Am 13.04.2010 10:55, schrieb Dave Blandston:

>> Completely off-topic: Your .png image is showing up with different gamma
>> 'values' depending if I look at it's 'preview' image here in the newsgroups vs.
>> clicking on the image link, to bring it up as a stand-alone image. (I'm using
>> the latest version of Firefox as my web browser, in both cases.) The preview has
>> much darker shadows, and more contrast overall. Anyway, I've never seen this
>> behavior here in the newsgroups before. Kind of a mystery! I can't tell which is
>> the 'correct' image. Makes me wonder if the newsgroup 'software' is altering the
>> gamma values of .png images when displaying a preview. OR, whether Firefox
>> itself has a gamma flaw.
>
> It's probably something I'm doing wrong - I get an error message about gamma
> settings every time I render a scene. One of these days I'll have to figure out
> the new settings... I've also noticed that sometimes when I select a POV-Ray
> v3.7 generated .png image as a desktop wallpaper for Windows XP the little
> preview shows a really washed-out looking image, but then the actual desktop
> image looks right.

Your .png file has a gAMA chunk indicating that you set File_Gamma=1.0, 
or are using #version 3.6 and assumed_gamma 2.2 (I'm presuming here that 
you're using POV-Ray 3.7); while this is not wrong /per se/, it has some 
drawbacks:

- You're not taking advantage of /gamma encoding/; as a consequence, at 
a depth of 8 bits per color channel you're risking noticeable color 
banding in dark areas.

- The images are not inherently /gamma pre-corrected/ for a typical 
computer; as a consequence, software that doesn't evaluate the gAMA 
chunk will present the midtones /darker/ than they should be (except on 
systems calibrated for a non-typical gamma, such as linear gamma).

With POV-Ray 3.7, it is strongly recommended to work without 
assumed_gamma, and set File_Gamma=2.2, for any image (or scene, for that 
matter) you plan to share with others.

That said, it seems to me that Firefox is doing it right (at least on 
systems with a display subsystem gamma of around 2.2): In the case of 
this particular file, it is the more washed-out version that is right.

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