What does Absorption color lightness mean?

[vicnaum]

Absorption has two parameters - color and thickness distance.

Physical absorption has a graph for all spectrum distance (usually displayed in cm^-1).
Example:


So there are different wavelengths = different hues & maybe "saturation" (closer to white = all wavelengths).

But there is no value, lightness, brightness on these graphs!

Because we set the "value" by Distance.

So why do we need another value parameter in color?

So what I mean - absorption should react to Hue & Saturation, but should ignore the Lightness.

Because now I don't know what should I set the Lightness to - it reacts similar to saturation, but when it's dark - the material is getting black faster (which can be done by decreasing the distance).

Why should we have another meaningless parameter? Maybe we should simplify this?

I don't ask for a CoronaSpectral material (althought that would be very nice to have!) - but let's at least follow common logic.

[maru]

I think it works the same as any other level and color parameter in Corona. It is level*color. For example setting diffuse level to 1.0 and color to 50% white is the same as setting level to 0.5 and color to 100% white. Same with absorption. Setting distance to 1cm and color with 50% lightness should be the same as setting distance to 0,5cm and color lightness to 100%.  You can just ignore color lightness and use distance only.

At least that's how I think it works. Correct me if I'm wrong!

[borisquezadaa]

I think that is tottaly necesary because of the simplification made to the "ligth emitters" in Corona.
Otherwise there could be no way of cheating the absortion color.
By the way of that graph only the visible spectrum sector you can use in Corona. It brings to  me an interesting experiment.

[Ondra]

Absorption has 2 parameters so it can be easier to set up - you set up COLOR that white ray will have after travelling specified DISTANCE in the medium. Otherwise you would have to deal with highly non-linear numbers - e.g. use something as 0.0000001 0.0000001 0.0000002 for air and 0.2 0.1 0.05 for glass.

value, lightness, brightness, etc. are just terms that make describing a color easier. Everything is transformed back to RGB first. You dont need to use these terms, and use R G B directly instead.

[borisquezadaa]

BEHOLD!!!---- AND BE AMAZED!!!---

The whole new Spectral absortion Corona Material on its "unoptimized" glory!!!!... from the same creators of "Corona can render Dispersion".

Thank you.... thank you very much...

[vicnaum]

Absorption has 2 parameters so it can be easier to set up - you set up COLOR that white ray will have after travelling specified DISTANCE in the medium. Otherwise you would have to deal with highly non-linear numbers - e.g. use something as 0.0000001 0.0000001 0.0000002 for air and 0.2 0.1 0.05 for glass.

value, lightness, brightness, etc. are just terms that make describing a color easier. Everything is transformed back to RGB first. You dont need to use these terms, and use R G B directly instead.

Distance is not ^-1 in Corona, so that's not a problem to input a 50000m for air, or 1mm for very toned glass.

I still see no meaning in value. It resembles something more like "saturation" to me, and doesn't have nothing similar with distance:


So that's like another "tricky" parameter, which no one knows what it does.

And RGB/HSV/LAB/whatever doesn't matter - in any system we are free to control the color's hue, it's saturation, and brightness -> but the brightness doesn't make sense to me in terms of absorption.

[juang3d]

I don't understand what you say, what is the parameter you don't understand?

I mean, both are pretty clear IMO, the color is the color you want at the end of the distance, wich you set with distance.

Obviously if you have a 3cm diameter sphere and you set a 2cm value the sphere will be pretty transparent, while if you set 0.1cm you will get a pretty obscure sphere, I'll try to illustrate this, but I think your picture illustrate it pretty well.

What is the parameter you don't understand?

Cheers.

[vicnaum]

Color is:
1) HUE - which is the tone of the color
2) SATURATION - which is the closer to gray, or closer to full hue saturation
3) VALUE - which is how dark is the color

Absorption doesn't need the VALUE parameter, because DISTANCE already controls the darkness.

You can see from my screenshot - there are 6 different VALUES, with 5 different DISTANCES. There are TONS of variations, and without even touching the hue & saturation yet... The HUE and SATURATION are the same everywhere, but if you look at different VALUES vs DISTANCES you'll see that the amount of "dullness" vs "redness" is changing when the VALUE is changed, or something like that...

[Ondra]

I already explained why the controls are redundant - but you are free to use only a subset if you think it will help you ;)

[vicnaum]

Yeah, but what VALUE is the correct one?

[Ondra]

Just use this:

Absorption has 2 parameters so it can be easier to set up - you set up COLOR that white ray will have after travelling specified DISTANCE in the medium.

[juang3d]

So here is my study about absorption.

That is how I understand how absorption works in Corona, if something is wrong, please correct me.

The curious thing is the loss of transparency, but I bet this is due to the loss of energy that the light ray suffer while traveling inside the volume, so if you change the color from red to white you will get a perfectly transparent object because the white light ray does not change.
Now the different in saturation is due to the same fact, the different energy of the ray while traveling inside the volume.

I hope this helps a bit :)

Cheers!

[Ondra]

So here is my study about absorption.

That is how I understand how absorption works in Corona, if something is wrong, please correct me.

The curious thing is the loss of transparency, but I bet this is due to the loss of energy that the light ray suffer while traveling inside the volume, so if you change the color from red to white you will get a perfectly transparent object because the white light ray does not change.
Now the different in saturation is due to the same fact, the different energy of the ray while traveling inside the volume.

I hope this helps a bit :)

Cheers!

yes, that is correct

[borisquezadaa]

Excelent explanation. That image should go to a material Wiki of some sort.

[vicnaum]

Ah, that's a surprise!

Hm... It looks that we can't get a white fog then?


Because white absorption is completely transparent, and a bit grayer is just getting black.

UPD: Ah, sorry. Forgot about Scattering. Well, yes, if you set it to WHITE, then you can get a white fog:


That adds a fifth unknown to the equation.

I assume the correct whiteness is the default 128, cause there's absolutely no point in setting it to 255 (lack of absorption).