ok, i´ve found a way via "pausstift" (trace). and then with the mask an the mask2shape script from redefinery (thx for the nice script)
if somebody know a better way, maybe with a script direkt from the alpha, please let me know.
Indirectly, there is a way. But it will require some intervention on your part.
The Auto-trace feature in After Effects (select your layer, then go to Layer > Auto-Trace) can generate mask paths from Alpha (or the color channels) in your layer.
And better yet, mask paths are compatible for copy/paste operations with shape layer paths (or you link a shape to a mask path, or viceversa, with the expression pick-whip). In fact, all vector path types in After Effects (motion paths, shape paths, mask paths, paint paths, etc) are now compatible with each other in terms of copy/paste or expression linking.
So, there is a way.
The hurdle you will face is: depending on the complexity of the alpha information, how high you set the detection threshold and the existence of overlaps, you will probably not get a single mask path, but several (sometimes a lot!). And the problem is, if the number of mask paths you copy is higher than the number of shape paths you target for pasting, then the remaining paths are pasted as masks. So the trick to make it work is, after you copy the mask paths, you have to make sure you manually create enough shape paths - as many as masks you want to paste. So, if auto-trace generated 5 masks, you have to make sure you create 5 shape paths. Then you have to target all the shape path properties (select the property names directly) and paste.
Remember that by lowering the precision in Auto-trace, you will get less mask paths as a result. This can help.
To create a shape path, you could go to the "Add" fly-out menu inside the shape layer, and choose "path". Or you could use the pen tool to create shape paths directly, which if selected before pasting the mask paths, will be replaced with those paths.
Using the Masks 2 shapes script will help so that converting the mask paths to shape paths is not so cumbersome.
You would still have to use auto-trace before.
It's not that complicated.
As explained, Live Trace is the only way to do that automatically inside AE. Everything else costs money or entails a lot of manual labor. If its a still, you could try online services like Vectormagic that do a much better job at vectorizing bitmaps, but for that you will also need Illustrator to be able to copy&paste paths from the resulting AI file over to AE. For motion content, you will require tools that offer trackable masks like Silhouette Roto or motor or work with compositing programs like Fusion that have better masking and roto features to begin with than AE.
Products like Vectormagic or the LiveTrace feature in Adobe Illustrator are very good at translating visual appearances when converting raster images to vectors, but they often produce hundreds of vector paths which would be very painful to handle with copy/paste. If you reduced the resolution to two colors (to tell apart opaque areas from transparency)... it wouldn't be so different from Auto-Trace in AE. Perhaps a bit more precise, but way less convenient.
thx for all the helpfull answerings.
i used the auto-trace feature (in german "pausstift") and then the mask2shape-script from redefinery.