I wanted to have anamorphic bloom for quite a long time now and I finally found a way to do it natively, without cheating with particles.
I detailed my findings in an articles on my personal blog : http://www.froyok.fr/blog/2017-05-anamorphique-bloom-with-unreal-engine-4
It seems that the engine already support it since a few versions, but I didn’t find any documentation for it. I simply discovered it by looking in the source code this week. However the default effect wasn’t enough for me so I went further and tweaked the engine source code to enhance the effect. I only tested it with the Unreal Engine 4.15, I did not yet tested the 4.16 version and the new Convolution Bloom. However the advantage of my modification is that I don’t change performances (unless I’m mistaken).
I would love to hear some feedback from a Rendering dev about it. I’m sure it is possible to do better but the engine code is a bit too complex for me unfortunately. Some hints could help.