On Tue, 29 Nov 2005 09:11:08 +0100, Kenneth <kdw### [at] earthlinknet>  
wrote:
> "Fredrik Eriksson" <noo### [at] nowherecom> wrote:
>> I am not sure if this is what is causing your problems, but PC software
>> has a tendency to assume that the monitor has a gamma of 2.2. Adjusting
>> your monitor to make it have a gamma of 1.8 can make such software  
>> produce
>> bad results.
>>
>
> Yet both .bmp and "unaltered" .png image_map images look identical on  
> both
> my PC and Mac (both monitors set to gamma of 1.8), in both versions of
> Photoshop and in the final POV rendered scene on the PC. It's only the
> "altered" .png file that causes problems.

Exactly. The bmp has no gamma information. The unaltered png has either no  
gamma information, or a gamma of 1.8 (the "correct" value). The altered  
png probably has a gamma of 2.2, which makes it misbehave because the  
image data is actually gamma 1.8.

Interestingly, I found this tidbit in the PS help files:

"Note: Images created in Photoshop 5.0 or later use the Windows gamma  
value (2.2) by default and are at the correct gamma for display in Windows  
with no adjustment."

I assume they meant just the Windows versions of PS though.



>> > I've purposely left out the "use assumed_gamma of 1.0" suggestion
>> > altogether
>>
>> I take this to mean that you have tried it, and it made no  
>> difference....
>>
> Actually, no. I'm still not totally convinced of its usefulness...mainly
> because I'm STILL trying to understand its pros and cons.  The set-up I  
> now
> have seems to "work for me, " so I haven't gone that route yet.   
> Although,
> I do (sort of!) understand that a scene file to be shared with others may
> need it, for rendering consistency.

That is indeed its purpose, as far as I understand it.



>> Have you examined the files to see what the gamma chunk says?
>
> I have to admit ignornace there; how does one go about doing that?

Off-hand, I can only point to one program, and I wrote that myself...

http://hem.bredband.net/b230591/hdr2exr/hdr2exr_win32.zip

Run it like this:
hdr2exr --verbose suspect_image_file.png %TEMP%\dummy.png

It should then tell you the gamma value encoded in the file.

You can also use that program to correct an erronous gamma value.



-- 
FE

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