Sky for iray render
Lot's of postings talk about how skydomes don't work with iray renders, or talk about workarounds (most of which are over my head). If iray renders aren't intended to work with skydomes, then what is the correct way to get a sky over your scene? Are there products for this? Or render settings I haven't seen or understood? Advice would be much appreciated.
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Actual modelled skydomes cause problems in Iray renders because Iray uses the real-world physical behaviour of light — you can't have light from an outside source passing through the skydome as if it's invisible. HDRI Environment lights (see the Render Settings>Environment tab) can supply a complete sky for your scene if you switch on "Draw Dome", and there are some good sets available in the store (also lots of freebie sets from other sources, of variable quality). Another possiblilty is making the skydome glow (apply the Emissive shader preset in the Iray shaders to the skydome's material), although this can be a bit fiddly and need a lot of tweaking to get it right. There are a few other ways to do it, depending on exactly how your scene is set up, and which kind of lights you're already using in it.
If all else fails, you might be able to do what I sometimes do, and cheat — pose and point your camera so that you can't actually see the sky in the Viewport.
Placing an HDRI in the proper channel is really the preferred method. It gives the base light set up and, if the image is of sufficient resolution, a 'sky' to work with.
In my sig is a thread with links to lots of the free HDRIs available, some of them are of very high resolution.
Another method is to use a backplate prop that has a high resolution 'ldr' (jpg, png, etc) image applied. But that only really works well with 'real' scene elements (like buildings and trees)...much like a 'sound stage'.
A third (or is it fifth) method...render to an image format that uses an alpha channel (not jpg) and add the sky/background in an image editor (Photshop/GIMP/etc).
If you really want to use a skydome, you can 'cheat' and turn the skydome into a giant light source, or in the case of Iray an 'emissive' surface. This essentially gives you an IBL based on whatever texture you have set for the dome. Make sure to set the colour temperature to around 6000, so that it doesn't tint the light too much.
Et voila, one working skydome. The advantage of this is there's less playing around to convert older 3DL scenes into Iray, since finding a matching IBL can be a challenge in and of itself. The disadvantage is that it can lead to some lighting 'blowout' if you're not careful, and obviously the light won't penetrate outside of the dome. Chances are you won't care about the latter, and you can easily tweak the former though.
The easier mnethod is to reduce the cutout oopacity of the skydome geometry to near 0. It's image will still render, but light provided by an HDRi or sun-sky will shine through. On making the dome an emitter: Light from an emissive source is always diffuse, which can produce bland and uninteresting shadow toning for the scene. HDRi or sun-sky will maintain shadow detail (given the right HDRi image and sun-sky settings).
Thanks for the comments. I'll try some of these, but I have to admit to being a real beginner at this. I think I see how to do some of these, but I don't find a "cutout opacity" setting. Any clues on where to find that?
I did try the "opacity strength" setting. When I reduce that, I no longer get a complete dark render, but I get a checkerboard pattern where the sky should be.
Cutout opacity is on the surface tab after you have converted your scene to iray materials using the DAZ uber shader.
Cheers,
Alex.
Sorry, I was thinking of another solution using skydomes... You want this one if you're looking to make the dome transparent to external light, but still show the image:
http://www.daz3d.com/forums/discussion/63014/yet-another-iray-skydome-technique
As with Alex's recommendation, be sure to first change the surface of the dome to Iray Uber. You need to see the "MaterialID" node near the top. If you don't, it's a 3Delight shader, and you need to convert it. Also be sure you plug the image into Glossy Color (which should be white). It won't do anything in Base Color.