In message , Mayayana
writes:
"J. P. Gilliver (John)" wrote

| a format for storing photos. Similarly with GIF: It's
| handy for creating small files and it's cross-platform,
| but it's lossy insofar as it reduces an image to 8-bit
| color.
| []
| Not quite: it reduces it to 8-bit _storage_, but it does that by using a
| palette. I think the palette entries are at least 16-bit. Basically, it
| reduce an image to 256 _colours_, but they're not the _same_ 256 for any
| given image: a picture of a sunset, for example, will have a lot of
| oranges and reds.

I wouldn't argue with that. But it's still reduced to a
max of 256 colors. It's best for logos, cartoons,
simple images. A sunset will dither. (Remember the
old days on Windows monitors? If you used a sunset
desktop photo you would have had stripes.)

Yes, I had such a monitor (well, laptop).

| And once the reduction has been done, there's no
| _further_ compression

As I understand it there is, but it's not lossy. It's
a formulaic system that will record things like "43
pixels of color #18" as a data record, rather than
using 43 * 3 bytes to record 43 pixels. It's very
efficient in that context because repeating pixels are
the norm.

My bad - I meant loss, not compression.

(though some image editors - like, unfortunately,
| IrfanView, which I think is great in most respects - tend to operate in
| maximum-colours mode, so edit actions in them _do_ cause degradation
| when resaved in GIF. But that's not the format's "fault"; if the editors
| could be constrained to work in 256-colour mode, there'd be no loss).

That's just not true. Few 24-bit images use only
256 colors. Try this one:
https://www.jsware.net/Files2/sunsetMV.jpg

What I meant was: once an image has been reduced to 256 colours, then
any editing _that did not change the number of colours_ (such as
brightness or _possibly_ contrast tweaking) would not result in further
corruption if saved as GIF; once it's been reduced to 256 colours, then
anything further you do to it, _provided it doesn't result in an
increase in the number of colours_, can still be saves as GIF without
further degradation. The sort of things that _do_ result in
number-of-colours increase include blurring, including resizing
(especially down).

IrfanView says there are 100,627 unique colors there.
If I reduce to 256 colors in PSP I get something
like a comic book image, where Superman is in 3
colors.

Agreed. (Though it's subtle: I didn't notice it at first.)

There are 3 reduction routines down to 256 colors
and the effect varies with each, but all drop out a
tremendous amount of data. If I save as GIF from
PSP I get a pointilistic image. PSP16 does a slightly
smoother job of it than PSP5, but both end up
looking like a print from an old printer. And that
image started as a low quality JPG that had already
been resaved at least twice, so it wasn't a great
picture to begin with. It had already dumped a
lot of the richness. The degradation from the original
would have been heartbreaking to see.

To me that's a great example of the role of JPG
and GIF: Great for onscreen images that need to
be small and that need to be accessible across
platforms. I use GIFs a lot for diagrams. But they're
not good for much else. It would be crazy to store
photos as GIF in order to save space.

Agreed (though there are _some_ images that _don't_ lose a lot: mainly
ones without gradual shading).

I find it kind of ironic when this topic comes up.
I don't think I've ever heard you say this, but whenever
I talk about conserving space on disk, many people
will respond with, "Ah, that's not worth the trouble.
Disks are so cheap these days!" Yet when it comes

No, you'll rarely hear me say that, as I come from the bygone era (my
first computer had 1K of memory; before that, the first one I worked on
had 16 memory locations). I will _sometimes_ concede that view when
discussion of time versus resources comes up, but given the choice and
time, I'll go for saving space where practicable. (Actually, more in MP3
files than images; my eyesight, touch wood, has not deteriorated with
age other than the ability to close-focus, but my hearing _has_ lost
top, and/or I haven't had speakers capable of great top for some time.)

to saving large images for a good reason, those
same people think it's crazy: "Takes up WAY too much
space!"

I suspect most people who feel that way are taking
loads of pictures with their phone. They just want
2,000 vacation photos to fit on disk. They have no
intention of doing any involved editing or printing of
those images, or even going through to dump the
bad ones, so they're happy with downgraded JPGs.

My 'phone - a cheap one (a DooGee) - has, IIRR, a 6M camera. It takes
pictures I consider considerably inferior to those I take with my 3M
Fuji with a reasonable lens - which I usually have set to 1M size (JPEG
that is).

When you say they want "vacation photos to fit on disk", do you mean "to
fit on _a_ disc", i. e. to make a CD (or, I guess these days, a DVD), to
give to friends/relatives?

(I remember using a Sony camera at work, that had a floppy drive built
in - and you could get several pictures on, of acceptable quality! [That
camera also had something I've never seen before or since: the ability
to use ambient light to backlight the display.])
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

If it's not on fire, it's a software problem.