# Tag Info

23

There are multiple areas in path tracing that can be importance sampled. In addition, each of those areas can also use Multiple Importance Sampling, first proposed in Veach and Guibas's 1995 paper. To better explain, let's look at a backwards path tracer: void RenderPixel(uint x, uint y, UniformSampler *sampler) { Ray ray = m_scene->Camera->...

16

I'm sadly not able to add a comment to the answer above (not enough reputation), so I will do it like this. I'd like to point out that what Dragonseel describes is simply an integral equation (specifically a Fredholm equation of the second kind). There are many such equations which do have an analytic solution; even some forms of the rendering equation have ...

13

The rendering equation is as follows: Now, the integral is over the sphere around the point $x$. You integrate over some attenuated light, incoming from every direction. But how much light comes in? This is the light $L(x',\omega_i)$ that some other point $x'$ reflects in the direction $\omega_i$ of point $x$. Now you have to calculate how much light that ...

9

As mentioned in the comments, I would highly suggest starting with Full Volumetric Scattering. This is two fold: Since you are doing path tracing, adding volumetrics isn't super difficult. Fully understanding how full volumetric scattering works will be a great basis for understanding the estimations. In addition, it can provide great "references" to see if ...

8

The box.obj file has no vertex normals, and by default Mitsuba will generate smooth normals for OBJ files that don't specify their own normals. This creates the magnification effect: the box with smooth normals forms a convex lens! By adding this line to the box object in the scene file: <boolean name="faceNormals" value="true"/> I got results that ...

7

Your first quote is referring to "Split-sum approximation" presented in "Real Shading in Unreal Engine 4" by Brian Karis, and also referred in the paper [Kar13]: \frac{1}{N}\sum_{k=1}^N \frac{L_i(l_k)f(l_k,v)cos\theta_{l_k}}{p(l_k,v)}\approx \bigg(\frac{1}{N}\sum_{k=1}^N L_i(l_k)\bigg)\bigg(\frac{1}{N}\sum_{k=1}^N \frac{f(l_k,v)cos\theta_{l_k}}{p(l_k,v)}\...

6

Overview Here is a short overview of the most used space representations, MLT variants and mutation strategies for these MLT variants. As you can see, there are quite some papers dating back to 2017 (e.g., three papers explore combining the Path Space and the Primary Sample Space by jumping back and forth between the two). Path Space (PS) representation, ...

6

The problem lies mainly in CIE1931XYZ::tristimulusValues() function, where you normalize the resulting color to the luminance of your illuminant which causes that directly observed light source has luminance 1, but everything else is much darker. That is a nice thing to do if you just want to visualize colours of various reflectance spectra under a given ...

6

The problem appears to be unintentionally transparent surfaces Although the image is grainy, it is sufficiently clear to estimate that all of the darker regions are due to surfaces facing away from the light, rather than due to shadows cast onto surfaces facing towards the light. So it does seem that there is a problem, and the lack of shadows is not just ...

6

The rendering equation aims to describe what the light distribution for a specific scene is, under several assumptions. The most important assumption is that we are working in a geometrical optics framework - so we do not consider the wave properties of light - meaning no diffraction for example, we also do not consider quantum effects, such as ...

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

is it correct having such a big light? I don't see any problem with having a big area light. That said, it also depends on the scale of your scene. If the light is large compared to it, shadows will tend to be more diffuse, like under an overcast sky. shadows are missing.. [...] Can you help me understanding why? I haven't found any blatant mistake, but ...

5

This bachelor thesis briefly reviews six SSAO techniques. It could be a good start. CryEngine 2 AO StarCraft II AO HBAO Volumetric Obscurance Alchemy AO Unreal Engine 4 AO http://frederikaalund.com/a-comparative-study-of-screen-space-ambient-occlusion-methods/

5

If you can make your samples deterministic (the same from run to run - don't base random sample points on time or any other non deterministic value), it will be a stable rendering that you get every time.

5

I'm not sure what exactly you mean by how to visualize a plane that use w value as one of this coordinates but here's a sketch that will, hopefully, clarify this sentence: a line through each edge of the input triangle as a plane in homogeneous (x c , y c , w c )-space I've used uppercase letters for clip-space, and lowercase letters for window ...

4

A note first From the look of your screen capture, I suspect there might still be a bug in your code. Noise is to be expected with only 16 spp, but your picture still looks surprisingly dark to me. For comparison, here is what my implementation of SmallPT looks like with 16 spp, 15 bounces, and no next event prediction: (here it is on ShaderToy) Noisy, but ...

4

The classic method is to uniformly sample the disc at the base of your hemisphere and to project your samples upwards on the hemisphere (eg. compute z from x and y). This yields a cosine weighted distribution. As the projection preserves stratification, you need only use stratified sampling of the disc to get a stratified cosine distribution.

4

I don't have that book to check the context of this, but from the equations you posted, yes, it looks like you're right. The $1/N$ factor should be applied to both terms. That agrees with the formula for variance from statistics, which is $E[X^2] - E[X]^2$.

4

For cases where the diffusion approximation is preferred over full volumetric path tracing, the method published by Solid Angle is fairly efficient: https://www.solidangle.com/research/s2013_bssrdf_slides.pdf It is implemented in the Arnold render engine, Blender's Cycles, and pbrt, the latter being open source. The file which implements it in PBRT is here:...

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

Illumination components If our scenes only contain point lights (e.g., omni lights, spotlights, etc.) and emissive surfaces, the illumination contributions at a surface position, $x$, are computed as follows: The self emission associated with the emissive surface (i.e. 0 bounces/surface interactions) is computed as usual without using the scene's ...

3

It's the normalization factor to make sure the BRDF always reflects the same amount of energy regardless of the value of the specular exponent n. Without that factor, changing the specular exponent changes the overall reflectance of the material. Further reading: The Blinn-Phong Normalization Zoo | The Tenth Planet Blog

3

I noticed three potential problems. First, this bit of code looks suspicious: Vector r = new Vector(PRNG.nextDouble(), PRNG.nextDouble(), PRNG.nextDouble()).normalize(); Vector v = r.cross(w); Vector u = v.cross(w); Vector wi = u.scale(sample.x).add(v.scale(sample.y)).add(w.scale(sample.z)); The vectors v and u are not normalized, so the construction of wi ...

3

Radiosity is a way to calculate diffuse GI, i.e. every surface is assumed to be Lambertian surface without specular component. In the radiosity algorithm you split surfaces into small patches and calculate "form factor" between two patches, which defines how much energy is transferred from one patch to the other. The form factor between patches is ...

3

The surface reflectance of a BRDF function doesn't depend on orientation or position of the surface in the world space (except for the view vector) but is defined relatively to the surface normal (or tangent space in case of anisotropic BRDF) so it should be defined in local space. Think of it as a hemispherical 2D function fixed to a point on the surface of ...

3

In this vein a bit, Killzone renders one bounce reflections, but uses temporal anti aliasing to combine the last frame with the current frame. In practice, this means you get MANY bounce reflections, since in effect each frame rendered catches the previous number of reflections as the new reflection - adding one each frame. Not quite GI, but i wouldn't be ...

3

This can be done in two ways - either use a sky with a sun in it as an cubemap-type light source outside your building, or use the windows as light sources. With BDPT, you could sort of do both, and consider the windows area light sources where the light differs depending upon the angle, using a sun and sky image/model, and then you could implement ...

3

I can find two possible reasons for the image not converging. #1. Every sample is the same For every sample, you generate random rays. You do that when you shoot the ray through a pixel (for anti-aliasing and DoF) and when you sample the hemisphere (for a new indirect bounce). The problem would be if for every sample, it would generate the same direction, ...

2

Yes, computation of global illumination is usually done in a stochastic way and as such it relies on randomly generated numbers. Typically pseudo random generators are employed, which generate deterministic sequences, and usually have so called "seed" parameter. The seed basically says where to start in the sequence, which in turn affects the noise pattern ...

2

I will not answer your questions directly, but will try to change your mindset a bit. I hope got your intentions reasonably well... The usual way the path tracing is implemented is that the reflection integral is split into direct and indirect illumination and they are estimated separately. The direct illumination can be estimated using more sampling ...

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