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As the title says how can I achieve a retroreflector (like you would see on bikes and street signs) material in Blender Internal? I haven't been able to figure it out, is something impossible to do in Blender?

I'd like to pull off something like this:

enter image description here

Or this:

enter image description here

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It is possible.

Retroreflectors reflect the light back in the direction it came from without regard of their surface normal.

In principle you can do it like this with a node material:

Node Material Setup

The material only uses Specular light and the surface normals are overwritten wih the View Vector of the active camera. This ensures that a specular reflection is only seen when the light comes from the direction of the View Vector.

The Multiply node is optional. It can be used in combination with the compositor to simulate the glow you can see in the photos.

I used these Specular properties:

Specular Properties

These are the test results from this material setup:

Same direction of light and camera: Scene 1 Render 1

Light has a bit different direction: Scene 2 Render 2

Light outside reflective angle: Scene 3 Render 3

Of course real retroreflective materials have some ambient reflections too. For brevity I left them out. It is easy to add some ambient components in the node material.

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