> WOW, that's... crazy. I don't even know what to make of it. *Thanks* for
> checking this out. One idea that occurs to me (which could be wrong, of course)
> is that posting a PNG to the newsgroups changes the gamma... perhaps twice, for
> the two different-sized previews. ?? Otherwise, it's a mysterious problem with
> several interrelated causes, my Photoshop being only one of them.
>

There *is* a problem with the web interface as the code that creates the 
downsampled preview always assumes an input image with 2.2 (or sRGB) 
gamma. It takes the original image data, resamples it and does tag the 
output as sRGB regardless of the original gamma information.
On the other hand, how the image itself appears solely depends on the 
way the browser handles it. And luckily we live in 2017, 10 years (or 
so) ago literally EVERY browser did handle it differently.
And BTW the problem is not limited to PNG images, it would also happen 
when you use for instance a JPEG with an ICC profile stating a non 2.2 
gamma.


> If you don't mind doing so, would you check the gamma chunk in the 'raw' POV-Ray
> PNG image I posted, the one that begins with this:
> "One more image test, again for my own purposes: This PNG is *directly* from
> POV-Ray (and NOT post-processed in Photoshop.)..."
>
> I'm curious to know what *that* turns out to be.

As expected - this one has a sRGB chunk.
(Rendered with POV-Ray 3.7.1-beta.9 at 22:22, 2017-08-04)


> Version 5.0, I'm embarassed to say. And yes, I also read somewhere that such
> 'older' versions had some kind of flaw in their handling of PNGs... although I
> don't know the technicalities.
>

I've worked with all versions of Photoshop before they turned to CC and 
Adobe's handling of some image file formats was always a mess. But for 
PNG it was the worst of all as Adobe did - when switching versions -  do 
it wrong again, but in a different way. So e.g. a PNG file written with 
PS 5.0 will look unexpected different when opened in CS1.
Good news is, as a user you can work around these issues. But as I've 
used version 5.0 quite a long time ago (20 years ?) I cannot tell 
anymore how the settings are called and where to find them.
Look somewhere at Preferences->Settings->Color management. There should 
be somewhere a checkbox saying something like "always ask when profile 
differs" and make sure it is checked. When opening now e.g. a PNG file 
written by POV-Ray a dialog should pop up asking you if you would like 
to apply a color transform and you should say NO.
But as mentioned, I'm not entirely sure, maybe this was the way v6.0 did 
handle it.

-Ive

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