Has anyone had any problems lighting objects inside a transparent object with HDRI as the only light source? I'm getting some weird results with different render settings (the sphere uses Carrara's default glass with the refraction value knocked down to 12). Light through transparency is enabled on all renders. I thought maybe it was my glass shader causing some refractive weirdness, but as a test (bottome left) i tried setting the glass shader to 100% transparent, no reflection, no refraction, and the result is definitely not what one would expect... Anybody else run into this, or am I just missing something in my render settings?

I tried this and got pretty much the same results. As you suggest, it seems to be a problem with refraction. If you use ambient light with the HDRI it gets rid of the problem. HDRI does seem to light non-reflective surfaces oddly sometimes. If you stick a mirror shader on your spline object it will render fine. HTH Mark

Unfortunately, adding ambient lighting negates the whole point of using HDRI. I'm not sure, but I think the deal with the reflective objects working inside a transparent object is the reflective object isn't actually recieveing any light from the HDRI, it's just reflecting the HDRI. If you put 2 identical objects in a scene, shade one chrome and one plastic, set HDRI as your only light source, and turn Sky Light off in render options, the chrome object will look look bright and shiny (odd), and the plastic one will be left in the dark (as expected). Turn Sky Light on, the chrome object looks just like it did in the previous render, and now the plastic one will be illuminated (as expected). Again, I could be totally off base here. If any of you guys that understand the underlying physics of this stuff have any ideas as to what might be going on, would appreciate the input. P.S. I also tried turning off "cast shadows" on the tranparent object, made no difference in the final renders.

Indeed, turning on ambient light is a rather desperate work around. But the problem is not so much with HDRI as how the sky light deals with light through transparency. You will find that if you use a map/colour/gradient instead of the HDRI you get the same result. It seems that it is definitely a bug. Mark

I was able to get around the problem by putting my color information in the glow channel instead. This is risky, since with indirect lighting on, the surface becomes an additional light source. My object was white, and I was able to get decent results with my glow value far lower than pure white. Don't have enough data to offer anything like a repeatable algorithm, but it's a place you mght start from. But you are right: hdri illumination doesn't understand transparency. I'd call that a bug.