I have had limited success in creating a 2-way mirror material, is it even possible to get it right?

side 1 :

side 2:

You've got partway there, but a one-way mirror gives total reflection on one side and partial reflection on the other, which is Normal dependent. For a two-way mirror, it's just partially reflective in both directions, so you just want a constant factor input. If you want it to be something other than 50% reflective in both directions, though, then you'll probably need to blend based on the normal as well.

Wouldn't you use the backfacing plug rather than the normals one?

using the backfacing plug gives the following results

I would like one side to be reflective and the other to be transparent with a little reflection - there is too much reflection atm with my current setup

According to search results, "for the two-way mirror to work it is essential that the room you want to look into is brighter than the room you are stood in". I'd suggest setting up the environment and see what the results are like before adjusting the material?

Conservation of energy says to me that you cannot have full transmission in both directions and get any reflection, additionally with real glass there will be some absorption.

If by two-way-mirror, you mean a partially reflective sheet of "glass", then a shallow box with BB's Tricky glass shader should be the starting point.

With the tricky glass, where you mix a GlassBSDF node with the TransparentBSDF node, default Glass has its own inherent Index of Refraction, about 1.45. That number, essentially, is what determines how much reflection and refraction happen at the glass surface. Push the number up to the point BB determined emulates metals, between 10 and 20, and you get a more or less completely reflective surface. Come back down to the levels of diamond, 2.1 and you get much more surface reflection than ordinary glass. Plugin 5 as the IOR, and you get something that gives roughly equal transmission and reflection.

Not to rain on yr parade or disparage the work you've done, but why? I can't think of any situation where both sides will be visible in the same render, so a simple adjustment of transparency should suffice between front & back based on viewing position.

With that out of the way, have you tried single sided planes facin' away from each other (or a box w/ separate MAT zones applied to opposite sides) w/ full or partial opacity assigned to the respective sides?

Luck on yr quest :)

it's an exercise in understanding material creation using the cyclesurface node

RedPhantom posted at 8:26AM Sun, 14 June 2020 - #4391494

Wouldn't you use the backfacing plug rather than the normals one?

Correct.

I never noticed before now that my twisted ribbon reflects the world rotated by 90 degrees, so that the ground is side-by-side with the sky. Very curious.

Thanks Geoff & Ted