View Full Version : IOR and Alpha

08-15-2016, 03:00 PM
It appears I've run into something VERY unexpected. Please confirm if my expectation is incorrect, I'm doing something wrong, or if a bug exists in LW 2015.3

Load a default sphere into Layout.
Make it 70% transparent.
Add any sort of object (a single poly) behind the sphere, halfway obscured with the remainder extending off screen, say to the left.
F9 Render (all ray trace render options are enabled).
Change previewer mode to alpha and note that the sphere's alpha is a gray circle and the background object is solid white, as expected.

Now, change Refraction Index to 1.4 and enable VPR.
Move background object right or left until you can see it refracting through the sphere if not already doing so.
F9 render and note that the sphere's alpha is solid white! What?

MY expectation is that the sphere's alpha should REMAIN gray AND solid white where the BG object is seen both inside and outside of the sphere. This unexpected result also happens using nodes, too.

08-15-2016, 03:27 PM
My expectation is that the internal reflections that are present in IOR > 1 materials bleed off ray intensity that is otherwise not bled off when IOR is 1, is what is influencing the real and perceived alpha level. There may actually be differences in there, but it will take a sample to reveal them. What happens when IOR is only slightly > 1 ... 1.01 perhaps? Does it go suddenly white or show some gray? Bear in mind that the closer the internal rays get to the "edge" of the sphere, the closer the internal rays get to Total Internal Reflection; past which the alpha will be completely white.

08-16-2016, 08:35 AM
Alpha suddenly goes solid white at even IOR = 1.0001. Ray Recursion = 6. Ray Precision = 6. Ray Cutoff = 0.01

DOH! Figured it out. It was the environment reflection sphere that was filling the sphere's alpha, even though it was set to Unseen by Camera. Setting it to Unseen by Rays or setting Object Dissolve = 100% "fixes" the alpha issue, but then, of course, removes the needed reflections.

Ah HA!! The final fix is to set the reflection sphere's surface Alpha Channel mode to Constant Value = 0 (does not work using Unaffected by Surface). NOW the glass sphere's alpha is correct, including the obscuring BG object's refracted shape. Back to waving, guys. Hope this short topic helps someone else (there were no topics found using the forum search with keyword "IOR").

08-16-2016, 01:27 PM
To clarify, do both Unseen by Rays or setting Object Dissolve = 100% remove the reflections or only the latter?

08-16-2016, 02:28 PM
Either of them do, for obvious reasons (ie. rays blocked from entering camera [Rays Unseen by Camera] or object set to invisible [Object Dissolved 100%], so no rays to enter camera).