4) Opacity determines how much light passes through the surface.
A material with an Opacity of 0 is completely transparent, while an Opacity of 1 is completely opaque. (There are values in between, just like with colors.)

To be able to use the Opacity output, you *MUST* configure the Transparency properties of the Material node properly (click on the Material node and scroll to the Transparency section in the Node Properties pane)
The options are as follows:
  • AlphaTestClipValue: if the Opacity value is below this, the material is completely "transparent" when BlendMode is set to AlphaTest. If BlendMode is not AlphaTest, this value has no effect.
  • BlendMode: Defines how alpha blending is done.
    • None: The Opacity output is not used, the material is completely opaque. This is the default value.
    • AlphaTest: The Opacity output is used for clipping using AlphaTestClipValue. The fragment is either completely opaque (Opacity >= AlphaTestClipValue) or completely transparent (Opacity < AlphaTestClipValue).
    • AlphaBlend: The color of the object is blended with the color "behind" it according to the Opacity value.
      Color = (MyOpacity * MyColor) + ((1 - MyOpacity) * BehindOpacity * BehindColor)
    • Additive: The color of the object is added to the color of the object(s) behind it.
      Color = MyColor + BehindColor


Node graph:
[Linked Image]

Results with Alpha Blending (left) and Additive Blending (right):
[Linked Image]
[Linked Image]


5) Refraction determines how much the surface changes the direction of light waves (think of water).
This takes a single float value as input, just like Opacity: the ratio of the sines of the angle of incidence and angle of refraction.
A list of refractive indices: http://en.wikipedia.org/wiki/List_of_refractive_indices

Example of refraction in action:
[Linked Image]

... to be continued.