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From: clipka
Subject: Re: Why is gamma-correction applied in povray, anyway?
Date: 5 Nov 2009 18:20:21
Message: <4af35db5$1@news.povray.org>
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Cousin Ricky schrieb:
> I have this gamma control dialog box on my system. I don't know whether it
> applies gamma correction to the output signal, or controls the LCD itself,
> though I suspect the former.
>
> I've never seen one of these on a CRT based system, but that might well be an
> artifact of history.
Yes, that looks just like one of the tools provided by graphics card
vendors to adjust the output signal of the graphics card.
If it is what it looks like, it essentially tampers with a look-up table
in the graphics card that maps framebuffer values to more precise values
sent to either the LCD via DVI connector or the DAC generating the VGA
signal.
My Nvidia graphics card also came with a tool that would help you set up
that curve properly.
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> I am having lots of problems with gamma correction on my system. At least
> there are a lot of iffy things.
>
> Contrary to what has been posted here, my video card's driver seems to do
> some gamma correction (Nvidea). At least I can do a some gamma correcting in
> the setup. However, it is a lot of voodoo: I don't know for sure what is
> corrected and when. I really don't see any difference.
The settings in your card driver are there to correct differences between
monitors, but unfortunately, they have no way to tell what your monitor is
actually doing, so the settings are only as good as your ability to tell if your
monitor is properly balanced. This in turn is influenced by a lot of things,
including the lighting where your monitor is. But if you send an image to your
printer and the colors are completely different than onscreen, then its a good
bet your colors aren't balanced.
Check out http://www.normankoren.com/
He has some stuff on how monitors produce color and how your settings actually
work, along with some tools to help you calibrate them correctly. In a
nutshell, he says your brightness sets the white level, contrast sets the black
level and then you use the gamma setting of your card to adjust the colors in
between.
Can't say anything about the science behind it, but I can say that after
following his advice my prints not only look more like onscreen, but I no longer
see the extreme differences in colors I used to see when viewing pictures on
different monitors. (A big issue in POV when creating textures... if you've ever
rendered a scene on another box only to find that your 'perfect' texture on box
A looks absolutely horrible on box B...)
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From: scott
Subject: Re: Why is gamma-correction applied in povray, anyway?
Date: 6 Nov 2009 02:48:06
Message: <4af3d4b6@news.povray.org>
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> Maybe. As I already posted in reply to clipka, my video card driver
> (Nvidia) allows to do gamma correcting.
You should try to set that to unity as a starting point, otherwise your OS
GUI and any web browsing will be messed up and not as they intended it to be
viewed. I guess it's more there to tweak your monitor by a very small
amount if it doesn't quite exactly match the standard sRGB gamma (a lot of
monitors nowadays have an "sRGB" setting which I assume is at least close to
the standard), obviously no consumer monitor is perfect unless manually
calibrated.
> Then there is colour temperature, which can be set on most monitors. How
> does this relate to gamma?
In theory not at all, but in practise it probably does a bit just due to the
way its implemented in some monitors. Colour temperature defines what the
white point should be, for accuracy you should set this to the D65 standard
(daylight 6500K) which is the sRGB standard. Obviously if you tell your
monitor to use 6500K it might not exactly use it due to manufacturing
tolerances (or there might not even be a scale on your monitor, just "warm"
to "cool"), that's why there are tools to calibrate your monitor.
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> I am having lots of problems with gamma correction on my system. At least
> there are a lot of iffy things.
>
> Contrary to what has been posted here, my video card's driver seems to do
> some gamma correction (Nvidea). At least I can do a some gamma correcting
> in the setup. However, it is a lot of voodoo: I don't know for sure what
> is corrected and when. I really don't see any difference.
Without access to a luminance meter, just show an image like this:
http://web.mit.edu/jmorzins/www/gamma/adilger/gamma.png
Squint your eyes to blur it a bit, and then where the plane grey area (under
the numbers) matches the black/white lines (should be in the 1.6-2.4 range),
you can read off the gamma value.
You can tweak your settings in the nVidia control panel until you achieve a
gamma of 2.2 with that image.
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>> Of course reflection on the left makes more sense than on the right. Is
>> this a gamma-related problem or is this an error in povray?
>
> Whoops - are you mixing up left & right here?
Oups! Well - back to painting "L" and "R" on my hands ;-)
Sorry about this.
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On 11/4/2009 9:52 PM, clipka wrote:
> I guess you'll agree there's something wrong with the reflection on the
> left side...
Actually, no, I have no idea what you're talking about.
Great opening line for a thread though.
Mike
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On 11/4/2009 9:52 PM, clipka wrote:
> I guess you'll agree there's something wrong with the reflection on the
> left side...
>
> Although artificially constructed, this scene may serve to showcase how
> raytracing computations can go wrong when ignoring gamma issues.
>
> Left side was rendered with POV-Ray 3.6 with default settings (i.e.
> without any gamma handling), right side was rendered with POV-Ray 3.7
> defaults (i.e. full-fledged gamma handling); the only difference between
> the scenes is in the brightness of the truly-grey squares, to roughly
> preserve the percieved contrast to the black-and-white-striped squares
> as seen on the unreflected plane (just as you would have to overhaul a
> legacy scene if you want to render it with full gamma handling).
Instead of arguing about it on this newsgroup, why don't one of you guys
write a wiki article so we can just go along step by step, starting from
the beginning?
Mike
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SharkD schrieb:
> Instead of arguing about it on this newsgroup, why don't one of you guys
> write a wiki article so we can just go along step by step, starting from
> the beginning?
Guess what: I'm presently on it. The left-side shot was designed to be
an illustration for that.
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SharkD schrieb:
> On 11/4/2009 9:52 PM, clipka wrote:
>> I guess you'll agree there's something wrong with the reflection on the
>> left side...
>
> Actually, no, I have no idea what you're talking about.
Do you see a checkered plane on the left side?
If you don't (even if you squint your eyes or take some steps back),
then your system really screws up PNG files.
If you do, then wouldn't you expect to see that checkered plane in the
reflection as well?
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On 11/6/2009 5:43 PM, clipka wrote:
> SharkD schrieb:
>
>> Instead of arguing about it on this newsgroup, why don't one of you
>> guys write a wiki article so we can just go along step by step,
>> starting from the beginning?
>
> Guess what: I'm presently on it. The left-side shot was designed to be
> an illustration for that.
Awesome!
Mike
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