Am 10.04.2014 20:58, schrieb Kenneth:
> I'm beginning to think that there is something wrong with my computer/monitor
> set-up (Windows XP on a CRT monitor): The .png images that various POV-Ray users
> are uploading to the newsgroups look 'dark' on my system, and I have no idea
> what the cause is. (Well, I'm guessing that the images *should* look brighter,
> although I don't know for sure.)

There are various possible reasons:


(A) Personal preference.

Some people simply love playing with darkness and light, and the images 
may be intended to be that dark, while you may prefer images to have 
less contrast.


(B) Your "display gamma".

In a nutshell, no two computer/display combinations natively show the 
same image in the same way, due to hardware tolerances, manufacturer 
preferences, and brightness/contrast control settings.

The only way to make sure your computer display shows .png images 
exactly the way they are supposed to be (according to the file format 
specification and the data in the file) is to /calibrate/ your display, 
which in essence means that you teach your computer to auto-correct for 
your display's peculiarities.

While proper calibration would require a colorimeter (and would also 
account for hue and saturation issues), if proper brightness is all you 
want then simple /gamma correction/ will suffice; most display hardware 
I know of comes with tools to perform gamma correction, and if I'm not 
mistaken Windows 7 and later (maybe also Vista) comes with such tools 
onboard.


(C) The author's display gamma.

Just as you can't be sure /you're/ seeing what's officially encoded in 
the .png file unless /your/ display is calibrated, you can't be sure 
that what's officially encoded in the .png is what the /author/ was 
seeing unless /their/ display is calibrated as well. However, in most 
cases this is obviously outside your area of influence.


(D) Your display's "black level".

Your display may be unable to show the same level of black that the 
original author saw, which might "drown" some almost-but-not-quite black 
details. However, while this is a common issue with LCDs, it is rarely 
seen in CRT displays.


In real life it's typically a combination of (A)-(C), as well as (D) for 
people who have an LCD.


> This dark appearance shows up in a range of applications, not just a specific
> one-- the latest Firefox (v28.0); my older Photoshop; and in Windows' Picture
> And Fax Viewer. My own .png images made in POV-Ray look fine on my system, as do
> ...jpegs.
>
> To better show this, I've made .jpeg screenshots of two recent uploaded images,
> by FractRacer and Pekka Aho. They are accurate reproductions of how the .png
> originals appear on my end. Can someone compare my .jpegs to the original
> uploads, to see if they visually match?

How on earth do you know that those JPEGs are "accurate reproductions of 
how the .png originals appear on [your] end"? After all, you'd need a 
calibrated display in the first place to be sure that what's encoded in 
the JPEGs accurately reproduces what you saw.

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