When Final Gather and Global illumination is turned off, how does the new “emit light” IBL works? does it work like a number of area lights? spot lights?
For me, IBL and skylights always belonged to indirect light, to see their influence you have to turn on GI or FG. Is that changing now?
I also noticed in 2015 that when using the mia_physicalsky, with FG turned ON, and rendering a sphere on a plane, the sphere looks super bright with FG turned ON as if the same light is being computed twice, and more natural when FG is turned Off, you can see the comparison here: https://drive.google.com/folderview?id=0BwsPPskcQl7BYUkzd2MydVREeUE&usp=sharing
Think of IBL as a huge sphere area light around your scene that shines inwards, instead of outwards. It will basically give you correct illumination from your environment map as a direct light, and FG will then do secondary bounces only.
ok, with that in mind, do you still add a directional light for the sun? or is it supposed to be used on its own contrary to the old workflow where you had to add lights?
I have seen these videos, pretty technical.
I guess this is a new way of doing and thinking lighting, a new state of mind since my brain is so used to only think of skydomes / IBL as a indirect source of light in the Maya/MentalRay rendering space.
Here how I see it, correct me if I am wrong: In thepast IBL would only cast light if FG was turned ON as it was considered as a secondary lighting source, but it was always actually both, and when FG was activated, the IBL would cast direct and indirect light into the scene even if in MentalRay terms it was considered as secondary.
Lighting, Shading and Rendering are hugely active areas of development. Not just in 3rd party renderers like VRay and Arnold, but also very much so within nVidia and Mental Ray. There are a lot of new changes coming for rendering in Maya with Mental Ray that should hopefully make things better for non-technical artists. But of course it does mean having to learn new workflows, so best be prepared.
Check the Elemental Ray blog and the Inside Mental Ray blogs to get a sense of where things are going to go.
Yes, previously, when you set up environment map, FG would send rays into the environment, and sample it. Imagine it a similar scenario to having self illuminated constant shader with environment map mapped on a huge sphere around your scene, and FG picking up that surface. That’s kind of what it was.
Now, environment is passed through a huge dome-like portal light, that will give you correct illumination, and FG will not send samples to environment anymore, instead it will pick up the light from the surface, which was delivered on that surface by IBL light, and will generate secondary bouncing.
So now, if you keep IBL emit light on, but disable FG, you will still get direct light. Previously, you would get such lighting using FG with diffuse bounces set to 0. Of course, FG is bad at sampling of pretty much everything (it’s bad GI solution in general) so IBL illumination is much more accurate.
You will need to use sun or other light source along with IBL only if your environment map has incorrect exposure bracketing (does not capture full dynamic range of for example sun disc), or if you use just 8bit image as your environment map, or if you use mr Physical sky. If you use correctly captured HDRI map, then there is no need to use any additional light sources to enhance environment lighting.
Basically, to put it more simply, the first bounce from the environment is not done by FG anymore, but is done in brute force fashion using IBL direct light. It’s just better, and more accurate result. There is not much more to it. It’s better quality, and it’s faster (compared to trying to achieve same quality with FG only)
So IBL is direct light now and FG will not sample it instead it will only sample the resulting generated indirect illumination like it would do for any other right (point, spot,area or directional).
if the HDR has a sun disc, will it generate correct sun illumination with sharp shadows like the mr_physical_sun does?
Also there is sun disc on mr physical sky, but mr sky was always meant to be used with mr sun, since FG was not nearly as accurate to pick up sun disc from the environment map and create correct crisp illumination out of it.
While IBL with emit option should be capable of doing it too, it uses special final gather version of the mr sky, which hides sun disc from final gather, as well as IBL. Therefore you will not get double illumination from sun, one from the MR sun light object, and other one from the bright spot on the environment map itself. This mechanism is used also to prevent FG from randomly sampling the sun disc spot on the MR sky and causing splotchy artifacts.
So basically, MR physical sky displays sun disc only for direct visibility, reflections and refractions, but for indirect rays, it sends version of the MR sky without sun disc. And IBL with emit light uses the same shader output of MR sky as the one FG uses, so it is also getting the version withou sun disc.
Although, back in the time when IBL was still experimental, there was often a bug that IBL would get the MR sky version with sun, and therefore double amount of sun light in the scene. This bug should not occur anymore, but it might, if someone messed up the implementation.
I understand that Mental ray can disregard the sun discof the mia_physical_sky and keep it only for eye rays,since its procedural but how can it disregard the sun disc in a random HDR map? how will it recognize the parts of the map representing the sun shape? how will it know what parts to disregard?
That’s the point. You don’t need to. IBL with emit light option is so accurate it can simply use that sun disc on the HDRI map itself to illuminate the scene. No need for any kind of infinite (sun) direct light source to substitute for sun. Only time you need to substitute for sun now, is when HDRI map was incorrectly captured (not enough dynamic range) or when you use just 8bit map (jpeg for example) as your environment map. In other words, if you capture HDRI map on a sunny day with correct exposure range, and use it with IBL emit light, you will get those strong sharp shadows out of that map.
The only reason IBL uses FG version of mr Sky is because MR sky is created along with MR sun, and if IBL received mr sky version with sundisc, it would double the strenght of sunlight (one from mr Sun, and another one from sun disc on the MR sky). It’s not because IBL would not be able to handle that.
