Here’s another popular question I frequently hear: My scene looks fine with the mrSun/mrSky system but when I use a HDR it’s black. Does mental ray, iray, vray, whatever-ray not support HDRs? If this has affected you, read on for the most common explanation.
I’ll go ahead and warn you now, this is one of those topics that may get kinda fuzzy. My primary goal here is to explain why HDR/EXR files may appear dark in a scene. To start off let’s say you have a scene you’ve created and now you’re ready for lighting. Typically a couple of common scenarios happen at this point. You may add a daylight system with the mrSun/MrSky/mrPhysical Sky combination or jump straight to a HDR/EXR for illumination. To begin with, let’s assume you start with the daylight system/mrSun/mrSky/mrPhysical Sky.
You fire off a render and get something like this:
Of course there will be slight variations to this based on your sun locations, etc. Anyway, nothing wrong with the lighting setup here, especially if your goal is a clear sunny day. However, after checking out your model in this type of environment you feel that you’d like to use a HDR/EXR file for the lighting instead of the mrSun/mrSky/mrPhysical Sky. You assign the HDR/EXR file to the environment and/or a skylight or whatever method you choose and fire off another render and get this:
What went wrong? In a word, exposure. Your exposure control is configured for a very bright daylight scene. Like this:
Typically, HDRs/EXRs that you have purchased or made do not default to the same intensity as the 3ds Max daylight system. If they did they would probably be pure white when you viewed them in a typical image viewing program. To help visualize this, if you disable the mrPhotographic Exposure control you’d probably see something similar to this if when you render the mrPhysical Sky vs. a HDR:
The mrPhysical Sky is super bright whereas the HDR/EXR looks spot on. Now if you enable the mrPhotographic Expsoure control with the outdoor daylight setting you’d see something like this:
Now the mrPhysical Sky looks right, but the HDR/EXR file is waaaay under exposed. What to do? Well you have a couple of options but basically you can either adjust the exposure control settings or the output/intensity of the HDR/EXR file to give it the proper intensity value.
For example, keeping the mrPhotographic exposure on Outdoor Daylight preset but with the HDR file instead of the daylight system I could use the unitless physical scale value to compensate for the HDR/EXR intensity like this:
A render with that yields this:
Yaay! Now the HDR/EXR file is visible. BUT…there’s always a but right? This setup isn’t a global setting that you can or should always use. I say this because at 90k the exposure settings are waaay to high for the time of day of this particular HDR – (dusk) environment. A problem would arise if I were to use this setup and wanted to add some lights to the scene I would need to boost their intensity unrealistically high before they would be visible at an exposure control EV value of 15.
How do you know what exposure settings to use with any HDRs/EXRs then? Well, that’s the rub here since HDRs/EXRs are built from multiple exposures and taken at various times of day in various conditions, etc.. Hard to nail it down to one exposure value, especially so if you plan on adding other light sources to your scene.
There’s one ‘trick’ that I use though. A lot of the HDRs/EXRs I have came with backplates. Those backplates are typically exposed properly and you can use their EXIF data to obtain a working exposure value.
In windows I can view the file properties of an image and in the details section easily find the camera info like this one that came with the HDR I used above:
With that info in hand I go back into 3ds Max and enter the Shutter Speed, ISO, and f-stop info into the mrPhotographic Exposure control.
Then I can adjust the unitless physical scale value to something that provides a close approximation to what I’d expect. For this particular HDR file 12,000 seemed to be fairly close to what I’d expect.
Of course it’s still a guesstimate, but with the backplate shutter/aperture/ISO info plugged into the mrPhotographic Exposure control I feel it’s somewhat close. It should also provide more realistic results when lights are added to the scene (like IES lights).
So the bottom line here is that more than likely if you’ve tried to light a scene with a HDR/EXR file and it turns out black or overly dark it’s probably because of a conflict in your exposure control settings and the intensity of the HDR/EXR file. You can adjust the exposure settings and/or the intensity of the HDR to help resolve the issue.
As with anything in 3ds Max there are many other ways to tackle this. I’m not in any way saying this is the perfect method, it’s not really…but it works for me and the scenes I work with. You may have your own preferred method for accurately adjusting the intensity of the HDR/EXR files for use with exposure control like not using exposure control at all and rendering out in linear format, etc..
NOTE: For the example images posted here I used an Evermotion scene.