I think the hemisphere environment mode in iray is kind of a cool deal. It’s another one of those things that’s hidden under the hood but in this blog post I’ll show how/what it is and provide a quick script to enable the mode.
What is it? The hemisphere environment mode in iray basically creates a hemispherical dome where the bottom is squished and uses the HDR like a texture map. Instead of using the default infinite environment you can enable this mode and give the illusion of your geometry in the HDR. It’s a neat looking ‘trick’ and especially cool when used in combination with the iray active shade mode.
Below are some images to help illustrate this mode. The first image is a typical render using the default iray infinite environment (in 3ds Max). The geometry is in the “middle” area of the HDR as it’s stretched around the scene. Therefore you’d need to take this render and composite the geometry onto a back plate or build geometry in the scene, etc..
In the next image I have rotated the camera around and you can see how the HDR environment & mesh do not align since the environment is such a large sphere.
In the next image I have enabled the hemisphere environment mode and set a size for it and it’s “ground” texture.
Pretty neat eh? The bottom of the HDR is flattened and it now appears as though the mesh is sitting on the road in the HDR. Of course there are some drawbacks, like the distortion of the ground. Still though, IMHO it’s a neat looking effect. In the following images I’ll rotate the camera around to show how it tracks in this mode.
As I move the camera around, the objects act as though they are in the environment now. 
I can imagine this hemisphere environment mode might be very helpful for preliminary design looks (especially with active shade), quick renders, or even in some cases final renders.
The iray manager script has this environment mode as an option, but I’ve not had any luck in getting it to work from within the manager. I think it’s because some of the control points are missing (like ground texture scale). Anyway, I put those string options into a simple maxscript. It doesn’t have a GUI, sorry I’m just not that far along with maxscript yet. Plus it should probably be included in the iray manager script anyway.
Usage: Setup your HDR scene as you normally would (create/import geometry, assign textures to geometry, create a camera, assign spherical HDR to environment, disable default lighting). Run the script, fire up iray active shade mode and have fun.
You can open the script to see the string commands I used, modify the scale/size, etc.. And to find out more about what the strings do (and other strings), see the “iray String Options” link here at the irayrender.com website.
UPDATE (12/15/11) – A few more examples using the hemisphere environment with various HDRs…MORE EXAMPLES.
Other notes:
In the script I have the environment set to 4k resolution (4096) to help make the HDR a bit sharper. You may want to decrease that to 2048 or less depending on how much GPU memory you have available (higher res = more memory used). This 4096 res environment will also probably load slower, so if load time becomes an issue you can reduce the environment resolution in the script to 2048 or less for faster load times.
Keep in mind the scene lighting is still generated from the infinite environment outside of this hemisphere dome. This is also evident in glass refractions, as it will still refract the infinite environment and not this hemispherical dome.
Mustang model used in this example from arte-3d.
HDR used in examples from HDRI-Locations.com. Some of the best HDR’s & backplates I’ve used.
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