flask_glass

Borosilicate: a thin wall that is almost nothing face-on and almost all of the picture at a grazing angle

The scene behind the wall is read displaced along the refracted ray, a Schlick fresnel decides how much of the wall is reflection rather than transmission, and every punctual light in the room leaves a hard specular on it.

The output is PREMULTIPLIED: the wall replaces what is behind it in proportion to how much of the view it actually bends, and its specular and its rim ADD on top of whatever survives. That split is why a highlight on glass stays a highlight when the glass itself is four percent opaque.