Hello there !
I tried, I tried, I really tried to understand how gamma works...
How could I switch it off ?... So this scene would give a perfectly mid-gray
image :

camera {
  location -z
  look_at 0
}

box {
  -0.5
  0.5
  pigment { rgb 0.5 }
  finish { ambient 1 }
}

I don't understand why this isn't the case when NO gamma settings is set
(evertything to default).

I tried File_Gamma=1 Display_Gamma=1, and it seemed to work fine in gimp and in
the window image viewer, but then I try to see the image in firefox and it is
way to bright - so firefox can't handle gamma data ?

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I just saw the gamma tutorial on the wiki, so I'll be reading it... Hopefully
I'll finally get to what I want ! :)

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If I understood correctly, the 'over-bright impression' is actually the correct
way images should be, right ?

But then, the default ambient light is waaaay to high, isn't it ?

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Am 26.06.2010 10:12, schrieb Gyscos:
> Hello there !
> I tried, I tried, I really tried to understand how gamma works...
> How could I switch it off ?... So this scene would give a perfectly mid-gray
> image :
>
> camera {
>    location -z
>    look_at 0
> }
>
> box {
>    -0.5
>    0.5
>    pigment { rgb 0.5 }
>    finish { ambient 1 }
> }

This scene actually /does/ give a perfectly mid-gray image... as long as 
we're talking about physical light intensity, rather than perceptual 
brightness ;-)

To really get the scene to look the way you want it to, use:

    pigment { rgb pow(0.5, 2.2) }

Future betas will also provide an extended color syntax for this purpose:

    pigment { rgb 0.5 gamma 2.2 }

> I tried File_Gamma=1 Display_Gamma=1, and it seemed to work fine in gimp and in
> the window image viewer, but then I try to see the image in firefox and it is
> way to bright - so firefox can't handle gamma data ?

Let me guess - you used PNG file output? In that case it is a sign that 
/Firefox/ is getting it right, while gimp and the image viewer are not. 
The File_Gamma setting does not affect PNG output very much (provided 
that your image processing software is handling gamma right), except for 
banding artifacts.

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On 26/06/2010 10:59 AM, Gyscos wrote:
> If I understood correctly, the 'over-bright impression' is actually the correct
> way images should be, right ?
>
> But then, the default ambient light is waaaay to high, isn't it ?
>
>

high. That means the box does not need a light source to be fully 
illuminated. If you are using radiosity set the ambient to zero. If not 
try an ambient between 0.1 and 0.2.

There have been a few discussions on this newsgroup about the changes in 
the way Pov 3.7 beta handles gamma so you could search here. Also the 
Wiki has a topic.
http://wiki.povray.org/content/HowTo:Fix_old_scenes_to_work_with_the_new_gamma_system


-- 

Best Regards,
	Stephen

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Am 26.06.2010 11:59, schrieb Gyscos:
> If I understood correctly, the 'over-bright impression' is actually the correct
> way images should be, right ?

Um... yes and no.

Yes: That's the right math.

No: If your scene looks overly bright, your parameters are wrong.


> But then, the default ambient light is waaaay to high, isn't it ?

If you're talking about the default global { ambient_light 1.0 }: No, 
that has nothing to do with gamma.

If you're talking about the default finish { ambient 0.1 }, and/or the 
ambient values in all those standard material .inc files coming with 
POV-Ray, then no, no and maybe.

No: Ambient light is pretty dependent on the actual scene, and actually 
the only way to /really/ do it right is using radiosity instead.

No: The effect of ambient light also depends on the object's pigment, 
which is waaaay too high in the default colors as well, as they were 
designed at times when gamma wasn't done right.

Maybe: The ambient light defaults, too, were chosen at a time when gamma 
wasn't done right, so they should be subject to re-evaluation.

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Am 26.06.2010 11:03, schrieb Gyscos:
> I just saw the gamma tutorial on the wiki, so I'll be reading it... Hopefully
> I'll finally get to what I want ! :)

Not sure which one you're talking about right now, but I'd like to take 
the opportunity to advertise my gamma tutorial WIP once again: 
<http://wiki.povray.org/content/User:Clipka/Gamma>

(Feedback appreciated, as usual.)

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Yup, that's the one I was reading. :)

About the ambient light, I meant that in a simple scene, with just a plane and a
box for instance, the ambient light was indeed designed to work with the old
settings ; and now, with the new gamma stuff, it might need some re-adjusting.

I don't want to adapt old scenes, I'm just trying to make new ones with what I
knew from before.

But I'm trying to make some nice scattering, and it is really harder now... I
used to use type 2, but now I can't get enough contrast between the high-angle
and the low-angle zone... So I use type 3, wich gives more or less the same
result as type 2 before, but... It doesn't seem right to change type for that.
:(

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clipka <ano### [at] anonymousorg> wrote:
[...]
> Let me guess - you used PNG file output? In that case it is a sign that
> /Firefox/ is getting it right, while gimp and the image viewer are not.
> The File_Gamma setting does not affect PNG output very much (provided
> that your image processing software is handling gamma right), except for
> banding artifacts.

But the POV-Ray preview was identical to what I get in Gimp or window viewer...
Is that normal ?
Display_gamma and File_Gamma being identical doesn't mean the preview and the
output image - when correctly decoded - are the same ?

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Am 26.06.2010 13:24, schrieb Gyscos:
> clipka<ano### [at] anonymousorg>  wrote:
> [...]
>> Let me guess - you used PNG file output? In that case it is a sign that
>> /Firefox/ is getting it right, while gimp and the image viewer are not.
>> The File_Gamma setting does not affect PNG output very much (provided
>> that your image processing software is handling gamma right), except for
>> banding artifacts.
>
> But the POV-Ray preview was identical to what I get in Gimp or window viewer...
> Is that normal ?

If you set Display_Gamma=1.0, then yes - you'll typically get a wrong 
preview...

> Display_gamma and File_Gamma being identical doesn't mean the preview and the
> output image - when correctly decoded - are the same ?

That depends.

Display_Gamma always specifies the gamma pre-correction to apply in 
order to show a correct preview.

File_Gamma strictly speaking doesn't specify gamma pre-correction, but 
gamma encoding (or, even more strictly speaking, the inverse of the 
encoding gamma). For some older file formats, like BMP or JPEG, where 
gamma encoding double-features as gamma pre-correction, this happens to 
be the same; PNG, on the other hand, includes information about the 
encoding gamma used in the file header, so that it can be left all up to 
the viewing software to perform both gamma decoding and gamma 
pre-correction for display. And for HDR and OpenEXR file formats, 
File_Gamma has no effect whatsoever because those file formats always 
use linear encoding.

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