When you’re working with refractive glass and a matte/environment/background in iray you may run into a problem if you’re using solid (refractive) glass. The problem is the glass refracts the “invisible” matte ground plane as illustrated in this first render:
You can avoid this by using a thin-walled glass instead of a refractive glass. However, that’s not always practical as you may need the refraction for a solid object, etc..
One thing I’ve been doing is placing a matte refraction object behind the solid glass object. Here’s a screen shot of the scene with the matte refraction object added:
I then assign a thin-walled glass preset to this refraction geometry (pink wireframe object). Then assign the geometry to the “Matte Objects” section in the iray Matte Environment shader. Screen shot:
Of course this becomes a bit more complicated if you’re rendering an animation. You could link the refraction geometry to the camera or perhaps use another method to prevent the matte/ground refraction issue.