Talisman,

Reducing shadow strength is a hack. It simply let's a certain amount of the light pass through objects as though they are partially transparent. This is better than a totally black shadow, but it's completely inferior to any other technique. It's a legacy from Poser 5 days when we had no way to easily manage some sort of ambient lighting.

With IBL, you can fill the shadows easily, albeit not with the precise and accurate results you can get with IDL.

When IDL fills the shadows, it doesn't do it by simply adding some light the way IBL does. It examines all the other object nearby and let's them contribute secondary light. So all manner of effects including occlusion and color bleeding work correctly. You don't get color bleeding with IBL+AO, and AO provides only an approximation of the effect of objects blocking ambient light. IDL fully takes everything into account, giving by far the most realistic results.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)