# Questions tagged [physically-based]

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I am studying the BRDF equation. From Wikipedia, the equation is as follows: I have one main question: Where does the $\cos{\theta_i}$ in the denominator come from? The denominator implies that $... 0 votes 0 answers 17 views ### Narrow-phase CCD in FEM topology query (finite element method) I am implementing IPC from siggraph2020, and one of its step is about CCD line search, where CCD is applied each iteration in the descent method in a part of the algorithm. I have implemented CCD ... 1 vote 0 answers 54 views ### Term confusion about volume scattering We have two different terms, for example, let's talk about scattering only. Scattering coefficient Scattering cross-section These two terms are supposed to have different physical meaning. I read it ... • 685 1 vote 1 answer 151 views ### Rendering equation integration variable change I'm watching the TU Wien rendering lectures in the course we were presented with a rendering equation (first one on the screenshot) and then another, which results from a change of variables (if I ... 1 vote 0 answers 86 views ### Difficulty in understanding the interpretation of propagation of an exitant function from the book Advanced Global Illumination (2nd ed) The book defines operator$\mathcal{T}$for the propagation of an exitant function as: if I'm assuming that the position of the point$x$is on the top-left surface, and that$\Psi$is considered as ... • 11 1 vote 1 answer 70 views ### How is the distribution of normals constructed from the distribution of slopes in 'Understanding the masking-shadowing function' paper? Recently I'm reading Eric Heitz's paper 'Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs', in section 5, 5.2, the paper defines the distribution of slopes and then says the ... 1 vote 1 answer 64 views ### Is the dichromatic model physically based? lately I have been learning about photometry, and everywhere I look I see a BRDF written according to the "dichromatic model". Roughly speaking: $$\rho= \rho_D+\rho_S \cdot (...)$$ for some$...
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I have spent the longest amount of time just taking the terminology for granted & have only just recently started studying or at least trying to understand the difference between radiometric and ...
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### Cosine in Rendering Equation and BRDF cancel out: Why cosine at all?

I am somewhat confused with the following issue reading about the rendering equation and BRDFs. That is, the BRDF is usually defined as surface radiance $Lr(ω_r)$ over irradiance $L_i(ω_i)cosθ_i$ for ...
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### How is the beam transmittance calculated in PBRT V3?

In pbrt v3, the book gives this description of beam transmittance, but I don't know how to solve the differential equation like it says to get Tr , can someone please tell me how to solve the ...
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### What's The Math Behind This Cloth Animation?

I progammed a simple cloth animation. Initially I tried to implement Hooke's law, but it wasn't numerically stable, presumably because I used explicit Euler. Anyway, I came up with something (WebGL2 ...
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### Rendering Hypercentric Perspective

I came across some Non-Euclidean Games which involve Hyperbolic Spaces, Spherical Spaces, Portals, etc. And, I noticed that they give quite deep feel of what Non-Euclidean Spaces feel like. So, I was ...
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### Motivation behind tesselation

This question is NOT about the hardware difficulties and shortcomings of tesselation shaders I am trying to understand the motivation behind tesselation shaders. Why/when/where tesselation is a good ...
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### PBR pipeline with multiple HDR images?

I believe I already know the answer to this question, but I want to make sure I'm understanding this correctly. Within a physically based rendering pipeline, you calculate your indirect lighting from ...
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### State of the art in real-time global illumination algorithms [closed]

What is the state of the art in global illumination algorithms in real time applications? Which algorithms or categories of algorithms are actually used in the wild and what are their characteristics (...
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### How to change a RGB value using a radiance value?

I am new to this awesome community, I am currently writing a ray tracer but I hit a problem... Let's say I have a surface(material doesn't matter) of colour [1, 0, 0] in RGB, and the radiance reaching ...
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### Ray casting and rasterization 101

Recently I've became an intern in company-name and my internship task is(for now) create rasterization using rays. I'm mostly interested in pure math, so basic ...
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### How to determine RGB values given a display spectral response curve?

I am reading Physically Based Rendering section 2.2.2 on RGB color. I am trying to understand how, given a display spectral response curves for rgb intensities, we can choose the intensities so that ...
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### Why is eye-based ray tracing preferred over light-based ray tracing?

It seems that virtually all path tracers use eye-based or view-based path tracing. That is, the light rays originate from the camera rather than the light source. The reason given for this everywhere ...
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1 vote
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When ray tracing, I find it intuitively clear that deeper paths have a lower contribution to the overal picture than shorter paths. This is the main reason why it is generally okay to render a picture ...
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### How is the image reconstruction filter on the path space defined in terms of the filter on the raster space?

Say we have a image reconstruction filter $h:R\to\mathbb R$ defined on the raster space $[0,w]\times[0,h]$, where $w,h\in\mathbb N$ define the width and height of the image plane in pixels, ...
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### Relation between camera sensor responsivity and image reconstruction filter associated with a pixel

Let $W_{\text e}$ denote the camera sensor responsivity (i.e. importance) and $h_j$ and $W_{\text e}^{(j)}$ denote the image reconstruction filter and sensor response associated with the $j$th pixel, ...
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### Properties of the image reconstruction filter in rendering

Let $f$ denote the measurement contribution function for paths of length $k\in\mathbb N$, i.e. $$f(x)=g(x_0\leftrightarrow x_1)W_{\text e}(x_1\to x_0)t_k(x_0,\ldots,x_k)L_{\text e}(x_k\to x_{k-1}),$$ ...
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### Is casting a shadow ray non-symmetric in pbrt?

According to the description in the book, Scene::IntersectP() "checks for the existence of intersections along the [passed] ray". This operatorion should be ...
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### PBR - How to handle multiple BSDFs and material type

So I am in the midst of writing my own path traced renderer and right now I have only implemented the old Blinn-Phong models where we separately calculated the diffuse and specular components of the ...
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### Interpreting Cornell Box data for path tracer

So I want to path trace a Cornel Box scene, but I don't understand the values given here in section 2 and 3. Cornell Box Data | Cornell University Program of Computer Graphics No.2 Lists surface ...
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### How to validate whether a reflection model is implemented correctly?

I think this is one of the most important part when you write a physically-based renderer. The solution I came up with is to put a sphere to the scene with a constant background radiance. Solve the ...
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### What is the physical interpretation of Normal, Roughness, Albedo and Metalness map?

In PBS context what is the metalness map? Are my interpretation of Normal, Roughness and Albedo map correct? I know almost for sure that: Normal map : Models the normals per differential region in ...
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### Ray tracing: transforming ray to object's local space for intersection test

Im writting a raytracer, for which I want to be able to test intersections with implicit surfaces (in my case spheres). Without applying any transformation it works great, but I would like to upgrade ...
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### kinematics and dynamics of a sphere in a spring mass system?

I am attempting to code a simple spring mass system simulation in which a sphere pendulus hangs from a string (a spring) by the top and then another sphere (controlled by the user) can move and ...
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