# Tag Info

16

Strictly speaking, dielectrics are not necessarily insulators. For example, salt water is a reasonable conductor but also a dielectric. The term "dielectric" tends to show up in discussion of the Fresnel effect—how reflectance and transmittance vary with angle. Dielectric materials (i.e. nonmetals) are contrasted with metallic materials as they have ...

15

While teapot, spheres and trusty Suzanne are not per se bad test scenes for materials, here are some things to consider. You can apply them to said examples to forge your own opinion. First and foremost, the viewer needs to be able to examine the behavior of the BRDF. Since it is dependent on both the direction of incident light and the direction from ...

9

You can choose whatever you want offcourse. A good material sample ideally has a few properties. It needs to show how the light falls of as the angle between light and surface normal changes. A sphere is pretty good for this. It needs to show how any possible environment, maps, reflects, refraction etc. A sphere is pretty good for this. It may need to ...

7

Wenzel Jakob et al presented a framework for layered Materials at SIGGRAPH 2014. Section 6.2 explains importance sampling. If you prefer code over equations, the method is implemented in the Mitsuba renderer.

7

Assuming that you are familiar with the concept of BSDFs, the usual way of modelling rough dielectric surfaces (i.e. glass, water, plastics) is to use microfacet-based models like Microfacet Models for Refraction through Rough Surfaces. To make it work efficiently in a path tracer you will need a good sampling strategy, like Importance Sampling Microfacet-...

6

The charts you show aren't showing two different phenomena - "glossy reflection" and "specular reflection" - they're showing two parameters of specular reflection. One is the specularity or specular colour and gives the amount or brightness of the specular reflection. The other is the glossiness or roughness and shows how sharp the specular reflection is. ...

5

You can use single sided triangles for the ceiling so that they are pointing towards the room. This way the ceiling influences the GI in the room but you can see through it when observing from outside

5

Diffuse colours on materials typically come from within the material, while the specular colour is from the very surface. Coloured plastic materials are made by embedding particles of dye inside a colourless medium, so the diffuse colour is the colour of the dye, while the specular colour is white from the colourless surface. With metals, all of the ...

4

For explicit light sampling: yep, you just evaluate the BRDF for that incoming direction and the output direction back toward the camera. There's probability involved in the case of an area light: you have to randomly choose a point on the light source, and include a factor to convert from probability density over light source area to a density over solid ...

4

BRDFs are not accurate for any dielectric surface, as all of them have some degree of subsurface scattering. A BSSRDF is also just an approximation based on the assumption of a semi-infinite slab. Ideally, you'd run a volumetric path trace for all surfaces, which would be much more accurate than BRDFs or BSSRDFs, but significantly more expensive. Although, ...

4

A really common case in the real world are industrially fabricated metal surfaces which often contain patterns (due to their creation process) with strongly anisotropic surface distributions, especially if they haven't been polished. Some of these patterns may only occur at the macro scale from common viewpoints (i.e. irrelevant for your BRDF), but on a lot ...

4

Like @lightxbulb said, a light source (i.e. an emissive surface whose L_e term in the rendering equation is greater than zero) can also have a BRDF. People usually do not model light sources really accurately down to the Tungsten filament geometry inside of a glass-enclosed gas chamber for incandescent light bulbs. Such light sources usually are modelled as ...

3

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 to a game engine might be to look at Unity's HDRP implementation. In the Lit.hlsl to see how to incorporate the iridescence Fresnel into a specular + diffuse ...

3

Most manufactured items that are made by grinder, milling, shaper or lathe that have not been polished. Due to tiny grooves caused by the cutting head. Intentionally brushed surfaces. Mainly to make it harder to see scratching. Used for example on metal panels of elevators, escalators etc. Hair, animal or human. Hair has a microstructure much like a surface ...

3

When implementing transmission into a path tracer you have to keep in mind that neither BRDF's nor your previous sampling method are gonna cut it. You will need to think about what happens to light on both sides of your surface, which is where BTDF's and, more generally, BSDF's come in (the T and the S stand for Transmittance and Scattering, respectively). ...

3

Both of your two proposed solutions are valid, with different properties. Which you choose is purely a design issue. The difference is that your probabilistic rays/particles (1) don't need to store their energy: they all have the same amount of energy, so you can just count particles to sum the energy. Your partial particles (2) need to store an energy so ...

2

It would be too expensive (both performance and memory wise) to evaluate BRDF first for each material layer and then blend the layers. Thus the material layer parameters & textures are rather first blended together and then written to the G-buffer for a single BRDF evaluation. This is feasible as long as all the layers use the same BRDF. This doesn't ...

2

AI_MATKEY_COLOR_TRANSPARENT is used to define the transparent colour and AI_MATKEY_REFRACTI is the refractive index.

2

It is not especially related to graphics but physics, and especially, interaction between electromagnetic waves (like light) and matter, i.e. microphysics of optics. Metals have free electrons, and thus it's quite a sea of almost free moving charges that are interacting with the EM field. Ideally, it would be totally reflected. In dielectrics the electrons ...

1

Yes, I think you're right. The values from a typical color picker would be in a gamma encoding or sRGB encoding and would need to be converted to linear to use as reflectance. This is probably just a mistake in the book. (BTW, in case you're not aware, sRGB isn't actually gamma 2.2 but a more complicated transfer function.)

1

According to the source (https://github.com/assimp/assimp/blob/b7df376836e36c4b6998ce95d97626e30f61c2f0/code/AssetLib/Obj/ObjFileImporter.cpp#L594) only illum values of 0, 1, 2 are supported: // convert illumination model int sm = 0; switch (pCurrentMaterial->illumination_model) { case 0: sm = aiShadingMode_NoShading; break;...

1

It turns out that the ring was happening from a negative divided by a negative, so adding nom = max(nom, 0.0); fixed the problem. The new highlight, amplified:

1

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.

1

@PaulHK explained it very well in the comment section: Ambient is a (rough) approximation of indirect lighting. For example, an outdoor object would be lit by the sun but the surfaces facing away from sun would be black if using only lambert shading, we can use dark gray for the ambient colour to mimic the effect of indirect lighting from the ground and ...

1

When applying various types of attenuation you are just multiplying by some number smaller than 1. This is akin to every atom of your material having some probability of absorbing the light that hits it. Something to note is that these calculations should be performed ( in the context of gamma correction and colour spaces ) in a linear space. If you don't ...

1

There is no single method of layering materials. Some implementations, as you have observed, simply interpolate shader input parameters. This helps especially in contexts where code divergence comes with performance penalties (GPUs for example), since all material layers use the same code path and only have different data. Other blend between the output of ...

1

Gloss and Specularity are features of the surface. In modern PBR terms we usually refer to the smoothness and metalness of a surface instead (unless you use a specular workflow, then metalness is still linked to specular). Basically, dielectric surfaces reflect around 4% of light in a specular way, and the rest is diffuse. Metallic surfaces reflect no light ...

1

https://courses.cs.washington.edu/courses/cse458/99sp/projects/help/phong_params.html Material Diff. slider Diff. color Spec. slider Spec. color Shininess Metal low color of metal high color of metal high Plastic medium color of plastic medium ...

Only top voted, non community-wiki answers of a minimum length are eligible