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By using specific settings for the UV transform and projection, you can use an image texture to create an arbitrary falloff:
In this example we use an image of 1px wide, with the color for grazing angles at the bottom, and the color for normal angles at the top.

• UV transform:
  • R.Z = 180°.
  • (S.X, S.Y) = (0.5, 0.5)
  • (T.X, T.Y) = (0, -0.25)
• Use spherical projection and set coordinate space to "Normal space"
• Set border mode to "Clamp value"

Now the image is mapped in the same way as the ramp of the falloff texture node.

You can change the rotation according to the orientation of the image. If your image contains the ramp along the X axis, use 90° or -90°.

--
Roeland

Thanks, Roeland

Please don't ever change this behaviour, I programmed a custom tool to be able to design fresnel curves and then project them using your method in Octane