Decreasing Arnold Physical Sky resolution causes it to become dark / have erratic lighting

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Ariff Jeff asked · Sep 17 at 03:26 AM

Decreasing Arnold Physical Sky resolution causes it to become dark / have erratic lighting

C4DtoA 2.6.1 [4186e70b]
Arnold core 5.4.0.0

New scene.

Add a floor and a box for reference.

Add an Arnold sky. Set it to Physical sky.

Now turn on the IPR and drag the Physical sky's resolution parameter arrows down so that the value decrements to ~500. Once you have scrolled from 1000 (the default) to 500 the lighting should at some point suddenly change from bright to dark.

This has to be a bug?

Increasing the sun size param acts as a threshold to prevent this weird effect from happening. Turning the physical sky resolution down further shows that the scene reverts from dark to light multiple times at random decrements. The resolution values at which the lighting flips seem to be different for different project scenes.

bug lighting sky

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Answer 1 · 2019-09-17T19:48:39Z

Ariff Jeff answered · Sep 17 at 07:48 PM

I'm not interested in using a skydome light object. Unless I have to? I still don't really understand what is causing the light flickering to occur just by adjusting the Physical Sky object's resolution.

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Thiago Ize · Sep 17 at 09:57 PM

This happens because the resolution is too low to correctly capture the small sun, which results in aliasing and these random looking jumps in brightness. More specifically, we're currently doing a nearest neighbor filter, which means that a pixel will either fully contain a sun value or no sun value (no partial sun is allowed). If the resolution is low enough that the sun covers only a few pixels, then you'll see a large jump in brightness depending on when a pixel flips from containing the sun to not containing the sun.

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Answer 2 · 2019-09-17T06:25:55Z

Lee Griggs answered · Sep 17 at 06:25 AM

From the docs:

"To use it as a light source you must attach it to a skydome_light with sufficient resolution to capture the small solar disk."

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Decreasing Arnold Physical Sky resolution causes it to become dark / have erratic lighting

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