Question for Render settings-- "Light Tracing"

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lucongmin
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Question for Render settings-- "Light Tracing"

Post by lucongmin » Mon Feb 24, 2020 12:07 pm

Hello everyone, I want to ask you a question, why is there a setting option for "Light Tracing" when using Path Tracing? How is it different from Bidir Path Tracing?
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Dade
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Re: Question for Render settings-- "Light Tracing"

Post by Dade » Mon Feb 24, 2020 12:17 pm

lucongmin wrote:
Mon Feb 24, 2020 12:07 pm
Hello everyone, I want to ask you a question, why is there a setting option for "Light Tracing" when using Path Tracing? How is it different from Bidir Path Tracing?
BiDir connect every single vertex of the light path with the eye path using MIS (Multiple Importance Sampling). The number of rays traced is O(<path depth> ^ 2): very slow.

Light tracing only connects the end of a light path with the eye and only if it renders caustics. No MIS between light tracing and path tracing, just static partition of the type of paths to render.

Enabling light tracing has a very little cost over normal path tracing but it renders caustics as well as BiDir (if not better) while being a LOOOOT faster.

The development thread about this topic is here: viewtopic.php?f=5&t=1187
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lucongmin
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Re: Question for Render settings-- "Light Tracing"

Post by lucongmin » Mon Feb 24, 2020 12:29 pm

Hi, Dade

It's incredible to get your reply so quickly.

As a render newbie, how should I understand MIS (Multiple Importance Sampling) ?

If "Light Tracing" has high performance compared with Bidir, why we still need Bidir ?

Thanks very much!

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Dade
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Re: Question for Render settings-- "Light Tracing"

Post by Dade » Mon Feb 24, 2020 4:35 pm

lucongmin wrote:
Mon Feb 24, 2020 12:29 pm
As a render newbie, how should I understand MIS (Multiple Importance Sampling) ?
Long story short: you compute the same result in many different ways, than you combine all the obtained results, in a "smart" way, to minimize noise. MIS is good because it is automatic: it works out of the box, without any user input. MIS is really slow because you have to compute multiple results to obtain single final one.
lucongmin wrote:
Mon Feb 24, 2020 12:29 pm
If "Light Tracing" has high performance compared with Bidir, why we still need Bidir ?
As result of having MIS, BiDir is "fire-and-forget" and works with nearly any scene but is very slow. While Path tracing + Light tracing has parameters to tune and may not work with all scenes but is a lot faster.

If you have (nearly) unlimited hardware resources and you seek automatic rendering of any scene: use BiDir.

For all other people (i.e. vast majority of users) there is path+light tracing, GPUs, PhotonGI cache, etc. Something that produced the same results but require a lot of know-how and it is usually 10-100 times faster.
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