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Hi there

I am puzzled by the fact that it DOES matter what luminance the color chips of refl 0 and refl 90 have (hue and saturation staying the same). The manual says the following:

Force Fresnel ensures that the reflectance ramp (curve) between Ref0 and Ref90 will be maintained by the given Nd. It discards the luminance of a given reflectance color and enforces physically correct Fresnel while only deriving the hues of the refl. 0° and refl. 90° color chips.

Can anyone please explain this to me?

Thanks
Rogurt

it is super truth with roughness = 0 I guess because I have no 2.x maxwell only 1.7.1 still

more roughness becomes higer then Nd. have smaller contribution in material apperance.

That part of the manual needs a lot clearer explanation I agree.

First of all, Force Fresnel or not, it is important to understand as zdeno says that the ND (index of refraction) of a material is enough to describe the reflectance of a material only if the roughness is low. As the roughness increases the reflectance from ND alone can't be calculated anymore and the reflectance 0,90 values matter more and more.

So, with low roughness, all you really need is the ND to specify how much light a material reflects back at different viewing angles. BUT, the reflectance 0,90 brightness of a color ALSO specify how much light a material can reflect back. So now there is a conflict - on one hand the ND is specifying a certain reflectance curve, and the 0,90 color brightness (or Value, if you work in Hue Saturation Value mode in the color picker) also specify a certain reflectance curve.

To solve this conflict, Force Fresnel is there to tell Maxwell - evaluate how reflective this surface is based on ND alone, and ignore the Value of the reflectance 0,90 colors, and use only the Hue. This will only work predictably though when roughness is low. As roughness increases, the reflectance values matter more and more because as mentioned before, the reflectance based on ND alone can't be evaluated properly for high roughness surfaces. For example set the surface to 99 roughness, changing the ND will have close to zero influence on the material, force fresnel on or off, doesn't matter.

Take a look at the glass wizard for example, it leaves the reflectance colors at black and has force fresnel on. Since the roughness is 0, the reflectance curve is derived purely from the ND.

So force fresnel is useful for low roughness materials (from 0 to about 15-20 I guess) to make sure any reflectance values you may have set do not interfere with the ND.

been away for a while. anyway - thanks for the explanation...
So this means that at very high roughness values the refl 90 and the ND above 0 get obsolete? ND below 0 really IS visible by the way.

Below 0? Well the minimum ND should really be 1, it means no reflectance/no refraction - it's vacuum, on which all other indices of refraction are based*. Going below 1 goes into the realm of Meta materials so no guarantees what happens then

* this is valid for all materials when K is not also used. For some metals like gold the ND is actually lower than 1 but then you also have to use the K value to determine the reflectance, so you still get a 'normal' reflective material.

right. 1 not 0. my bad...
still I am not yet comfortable with the nd/luminance thing in mxed (let alone k value).