14
votes
Accepted
How to properly combine the diffuse and specular terms?
Using two Fresnel terms is correct in the sense that any given diffuse path will pass through the surface twice. If you're solving diffusion by tracing a path through the medium until it bounces out ...
- 330
12
votes
Accepted
What explains the high specularity of metals?
Warning: I am not a physicist.
As Dan Hulme already explained, light can't travel through metals, so dealing with IOR is a lot more... complex. I will answer why that happens and how to calculate the ...
- 546
10
votes
Accepted
In a physically based BRDF, what vector should be used to compute the Fresnel coefficient?
In Schlick's 1994 paper, "An Inexpensive Model for Physically-Based Rendering", where they derive the approximation, the formula is:
$$F_{\lambda}(u) = f_{\lambda} + (1 - f_{\lambda})(1 - u)^...
- 3,732
8
votes
Accepted
Why is the half vector not used in diffuse BRDF Fresnel calculations?
I finally figured out a flaw in my argumentation to use the half vector for the diffuse part.
tl;dr version:
$\alpha_{hi}$ and $\alpha_{ho}$ are not equal, this assumption only works for the specular ...
- 1,541
8
votes
In a physically based BRDF, what vector should be used to compute the Fresnel coefficient?
The Fresnel coefficient should be evaluated using $H$, not $N$.
You wrote,
I have trouble seeing why we can still use that formula in a BRDF, which is supposed to approximate the integral over all ...
- 24.7k
8
votes
How to properly combine the diffuse and specular terms?
While browsing to properly write my question, I actually found the answer, which happens to be very simple.
Another Fresnel term is also going to weight in as the photons make their way out of the ...
- 4,380
6
votes
Accepted
Fresnel and specular colour
Yes, because refractive index can vary with wavelength. This is the origin of colored specular reflection in metals such as gold and copper; most other materials have essentially uncolored specular. ...
- 24.7k
6
votes
What explains the high specularity of metals?
Look at the refractive index of several metals. They are all complex numbers and the math does work out when you put this into the fresnel equation: you get the expected high reflectivity at all ...
- 1,585
6
votes
Accepted
"Artist Friendly Metallic Fresnel" reversible?
I wasn't really expecting that, no. The formula in the paper is not the most elegant - there's quite a few parentheses in there. In this case I think it's just a matter of shuffling the parentheses ...
- 76
6
votes
Accepted
Fresnel equation with incident angle, n and k as input
@PaulHK's answer is correct I'm sure, here's a bit of a check to show that the IOR() function is calculating the reflection coefficients for $s$ and $p$ ...
- 330
4
votes
Fresnel equation with incident angle, n and k as input
This is the complex number version of refraction, were K is the extinction coefficient. This is commonly used for metals.
You can check the Wikipedia on refraction: Complex refractive index | ...
- 2,312
3
votes
Accepted
Does fresnel reflection make sense for metals?
I think this question is a bit tricky to answer, since I have some seemingly contradictory knowledge about your implementation. If I recall correctly, you are using Schlick approximation for the ...
- 575
3
votes
Accepted
Iridescence/Thin-film interference integration as a layer
Unfortunately, the iridescence model is not made to be applied to a diffuse term. Pascal and I made it for microfacet models only (that is the specular term).
One way to understand how to include it ...
2
votes
What explains the high specularity of metals?
The refractive index is related to the speed at which light travels through the medium, and only applies to materials which are at least partially transparent. Metals are electrically conductive, so ...
- 6,700
2
votes
More accurate Fresnel approximation for a RGB-based Path Tracer
This would be a response to why is it possible to use the Fresnel equations in an RGB path tracer. Though evaluating Fresnel equation directly requires some what more information about the environment ...
- 461
2
votes
Equivalent of Fresnel law for determining transmitted/diffuse split
Physically, the origin of diffuse light is subsurface scattering, which happens continuously as light travels through a material. So, the proportion of transmitted light depends on the thickness of ...
- 24.7k
2
votes
Why is the half vector not used in diffuse BRDF Fresnel calculations?
We're actually going through that paper for our own GGX BRDF metallic & edge_tint model, and we've spotted one crucial problem.
Hammon is still using the Fresnel Schlick approximation for his ...
- 21
1
vote
BRDF sampling and evaluation of diffuse vs specular component
So I found a solution on a shader toy.
I choose randomly from sampling diffuse or sampling specular with distribution 50:50. But I calculate PDF of both and average then. If you want to use a ...
- 65
1
vote
Accepted
Correct way to think about Fresnel effect
If you just implement Fresnel alone you'll see that what you think in your original question is mainly correct for dieletric materials.
However don't calculate Fresnel ahead and then stochastically ...
- 292
1
vote
Correct way to think about Fresnel effect
BIG UPDATE and explanation of BRDF model
While I was cleaning my browser and closing about 100 tabs I found this awesome website (it was there 3 days so I could already found an answer...) which ...
- 65
1
vote
Correct way to think about Fresnel effect
I will answer based on my experience with Fresnel models. Due to Kirchhoff's law of thermal radiation, in an opaque material the emissivity plus reflectivity plus absorptivity equals to one for every ...
- 111
1
vote
Accepted
Confused on reflection coeffcient of Fresnel equations
We have $t_{12} = t_{21}$, by the law of reciprocity. Also, $$r_{12} + t_{12} = 1\\r_{21} + t_{21} = 1$$
It follows that $r_{12} = r_{21}$ as well.
- 24.7k
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