Kenneth wrote:
> Christian Walther <cwa### [at] gmxch> wrote:
>><http://www.optics.arizona.edu/Palmer/moon/lunacy.htm> (Caution: his GIF
>>images of a lambertian sphere are encoded with a gamma of 1 and don't
>>appear correctly on usual displays! In fact, I wouldn't be too surprised
>>if they were made using POV-Ray without an assumed_gamma statement.)
>
> But if that image was created incorrectly (or displays incorrectly),
> wouldn't his entire argument fall flat? He's using it as the core of
> his discussion, so I expect that he went to some pains to reproduce
> true lambertian characteristics.

My impression is that he bases the discussion on a hand-calculated 
result, not on these images. He calculated that the intensity ratio 
between quarter and full moon should be pi (that calculation is correct, 
unless I made a mistake when I did it myself), while he measured it to 
be about 10.

It's possible that he didn't know about gamma encoding, had his display 
calibrated to gamma 1, or just trusted his software to do the right thing.

> Well, I had begun to think that different versions of POV (on different
> platforms) might be producing images differently, gamma-wise--leading to
> wildly opposite viewpoints such as we have.  Although there may be
> something to that (don't really know, but I'm guessing not), I've put aside
> that line of thought for the time being.

There's no reason for such an assumption - the versions for different 
platforms are compiled from one and the same source code. To the best of 
my knowledge, the only difference between platforms is the default 
display_gamma setting. IIRC you have both a Mac and a PC, so why not try 
it yourself?

> But while we're on the subject--would you mind telling me what overall gamma
> your various monitor/OS systems are set to? If they're "largely" different
> from mine, that might explain part of our disagreement.

My Mac (my main day-to-day machine) is calibrated to gamma 1.8 because 
that's what the user interface elements of Mac OS X are made for. I 
never calibrated the PC, as I seldom use it, but I guess it's around 2.2 
as PCs usually are - if it's a power function at all, and not just 
uncorrected LCD response.

I don't think that explains our disagreement - if your statement that 
your displays are calibrated to gamma 2.0 is correct, I know how the 
images look there.

  -Christian

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