The Blender documentation says the following about the Light Path node support in Eevee:
Eevee has no real concept of rays. But in order to ease the workflow between Cycles and Eevee some of the outputs are supported in particular cases. This node makes it possible to tweak indirect lighting in the shader.
It further provides a list in which
Is Diffuse Ray is indicated as "Supported" in Eevee.
What does this mean, exactly? What are some use cases in Eevee in which
Is Diffuse Ray is valuable?
For context, this question was motivated by my noting that I can use
Is Diffuse Ray in Cycles to render an object's mesh as transparent but to retain the effect of light hitting the object's mesh and bouncing onto other objects. For example, I have this simple scene:
If I switch to render preview in Cycles with the material pictured below on the sphere, then the sphere itself doesn't appear in the render, but the light scattered through its surface onto the rectangles does:
If I switch instead to Eevee (note: with
Screenspace Refraction, and
Blend Mode=Alpha Blend set in
Material Properties, and with
Screenspace Reflections enabled in
Render Properties), then in the render preview, the sphere still "disappears"/is fully transparent, but the green light scattered off/through the object's surface and onto the rectangular objects does not appear:
This example left me wondering: this does not appear to be an intended use case for
Is Diffuse Ray in Eevee -- but, if that's so, then what are some examples of its intended uses in Eevee? Or, is this an example of an intended use case, and I am missing a relevant Eevee setting(s)? Mathematical/algorithmic explanations of the difference between Eevee and Cycles behavior in this case would also be appreciated, if available.
Version notes: the above was produced in Blender 2.90.0 alpha, but I have seen similar behavior in 2.83 and 2.90 (non-alpha) as well.