Spiral Copper?

[spathi]

Wondering how "spiral copper" effects are done, where it will appear that several copper bars are twining together in a double or triple helix.  There are also a lot of other effects that use this, like square columns that seem to be twisting and untwisting, that sort of thing.

Here is an example of a crazy effect using this, starts at 14:50 or so:

https://www.youtube.com/watch?v=pssH6moTGLc 

Obviously the individual columns of a bitmap are being shifted based on a sine wave to get the cylindrical effect, but how exactly?

Perhaps I am being foolish and it isn't really being mapped onto a cylinder...

[spitfire]

That's a pretty cool effect, and a neat demo thanks for posting the link.

I agree its probably a sine distortion in the y direction of some kind but I'm terrible at this kind of mapping stuff. Moving pixel rows up and down out of phase and using positive/negative values to determine if its the front or the back and then adding some white or dark highlights near the 0 values of the sine.

[spathi]

It could very well just be a non-perspective horizontal shift.  Perhaps the shifting produces a forced perspective effect.