I had set up a few meta-balls in a scene with nothing else. I rendered the group and all was well in the land of Bryce. HOWEVER, when I asked for a object mask of the selected meta-balls, all I got was a black picture - no mask, just black. So I tried an distance mask and it worked. Why won't meta-balls render a object mask? PS. needed the mask for translating the background of the image into an alpha channel. I was able to massage the distance mask and get an object mask from it.

Dunno either, but i DO know one thing, when you ungroup them, then object mask will render correctly, guess it has something to do with the fact that individual object masks (which intersect each other, BTW) get canceled out when meatballs are grouped !!! :) Hope this helped ! :)

Correct, grouped metaballs will not show up in object mask renders. You have to ungroup them. AS

Well! Looks like I accidently found a way around the problem. If the meta-ball group is a relatively uniform distance from the camera, the distance mask will give you an image which can be massaged (in Gimp or Photoshop) to produce a object mask. Now everyone who reads this will know.

You don't have to ungroup them, the metaballs must beslected individually (not the group) to get an object mask. To easily do this 1. Select the metaball group as a whole 2. go to solo selection 3. unselect the group 4. at the botom of the screen clcik the metaball icon this should select all the metaballs in the group that you want and now a mask can be rendered. Same problem happens with grouped trees, which makes sense considering they are metballs too.