I am posting this here because Iray and OptiX teams are in my understanding dependent and thus work closely together so I hope this will be read by NVIDIA developers and managers who decided to make those changes, rethink their decisions, and provide workarounds.
Iray versions after version 334300.9558 have two significant changes which were made in an effort to make the raytracing more physically correct.
One of them is thin film calculations and I won’t get into that, because DAZ team already has a partial workaround, though the previous color result cannot be restored completely in more complex scenarios.
The other change is about luminosity calculation change and how it now affects emissive surfaces – it is much bigger change and it breaks a lot of existing 3D assets in many stores not just DAZ3D store, not to mention all the scene files already saved using those affected assets.
To understand why it is a breaking change, you need to understand a bit of history. Iray was (and in my opinion still is despite the improvements that made use of light portals obsolete) not really great when it comes to interior lighting.
To work around that, people using DAZ Studio and Iray used a simple trick – they used something called a Ghost Light Kit. Those are essentially 2D planes with emissive surface and cutout opacity set to a really low value of 0.0000001.
Since cutout opacity up to and including the Iray version 334300.9558 wasn’t affecting luminosity, that esentially makes ghost lights invisible to primary rays AND reflections (they are not visible in mirrors and on glossy surfaces) while providing nice diffuse light source of arbitrary size which produces soft light casts no shadows, and which can be placed anywhere in your indoor scene – directly under ceiling, even over the floor or on a wall to get the desired light level and to speed up the render without having to fiddle too much with tonemapping or to waste an exorbitant amount of time setting up dozens of individual point and/or spot lights which don’t even work if sun/sky lighting mode is used while emissive surfaces always do regardless of the scene lighting mode.
Anyone who ever saw a gaffer on a movie set can understand the value of such a light source – gaffers go to great lengths to eliminate unwanted shadows and reflections when lighting a movie scene even if that is not how the scene would look naturally.
The Iray change causing the problem with ghost lights is that now luminosity is multiplied with cutout opacity and the result is that they no longer emit light until cutout opacity is restored. There is a new advanced Iray node property that allows making an emissive surface invisible to primary rays but they are still visible in reflections and on glossy surfaces so that doesn’t help at all because the main advantage of those ghost lights was that they were invisible and didn’t cast shadows.
There are a lot of 3D assets (mainly environments, but also Sci-Fi clothing and weapon props, candle props, fire props) that used emissive surfaces with lowered opacity. An example off the top of my head would be an apartment with lightbulbs in light fixture in every room. They are modelled with glass ball which is transparent and emissive (at a lower level than the filament) and now they don’t emit any light. Another example are windows which have loadable emissive presets for day and night which no longer work because of the luminosity = luminosity x opacity change. Clothing with glowing parts such as armors and spacesuits are also affected as are weapon props, displays, etc.
Now I understand the desire to get raytracing as physically correct as possible, however I don’t understand blatant disregard for the consequences of such a change on the 3D asset ecosystem centered around rendering applications using Iray.
If it is really impossible to keep the old behavior for backward compatibilty with all the affected 3D assets and existing scenes, I would urge Iray team to at least consider adding a new light source type – an invisible to primary rays and reflections emissive surface with 2D plane geometry which doesn’t cast shadows so we can at least have a built-in replacement for ghost lights going forward.