This is initial setup. Red plane and green cube is a single object, red plane has flipped normal (directed inside cube). Another plane is half glossy half diffuse material:

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It becomes interesting... Subtracting Backface from Shadow ray blocks shadow casting from red plane but alows shadowing from green cube (dark spot behind):

enter image description here

Then I tried with all other rays and such behaviour didn't occur. Every bounced ray is gone away except shadows:

enter image description here

I'm curious what is going on here, because I didn't expect Shadow ray to differ from others.

  • 1
    $\begingroup$ When you make the backfacing side of the cube transparent, the front and top facing sides still have a diffuse material and will cast a shadow. $\endgroup$
    – Robert Gützkow
    Sep 26, 2019 at 12:43
  • $\begingroup$ You are right, post an answer if you want. $\endgroup$
    – Serge L
    Oct 3, 2019 at 18:36


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