Set Alpha Node Seems To Fail?

Hi All,

I have a scene with multiple objects. One of the objects is a red beam with it’s index set to #5. So I wanted to isolate the red beam using a nodes system so I could blur it out a bit. But Blender seems to fail to do this correctly.

Perhaps it is my misunderstanding of the SetAlpha node (the description is blank in the documentation). I assumed that I could give the node an image input and an alpha input and the resulting image would be a RGBA image with the RGB coming from the image and the A coming from the supplied alpha. This does not seem to be the case.

In the image, you can clearly see that ID #5 only has the beam. Then after the SetAlpha the image has a ghost of other object in the scene as well. It looks like SetAlpha should really be called “MixAlpha”. Is there any work around for this? A node like SetAlpha, but one that actually works?