I actually found a way to get a smooth, frosted translucent glass effect to work very cheaply with static cubemaps on opaque surfaces. My methodology was:
1 - Create and light a scene requiring a frosted pane of glass, but don’t implement the glass in the model (yet).
2 - Use a scene capture cubemap actor to generate a static HDR cubemap right on the window pane. This method only works with static cubemaps.
3 - In Photoshop, shift the cubemap 256 pixels to the right. This should invert it. Blur the texture using gaussian blur as well.
4 - Re-implement the glass material as a separate object. Do not let it cast shadows.
5 - In the glass material, plug a reflection vector into the cubemap. The end result is, instead of reflecting the cubemap over the surface, it’s refracting it, and should appear to almost match up with the environment on an opaque surface.
6 - Multiply down the values and colors, and plug it into emissive. The more you lower the colors, the more opaque the glass becomes.
7 - Use other material settings for recreating a reflective glass surface, like very low roughness and metallic set to 1. Balance the base color value with the emissive value to get both direct reflection and translucency.
Limitations:
*Not dynamic. Cannot be updated in realtime.
*Requires blurring function to be performed in Photoshop instead of realtime in the editor: if you need to change how blurry the translucency is, you need to go back to the original scene capture in Photoshop.
*Requires HDR photo editing suite with blurring capabilities.
*Translucent effect not entirely accurate, and loses accuracy as players move away from the window (only completely accurate when player assumes same position as original scene capture location.
Pros:
*Extremely efficient: because it uses opaque surfaces, it’s very easy to render with minimal instructions. And an effect like this can even work on unlit opaque surfaces. Like I said, EXTREMELY efficient!
*Compatible with normal maps, but can also be used on smooth, flat surfaces.
*Texture-based blurring can be more accurate than blurring functions in realtime.
Most of my material setup is actually just to tile the normal map and soften it. The cubemap and reflection vector multiplied by a color is all you need in the shader for getting the translucent effect to work properly.
