> 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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> 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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"TC" <do-not-reply@i-do get-enough-spam-already-2498.com> wrote:

> I am having lots of problems with gamma correction on my system. At least
> there are a lot of iffy things.
>
> [...]
>
> Now, I am using Corel quite a lot. Corel has a lot of colour correcting,
> too. I downloaded the proper colour-management system files for my monitor
> and my printer, respectively, but the printed result did not look much like
> the screen. The PDF-file I sent to my printer (I mean the real one, the guy
> who doesn't bother printing anything below 1000 copies ;-) did look like
> neither my printout, nor the screen.
>
> So I did trash all colour-correcting. All set to "ignore" - from video card
> driver to Corel to printer profile and printer driver. And behold: now I get
> on my printer what I see on my TFT and the guys at the printing press
> produce something that comes close enough to what I want.


FWIW, my experience matches yours to the dot: The more "color management" is
going on, the rattier and less predictable the result becomes. Which is why I
keep asking folks who write software for a switch that goes "turn off / ignore /
keep your hands off all this"...

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