Why do we need range 43-240 for diffuse?

[astudio]

Please in simple words if possible.

There is no glossy in my test. IOR=1

[maru]

Umm, sorry, but can you explain what is this?

[astudio]

Left and right corona color wit RGB 0,0,0 and 43,43,43
Middle bitmap with same values

[Jpjapers]

Are you asking why you need anything other than black to dark grey for the diffuse channel?

[astudio]

There is a lot of tutorials that we need to use albedo in this range 43-240. Why?

[Fluss]

Because that's the range observed empirically in real life. Coal is regarded as the darkest (common) material on earth and has a approx measured albedo value of 43. Snow is regarded as the brightest and has an approx measured albedo value of 243 (sRGB). Even if today, we have made some spectacular materials like the well-named Vantablack.

Edit : For the curious : http://petapixel.com/2016/03/07/the-darkest-material-on-earth-is-now-even-darker-than-before/

[astudio]

It's right usually for common materials. But I need now black paint with LRV 6%. So I need RGB 15,15,15. Is it OK in terms of PBR? Because 43,43,43 is just grey near it.

[Fluss]

Be carefull, [43,240] is the sRGB range which translates into [8,230] RGB.

[astudio]

I know. And this things confuse me.
I made bitmap for my test in photoshop (it means sRGB) and load it into diffuse as automatic. (it means gamma 2.2) .
And for another cubes I used CoronaColor instead of diffuse color.
So I really tested sRGB value, isn't it?

My question is: Is it possible that in my case CoronaColor 15,15,15 would be physically correct?

[Fluss]

yes, with "Input values are in linear color space" checkbox checked. Which translates into [70,70,70] sRGB.

[astudio]

No. I input a hex color here, wich I take in photoshop picker.
I see in bottom color rectangle  sRGB 43,43,43.

Is a reason for 43 just color of charcoal or there is any mathematics reason?

[PROH]

@astudio: have you seen this: https://forum.corona-renderer.com/index.php/topic,13833.0.html

[astudio]

Yes, I read. And install Corona picker. So I sure that I use sRGB.

Offtopic. As I understand hex number of color is a function of color model, not color space. When I put hex number, "insert value in RGB" checkbox is not active, and you see real sRGB value in the bottom rectangle.
By same reason for calibrating albedo value I check lightness value in Photoshop in Lab mode. You deal with color model, not color space, so you don't need to think about RGB-sRGB. You free from hue and saturation.

To the thread. Is it ok to use color darker then 43,43,43 sRGB or there are some mathematical restriction? Or may be a range 43-240 is RGB range and not sRGB?

[Fluss]

OK now i see what you try to acheive. You confuse Lightness from Lab color space and LRV (Light reflectance value). LAB is a way to describe colors and LRV is a measure of  light that is reflected from a surface when illuminated by a light source. So 4% LRV does not mean you have to put 4% lightness in LAB color space and get the corresponding sRGB value. That's not the way it works. You can see this link, from a coating manufacturer, where you can find some LRV - RGB correspondence (They say RGB but i assume it is sRGB in reality) :

http://www.duluxpowdercoatings.co.nz/wp/wp-content/uploads/2013/11/Copy-of-RGB-References2.pdf

[astudio]

I don't try to connect LRV and Lab of course. There are two different things, as green and warm.

I use Lab when I need to use known albedo value from reference book. Unfortunately there is not a lot of information :(
For example I know that albedo(sRGB) of red brick is 148. In this case I guess that I need adjust my texture in Lab mode till I get lightness 148. If I work in RGB mode (sRGB of course), all parameters are depending. You may change luminosity in 3 ways by changing hue, saturation and lightness. It always confuse me lightness, brightness, luminosity :).  In Lab mode there is only one way.

I don't know what does it mean  Albedo (sRGB) = 148 and why they give monochrome information. But I guess that green or blue bricks will have same lightness in Lab mode. Correct me if I wrong. In RGB mode lightness depends from color.

EDIT: Tests show that something wrong in this workflow. I don't delete it from here. Show me please my mistake. Have you an idea how to use monochrome albedo (sRGB) Values?

In my particular case I guess that paint is black and if LRV is low - I need low glossy, low IOR material. (IOR is another long story for another thread which I'll open for sure after understanding albedo). But the question is how much black. With low LRV I can imagine that it is darker then charcoal. So my question is if we have some restrictions to use RGB less then 43?