### Volumetric Pathtracing / Small UPBP

Posted:

**Wed Dec 14, 2016 6:51 pm**Hello,

I have a question to the PDF from the Distance-Sampling-Function in particpating media.

In the SmallUPBP-Example is a function, that sample the ray-distance:

My question is: If the ray is shorter than the distance to the medium-boundary (Sample inside the medium) than he returns a PDF. If the ray is longer than the max-distance, then he returns a Probabilty.

A PDF is not the same as a Probabilty. So why can he mix this two things?

If I use his explanations

http://www.smallupbp.com/thesis.pdf Page 33

http://www.cs.cornell.edu/courses/cs663 ... olpath.pdf Page 3

In the 'normal' pathtracer I get the Path-Pdf by multiplying all the Sampling-Pdfs (Direction-Sampling / Survace-Area-Sampling). This Path-Pdf is needet for the MC-Estimator. But what happens, if one of this Sampling-Pdfs is not a PDF but a Probability?

Can someone explain me why he can mix this two things, if he want to MonteCarlo-Integrate?

I have a question to the PDF from the Distance-Sampling-Function in particpating media.

In the SmallUPBP-Example is a function, that sample the ray-distance:

Code: Select all

```
virtual float SampleRay(
const Ray &/*aRay*/,
const float aDistToBoundary,
const float aRandom,
float *oPdf,
const uint aRaySamplingFlags = 0,
float *oRevPdf = NULL) const
{
float s = -std::log(aRandom) / mMinPositiveAttenuationCoefComp();
if (s < aDistToBoundary) // sample is before the boundary intersection
{
float att = EvalAttenuationInOneDim(mMinPositiveAttenuationCoefComp(), s);
if (oPdf) *oPdf = mMinPositiveAttenuationCoefComp() * att;
...
}
else // sample is behind the boundary intersection
{
float att = EvalAttenuationInOneDim(mMinPositiveAttenuationCoefComp(), aDistToBoundary);
if (oPdf) *oPdf = att;
...
}
}
float EvalAttenuationInOneDim(
const float aAttenuationCoefComp,
const float aDistanceAlongRay) const
{
return std::exp(-aAttenuationCoefComp* aDistanceAlongRay);
}
```

A PDF is not the same as a Probabilty. So why can he mix this two things?

If I use his explanations

http://www.smallupbp.com/thesis.pdf Page 33

Or a other text to this topic:If the sampled distance

d is lesser than or equal to dmax, the method returns d along with the desired pdf p¯(d) = σt,mT0

r,m(d). If it is longer, dmax is returned with a probability Pr{d>dmax} = 1 − P(dmax) = T0r,m(dmax).

http://www.cs.cornell.edu/courses/cs663 ... olpath.pdf Page 3

He also use this formula but I don't understand why he can mix PDF and Probability.The probability

that s > smax is 1 – P(smax), which is exp(–σt smax): exactly the attenuation that needs to be applied

to the radiance from behind the medium

In the 'normal' pathtracer I get the Path-Pdf by multiplying all the Sampling-Pdfs (Direction-Sampling / Survace-Area-Sampling). This Path-Pdf is needet for the MC-Estimator. But what happens, if one of this Sampling-Pdfs is not a PDF but a Probability?

Can someone explain me why he can mix this two things, if he want to MonteCarlo-Integrate?