For my research, I'm using Blender to generate images. I wanted to know how visible a certain object is in the final render (i.e. how many pixels it occupies). For this there is the "object index" render pass (aka "IndexOB" in the compositor). I've been struggling with it, since it always outputs that the index is 0, even though there are multiple objects in the scene.

Well, with the help of mfoxdogg on the #blender IRC channel, we found a solution: You need to set the index by hand, for every object you're interested in. If you go to the object properties (in the properties explorer), in the section "Relations" there is a slider "Pass Index". This is set to 0 by default, and you can set it to any positive number you want. This is then reflected in the output of the "IndexOB" render pass.