Shadow Catcher Artifacts - Mesh Lights with improper shadows Tweet

[tcheng00]

Hi Guys,

I've been testing out a goemetry plane using Universal material and a Emission color value. The light that comes from it does not cast shadows correctly onto a Shadow Catcher. Please take a look.

With no Mesh Lights, the shadow catcher looks proper from the HDRI.

[frankmci]

If I understand you correctly, it's behaving as intended. You don't see any shadows, because there are no shadow rays cast in those areas. Shadow catchers catch shadows, they don't catch overall illumination. Try enabling, "double sided" in the emission node. It will still give you some artifacts if you are using a single plane as an emitter, though, since it's infinitely thin and does not emit sideways. To eliminate that artifact, use geometry with some significant thickness.

[tcheng00]

Your example is correct behavior, but it doesn't work for my HDRI backdrop and shadow catcher combination (I tried mulitple HDRIs too). Light should be an additive effect, and shouldn't make shadows even darker compare to the image lit only with the HDRI. If you change the shadow catcher to a diffuse surface for comparison, it shows the shadows correctly (no hard edge artifact that is darker than what was cast by the HDRI alone). I tried double-sided, it just makes the ground too dark overall, which shouldn't be the case. This doesn't happen with a normal octane light plane, it only happens with mesh emission lights and shadow catcher combination. I tried increasing the samples, but the artifact still remains.