Texture Exponent Questions!

[zeitmeister]

Hi there,

I'd like to know which role exactly the texture exponent plays.
I am using a Mac with 10.6, plus C4D R11.5. In this case, my monitor has a 2.2 gamma, while C4D uses 1.8 by default.
Which gamma does Indigo on the mac use, and which texture exponent do I have to choose? 2.2 or 1.8?
And why do some materials have only a exponent of 1 in bump or exponent slots, and some not?
I suppose that every texture in every slot has to have the same texture exponent by default, don't they?
And if not, why?

Could someone please explain this to me?
Thank you very much!

[galinette]

Gamma is used to convert light intensities to integer sRGB values, because if you do this non linearly (ie with a gamma > 1) you improve the dynamic range while limiting the stepping effect in dark gradients. This is why a light intensity of 50% (relative) in a picture is not encoded to 128 (=256 * 0.5) but to 187 (=256 * 0.5^(1/2.2)) with a gamma of 2.2

I believe the indigo texture exponent is used to back-convert the sRGB texture values to physical coefficient (ie light intensity diffusion/transmission/reflection... coefficients)

If true, the value to be set has nothing to do with your monitor or output settings, but only with how was encoded the texture image. Most scanners and picture cameras encode their images with 2.2 gamma, I believe. If you paint a texture yourself on your monitor, then use your display gamma.

For the bump maps, you want to convert the sRGB values to a physical height (in meters). In that case, there is not much reason not to do it linearly (ie gamma=1).



Etienne

[zeitmeister]

Hm.
And if I use textures I bought or downloaded? Where can I see in Photoshop which gamma is embedded?
Or, if I resave them, is automatically a 2.2 gamma is being embedded?
I am a little bit confused.

[galinette]

I just edited my previous post so that it's more clear...

That's an issue when downloading or purchasing textures, as gamma is usually not mentionned (but if you purchase pro-grade textures, they should be able to provide the gamma to you, otherwise they are non-pro).
Resaving will not change your gamma in most cases : tools like photoshop do not work in physical units but directly in the colorspace (unless you explicitely do some conversions)

If you are very lucky, the gamma can be recorded in the EXIF data. But I doubt for non-camera pictures.

However, this is not a big issue for textures I think. In most cases, if your texmap looks nice on a computer monitor, then 2.2 should be usable. I do not think you look for colorproof matching when using textures.

Gamma is much more relevant when you want to make picture publication, video, professionnal prints etc...

Etienne

[zeitmeister]

Alright...
so, exponent maps also have to be set to 2.2. Right?

[galinette]

Yep, in 99.99% of the cases I would say.

[fused]

Hey galinette,

thanks for your explanations!

@zeitmeister:

I will change the default gamma value for the exponent map.

[CTZn]

Thanks +1 Galinette, I will not dismiss this step to balance textures (rgb) gamma in the future, your explanations saved me testings I had planned.

- you underlined that textures from different sources might use a different gamma (capture devices discrepancies)
- and that the rgb exponent has a great impact on the final look (dark corners)

Those were the points I wanted to stress.

Thanks zeitmeister, also !