question

Jun Kim avatar image
Jun Kim asked ·

Inconsistant shadow falloff in shadow_mask aov

Hi!


This is something I noticed when I was testing aiShadowMatte shader.

As I increase the areaLight sample, the fall off area of ShadowMask changes.

Is this a bug or is this something to be expected of?


Thanks

Jun


[Initial setup: sphere, plane and areaLight]


[aiShadowMatte applied to the plane]

[The areaLight settings ]

[As light sample gets increased, the shadow fall off changes]

lightsamplingaishadowmatte
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Alan King avatar image
Alan King answered ·

Hello Jun,

The shadow_mask AOV is designed to reveal the presence of any degree of shadowing on direct light samples and returns a white output when this happens. The detection of shadows relies on a sort of random sampling, so shadowing in the penumbra region of area lights can end up taking samples that are either shadowed or unshadowed entirely by chance. This chance increases as the number of light samples and/or light sources increases, so the kind of output you're seeing makes sense.

If this is not what you expected, I think you might be might be more satisfied with the output of the shadow_matte AOV instead of the shadow_mask, since it should be able to consistently capture the degree as well as the color of shadows at different light sample rates. The shadow diff AOV goes further still by giving you the differences in the shadowed and unshadowed output, which better captures the exact brightness and color variation of shadows, although it can be harder to set up in a useful comp.

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