I am making an animated graphic showing oceanic upwelling. Here's what I have so far:
The viewport shows three circular sets of arrows -- each is on a separate mesh and is a PNG with alpha.
The UV Editor shows an alpha mask, also PNG with alpha. The node editor shows my arrow mesh material thus far.
The mask fades the arrows in and out as expected, but when used as I have it connected in the node editor, the alpha mask rotates right along with the arrow image texture... so I do not get the arrows fading in and out. The desired effect is for the arrows to fade in from transparent and appear at the bottom of the rotating wheel, and to disappear by fading out to transparent at the top.
The question: How can I not rotate the alpha mask as the arrow "wheels" rotate? The arrow meshes rotate on Y, and I am sure there must be an elegant way to counter animate or driver the alpha so it actually stays in place, but I have not found it.
EDIT: And I guess the other way to ask this is: I want a circle of arrows to fade in at about 6 o'clock in rotation and fade out at about 12 o'clock rotation. What's the right way to do this?
Adding a sample file containing the sample mesh, material, arrow PNG and
EDIT3: Rumen's answer is correct, although it took me a while to figure it out. I have posted a resolution file showing how I did so.