Hi,

I'm slightly confused. Any help appreciated.

Only recently I added Display_Gamma=1.7 to my config (it's probably
1.8 but that was what I arrived at).

then today I grabbed some source from p.b.sf (the balls collection)
and rendered it.

The image I got was considerably lighter (and better for it) than
the one posted on p.b.i as viewed on my monitor.

It has assumed_gamma 1.0 in its global_settings, I can only assume
the author (Nico) also has an appropriate Display_Gamma locally.

so, I have read all I can find on gamma in the pov docs,
but I'm not sure if I understand it correctly.
Could someone please correct me if I'm wrong on what I
gleaned:

1. When you look at an image that was generated on a different
system, you are at the mercy of the gamma settings of that
system, *unless* it is a png image, which *might* allow
gamma correction when you view it.

2. However if you render the same image on your own system,
with your own Display_Gamma settings and assumed_gamma 1.0,
then you pretty much see what the other person saw when they
rendered the image.

Is that right?

-- 
Bill Hails

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In article <3fc52ebd@news.povray.org>,
 Bill Hails <bil### [at] europeyahoo-inccom> wrote:

> 1. When you look at an image that was generated on a different
> system, you are at the mercy of the gamma settings of that
> system, *unless* it is a png image, which *might* allow
> gamma correction when you view it.

Right. If it is a PNG image and the gamma information was written 
correctly, your PNG viewing software reads and uses it, and your viewing 
software is set up correctly with your system's gamma, it will display 
with the proper gamma correction. Unfortunately, it seems like at least 
one of these is usually wrong.


> 2. However if you render the same image on your own system,
> with your own Display_Gamma settings and assumed_gamma 1.0,
> then you pretty much see what the other person saw when they
> rendered the image.

Yes.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Thanks for asking your questions, I learned something!

Tim

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Christopher James Huff wrote:

> In article <3fc52ebd@news.povray.org>,
>  Bill Hails <bil### [at] europeyahoo-inccom> wrote:
> 
>> 1. When you look at an image that was generated on a different
>> system, you are at the mercy of the gamma settings of that
>> system, *unless* it is a png image, which *might* allow
>> gamma correction when you view it.
> 
> Right. If it is a PNG image and the gamma information was written
> correctly, your PNG viewing software reads and uses it, and your viewing
> software is set up correctly with your system's gamma, it will display
> with the proper gamma correction. Unfortunately, it seems like at least
> one of these is usually wrong.
> 
> 
>> 2. However if you render the same image on your own system,
>> with your own Display_Gamma settings and assumed_gamma 1.0,
>> then you pretty much see what the other person saw when they
>> rendered the image.
> 
> Yes.
> 

Great, thanks.

In that case, if when someone posts an image they mention their
own gamma settings, it should be possible to calculate a gamma
setting for viewing the image locally, either my_gamma / their_gamma
or its inverse, not sure which :-), assuming a viewer
that allows a numerical setting of the gamma.

-- 
Bill Hails

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In article <3fc5b224@news.povray.org>,
 Bill Hails <bil### [at] europeyahoo-inccom> wrote:

> In that case, if when someone posts an image they mention their
> own gamma settings, it should be possible to calculate a gamma
> setting for viewing the image locally, either my_gamma / their_gamma
> or its inverse, not sure which :-), assuming a viewer
> that allows a numerical setting of the gamma.

Yes, if you know the gamma the image was viewed with, you can correct it 
to display properly with your gamma, doing manually what the PNG gamma 
is supposed to do automatically.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Tim McMurdo wrote:

> Thanks for asking your questions, I learned something!
> 
> Tim

Thank C. J. Huff for answering them :-)

-- 
Bill Hails

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On Wed, 26 Nov 2003 22:57:04 +0000, Bill Hails
<bil### [at] europeyahoo-inccom> wrote:

>Hi,
>
>I'm slightly confused. Any help appreciated.
>
>Only recently I added Display_Gamma=1.7 to my config (it's probably
>1.8 but that was what I arrived at).
>

Just my two cents here ...

In my .pov file, I set the assumed_gamma in my
display settings to be the same as my display gamma.
This allows me to "feedback" output images back into
a render so that they don't get lighten or shortened
by successive re-renderings.

I avoid PNG like the plague.  The idea (and theory)
of PNG is wonderful, but no two programs seem to
implement gamma correction the same way.  They never
seem to agree on gamma.  The means that the
appearance of the images gets messed up worse
between programs than if I had not used PNG.  The
TGA format has always worked well for me.  BMP has
also worked well but is windows-specific and does
not seem to allow an alpha channel.

Your mileage may vary

Hope it helps, even if only partially

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nospam wrote:

> [...]
> 
> Just my two cents here ...
> 
> In my .pov file, I set the assumed_gamma in my
> display settings to be the same as my display gamma.
> This allows me to "feedback" output images back into
> a render so that they don't get lighten or shortened
> by successive re-renderings.
> 
> I avoid PNG like the plague.  The idea (and theory)
> of PNG is wonderful, but no two programs seem to
> implement gamma correction the same way.  They never
> seem to agree on gamma.  The means that the
> appearance of the images gets messed up worse
> between programs than if I had not used PNG.  The
> TGA format has always worked well for me.  BMP has
> also worked well but is windows-specific and does
> not seem to allow an alpha channel.
> 
> Your mileage may vary
> 
> Hope it helps, even if only partially

Thanks. I use png mainly just because it's the default
for Povray on Linux, also it's lossless (though I guess
TGA is too).

Apart from height fields I'm not often recycling images
through subsequent renders, and I'm not doing any
post-processing in most cases.

-- 
Bill Hails

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In article <3fc6d59f.4578013@localhost>,
 Pet### [at] nymaliasnetalmost (nospam) wrote:

> In my .pov file, I set the assumed_gamma in my
> display settings to be the same as my display gamma.
> This allows me to "feedback" output images back into
> a render so that they don't get lighten or shortened
> by successive re-renderings.

Just don't specify assumed_gamma at all. This disables gamma correction 
without the possibility of accidentally specifying the wrong value, plus 
you can render it on multiple machines without editing.


> I avoid PNG like the plague.  The idea (and theory)
> of PNG is wonderful, but no two programs seem to
> implement gamma correction the same way.  They never
> seem to agree on gamma.  The means that the
> appearance of the images gets messed up worse
> between programs than if I had not used PNG.  The
> TGA format has always worked well for me.  BMP has
> also worked well but is windows-specific and does
> not seem to allow an alpha channel.

Well, just render without gamma correction, or otherwise disable the 
gamma value. You can handle gamma however you want, and still get the 
compression and other benefits of PNG. And avoid BMP like the plague. ;-)

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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