Mailinglist Archive: opensuse (958 mails)

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Re: [opensuse] Re: pngquant - PNG compressor
On 04/05/2019 07.56, L A Walsh wrote:
On 5/3/2019 2:07 PM, Carlos E. R. wrote:

And the result is pretty good for sending informative screenshots.

However, all of them seem to be 8 bit, according to "file":

cer@Elesar:~/Pictures/Screenshots> file *png through extraneous info deletion...amazing what you can do
if you go for the lossy option:
PNG, 8-bit/color RGB, non-interlaced <-- original
PNG, 8-bit colormap, non-interlaced <-- pngquant
PNG, 8-bit/color RGB, non-interlaced <-- optipng

I suspect you would get similar results if you did a conversion
to GIF format as that's essentially what it appears to be doing.

I.e. an 8-bit colormap = 256-color image vs. 24-bit colormap
with 2**24 (4 mega colors).

What about 16 bit colors?

For the stated purpose of a color limited screenshot, pngquant is
clearly superior, however it would be nice if we knew if it was
losing color information or not -- i.e. if there really are no
colors that are dropped, approximated or dithered, then you have
an accurate reproduction at a smaller size and that's cool!

But it if throws away info and doesn't tell you, color me less
than impressed.

Yes, that's a point.

On the otherhand -- people lived with GIF's and 256-bit color for
several years back in the late 80's, early 90's...wasn't until
VRAM got cheap enough for 24-bit color that interest faded for most

And less.

I had occasion to test some astronomical photograph samples that came
with a CCD camera for a telescope (it was cooled with peltzier cells,
around 1990, B&W. The software came in basic, so I analyzed it, and
started doing an equivalent in turbo pascal.

A problem was that the VGA (SVGA?) displays of that the time could not
display 256 levels of gray (or any single colour), just 64 (with a color
map). So what it did was create variations adding a bit of blue, red or
yellow to change the light value of the pixel, thus "emulating" 256
levels of gray.

Even so, it did "impressive" manipulation of the photos. Altering the
rendition curve, for instance, hidden important details appeared on the
photo (taking a minute). Of course, tools like gimp can do that now in
"really" impressive manner and in a second on a big photo.

Still, if the picture you encoded really had only 256 colors, I've
seen png files compress alot better with low-detail pictures.


Cheers / Saludos,

Carlos E. R.

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