Can't see through BSDF principled/glass 2.8

Question

High risk this question was asked a billion times and I am just too stupid to find the correct answer, so apologiez. I am trying to create a drinking glass, the inside faces are colored red to simulate a sort of fluid, now how come that I cannot see it through the glass. Not in cycles, nor eevee in dev, or rendered view. Not by using principled, nor glass BSDF and I made sure that Screen Space Refraction is enabled. Any further suggestions are more than welcome.

The image you see is made in dev view and colored faces behind the glass do color the glass itself, but not what happens to be inside of it.

Answer 1

I was able to create a transparent glass that you can see through by only changing a few options in the material and in the render (I'm using Eeve, not sure if it applies to cycle as well):

First enable reflections and refractions in the render:

And in my material I took a simple Glass BSDF, and you must also set Blend mode=Alpha blend and enable Screen Space Refraction:

Answer 2

I do it by using a Glass BSDF and a Transparent BSDF. Then I mix both using the Mix Shader before attaching to the Material Output.

Answer 3

You can't see through any shader that was opaque.

The BSDF shader work on Surface Only, as Blender is a surface mesh 3D tool.

If you assign a opaque color to inner surface, the glass BSDF will become useless since you are literally painting the inner glass with that color.

A better approach will be adding a surface on the liquid surface, and change that surface color. Also blend shader with Glass or other transparent shader to make better.