Next steps - lights, can't quite understand the difference between shadow rays and biased rays.

[KnockerPulsar]

Hello, I've been having trouble trying to understand the second option for biased lighting.
The first approach (shadow rays) seems quite common online. But the second one

... it can be done implicitly by making some objects emit light, biasing scattered rays toward them, and then downweighting those rays to cancel out the bias. ...

I can't quite wrap my head around it. Here are the two main interpretations I have:

1. Shadow rays: for each light source, cast another ray for each hit during recursion, aimed straight at the light source. I don't believe this is the correct interpretation (since we just mentioned shadow rays before this quoted part), but it's always the first one to pop into my head when re-reading this quoted part.
2. Something similar to shadow rays, but instead of shooting straight at light sources, we scatter like we've been, but push the rays slightly towards each light source (biasing), then do the down-weighting for the ones that hit. This seems more logical than the first interpretation, but it's also less efficient since we're not guaranteed to aim exactly for each light.

[trevordblack]

Lights are covered in book 2 and biasing is covered in book 3

As far as:

Shadow rays: for each light source, cast another ray for each hit during recursion, aimed straight at the light source.
I don't believe this is the correct interpretation (since we just mentioned shadow rays before this quoted part), but
it's always the first one to pop into my head when re-reading this quoted part.

You pretty much have it correct. At every surface, just before the scatter, you'll shoot a "shadow ray" toward a light. Where typically you'll choose a number of shadow rays 1,2,3,4,8,16, etc and point them at a random point on any of the lights. There are structures that exist to prioritize certain lights over others, but the bottom line is that you are explicitly shooting a ray at a light.

Then, you just check if the ray hits anything other than the light on the way there. If the ray hits the light, that light is used to calculate the luminance of the scatter point, if the shadow ray hits another object before the light, it is ocluded and it is assumed that the light is not illuminating the scatter point.

If you do this many times for each scatter point, you can get a really good, noise-reduced approximation for the luminance in a scene.

After you send out all of your shadow rays, the ray continues on, scattering and continues onto he next step of the recursion.

Shadow rays tend to be desirable because it's a single ray where all you care about is a single intersection, no solving for the scattering direction, no solving for additional bounces, no solving for colors. So you'll typically see most renders use more shadow rays than bounce rays.

Something similar to shadow rays, but instead of shooting straight at light sources,
we scatter like we've been, but push the rays slightly towards each light source (biasing),
then do the down-weighting for the ones that hit. This seems more logical than the first
interpretation, but it's also less efficient since we're not guaranteed to aim exactly for each light.

Not exactly.

As stated above, a shadow ray is actually a simpler kind of ray than exists in the rest of the book.

For biasing, we are literally biasing our rays toward the light.

In the example of book 3 the way that we bias is that when solving for how a ray should scatter off of an object we flip a coin.

If it's heads, we just shoot the ray straight toward the light (and since it's a regular ray, can just be bounced around or intersect at the light)

if it's tails, we just scatter normally.

It'd take a book (book 3 to be exact) to explain why this heads-tails thing introduces inaccuracies into the final render, but since we're unnaturrally making rays more likely to go toward light, the scene will appear waaaayy brighter than it actually is.

[KnockerPulsar]

I now understand, thank you for the clear and quick answer.