Per object transparency

Is it possible to control alpha at the object level, instead of using material settings? I need to fade in and fade out several arrows in an animated diagram, at different points in the timeline. I would rather avoid making duplicate materials just so I can keyframe their alpha levels independently for each object.
Alternatively, is it possible to create multiple materials where all parameters other than the alpha are centrally controlled, so they could all be adjusted together?

Can you use object info/color to drive the transparency node/mix node?
Alternatively, you can create passthrough nodegroups you control from world. The node groups don’t do anything in world, they just sit there for easy access. I use this to drive groups of lights. Will work with any value that is not exposed. To adjust it, you need to go inside and change it there. Then it’s a matter of duplicating the node group and using it per group of materials you want to control.

You can make a node group with just single value node connected to output and used as alpha in material.
And another group, for all other parameters.
You can then reuse the node groups in several materials.

yes it is by object index:

Step in this node means which index is next.
so 1 means 1 object is invis 2.5 means 2 objects are invis + one half one


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Thanks everyone for your suggestions. I went with a modified version of @Weekend’s:

Since the idea was to have the ability to have the arrows appear independently, instead of simple subtraction, I fed the result through nodes equalling 1-4x^2, which is an inverted parabola with a y-intercept of 1 and x-intercepts of 0.5 and -0.5:

Depending on the pass index, each object can be faded from 0 to 1 by changing the Index value above (at +/-0.5 all items have an alpha of 0).
Animation Test.blend (864.3 KB)

Here’s what it looks like animated (just a test, not the finished product):

Again, thanks for everyone’s help.