08, Paul wrote:
Brian Gregory wrote:
On 17/02/2020 21:23, Big Al wrote:
optipng doesn't change the image quality any?Â*Â* I've never used it,
just installed it on my system.Â* I ran a test and some images were
cut in half.Â* 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.

Â*Â* Paul


Look for gods sake just TRY IT and stop endlessly speculating on what
you kind of feel is probably the case.

Get a selection of PNG files, some photos and some graphics and you'll
see how much of an improvement optipng can make. Usually not much, maybe
2% if you're lucky but when a PNG is going on a website why not do it?
If you see more than that it probably means the original PNG wasn't
compressed properly.

--
Brian Gregory (in England).
Ads