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Rodrigo Mizuno shared a quick guide for UE4 users that focuses on faking light shafts, glow and stuff using planes.
The idea is to first find the relationship between the object, the world and the camera. Transforming the vectors from world space in to local space is really important for it to work no mater where the object is (now that I look at it, I have the feeling the TransformVector node after the ObjectOrientation could be removed).
After it is just a mater of manipulating those vectors to achieve the desirable result.
The Aboslute Value node is transforming the gradient that goes from right side to the left side of the object in to a gradient that starts from the front and goes to both sides equally.
absolute_exemple_01
Example of how Abs is affecting your mesh, the sides of the square being the left and right angle of your mesh and the center being the front
If for some reason you want the sides masked but not the top, remove the CrossProduct node and add OneMinus instead.
That should give you the mask you want, now you can control how much to the sides and how strong you want the effect to be.
biasScale_exemple_01
Simple way to manipulate a gradient
If you only want your geo to fade from the top all you need to do is isolate the blue channel (Z) and use it as your mask.
objTopFade_shader_01
I think that is all, let me know if you have any question, suggestions or know some other way of doing the same trick.
fade001
Rodrigo Mizuno
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