sorry to intervene, but is someone still interested in my findings?

I've found the following, after toggling about all lighting aspects and IDL properties of things.

The issue of a deep shadowed stripe over the couch (cube) that sticks somewhat out of the porch, is caused by the interference of the direct steep incidenting sunlight & shadow at one hand, with the indirect light - mainly from the sun also - bouncing from the ground onto the ceiling downwards onto the couch.

The first: direct sunlight, creates a bright portion on the "frontside" of the couch as far as the couch is in the light, then the brightness drops rather quickly due to the shadow from the ceiling/roof. Orange line in the graph for intensity.

The second is indirect light bouncing especially from the ground towards the ceiling. This turns the ceiling into a light source, with the largest intensity at the frontside, dropping towards the backside of the porch. But also dropping drastically above the couch itself, as the ceiling stops just there.
So when moving from the outside inwards, this portion of the lighting first rises as we get more ceiling above us, and then reduces slowly as things get slightly darker towards the back of the porch. Blue line in the graph for intensity.

Combining these two effects creates a high intensity level first, then a brighness drop as the direct sun-shadow kicks in, then an increase from the IDL light from the ceiling, and a slow drop as things get darker towards the end. Green line in the graph.
In other words: from some point the brighness from the ceiling (IDL) increases faster than the shadow from the sun (direct) decreases.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though