I’ve made this simple expanding spiral (attached) and now it would be cool if it could fade away as it goes, so only the front part is “solid” transparent.
The problem is that my solution relies on the array modifier being active. Is there still a way to make it fade away in the back?
SpiralSpiral.blend (463 KB)
Alright… I got an idea. It’s not perfect but I think that it’s going in the direction of what you want. (Look at the big picture.) (Ignore the glowing round thingy. I just needed to copy what I did for this material and I was too busy fighting with Blender to delete the object… until I re-uploaded your file.)
Second picture: What the material nodes do is to apply a part of color ramp along one axis of your spiral, as it happens the X axis. (Don’t ask me if it’s the local axis, I’ve never needed to know so far. I mean that you can try to rotate your spiral but I won’t guaranty the result.) The value of 0.5 that I add and animate toward zero means that Blender will use the half end [0.5 to 1] of the color ramp on the first frame and the whole ramp [0 to 1] on frame 100. It explains why the color ramp is so “tight”. It has to be this way to keep visible only the front part of the spiral.You can increase that value of 0.5 (Don’t forget to re-insert a keyframe on frame 1!) to narrow the visible part of the spiral. (Values close from 1 and beyond are useless.)
BUG: There is a bug somewhere. It appears that Blender renders some shadows along the middle of the spiral even when it’s supposed to be invisible. I tried hard to remove it… but I couldn’t. I didn’t try but I think that giving some thickness to the spiral might fix it… maybe.
Anyway… I rendered the whole animation (in 360p only) to check the result and it’s very interesting when you know how it’s done… despite the bug.
Fantastic! The bug I can absolutely live with, since I thought it would be impossible altogether with the setup i chose.
Pictures, explanation, blend-file; Your level of helpfulness is equally fantastic! Thank you!
By the way, subdividing the mesh did the trick with the glitch. It seems like i tried bending it a little to much as it where.
You’re welcome. And I’m glad to head the bug was only a minor glitch. It was bugging me. 
