appleseed Users Forum

How to display Light Paths in

I do not know the use of light pathing tab. I render the build in cornell scene, and get its light path

the corresponding light path image is :

what is the meaning of this image and what is the usage of it?
And ,the log says

Are there 27757348 light paths( this is really huge!!) ??

Displaying Light Paths is a powerful tool within to visualize individual rays originating from a selected surface point and for fireflies and stray-light analysis.

A step-by-step instruction from @franz how to use it is here:

In the near future this feature will be extensively beefed up with faster display, selectable ray transparency, color-coded rays according to e.g. number of bounces and more…

Wow yes, this is what I needed, this is really powerful features!

Thank you very much

So as I said, there are 27757348 light paths?!

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Use only 1 render pass and 1 sample/pixel to limit the number of recorded light paths.

Here is a nice demo video:

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Another question. I set the configuration of final render as you said

After finish rendering, I click one point and see the light path, but there are sometims more than 1 light path. It may be 1, 2, 3 light paths.

I think there should always be 1 light path for 1 pixel, right?Why there are more than 1 light path? Is this due to some other settings such as

Usually several rays contribute to the final colour value of a pixel. How many depends on the bounce limits settings and the scene materials and objects. A ray emanating from the camera into the scene will hit an object and then may be transmitted/reflected or absorbed depending on the surface material.
Also so called shadow rays are sent from the diffuse hit-points directly to the scene lights to estimate the lighting contribution (Next Event Estimation).

You can disable Global Illumination by setting the Diffuse Bounces to 0 then only direct lighting will contribute to the pixel colour values (see screenshot) and the Light Paths will show clearly the shadow rays.

Got it. I was confused because I forgot the camera!! There is one light trace from the camera to the hit point, and from the hit point there will be many rays scattered due to the BSDF and so on. So “One sample One pixel” means “one” light trace" from the “camera” to the hit point. Thanks

One comment:

  • The default lighting engine in appleseed is Unidirectional Path Tracer and it behaves exactly as you wrote above, sending rays from the camera into the scene (similar to Cycles or Arnold).

  • We have a second lighting engine Stochastic Progressive Photon Mapping where rays are also sent from the lights into the scene (in the photon passes). This helps rendering complex ray paths such as specular-diffuse-specular ones and difficult lighting situations such as caustics.