idea #403: Option to cleanly combine "ID Mask" output (or get the non antialised version)

Often one wants to combine 2 or more (bordering or overlapping) ID Masks.
The problem is that they often are anitalised and mixing them with nodes (i.e. "Mix" node) often results in mask-"halos" around some objects.
(EDIT: [Unusable] example of node-mixing http://www.flickr.com/photos/63231715@N00/2494419158)

So a way to combine them before they are converted to a bitmap (if that is even possible) would be quite handy here.

Of course a way to get a non antialised output of the "ID Mask" node might work as well and it may even be more flexible ("Mix" nodes will work correctly on purely BW images)

Example:
Combining mask 1 and mask 2 in this image:
http://wiki.blender.org/index.php/Image:Manual-Compositing-Node-IDMask_ex.jpg
Note: No, one can't simple take an inverted 0 mask here, because this is a simplified example and it will break if there are more masks.
The second example (with "halo" around borders") was already linked above.

Disclaimer: I'm still pretty much a noob when it comes to the compositor, so there may already be a way to do this cleanly. C&C welcome.

Werner

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