A recurring problem I get is this one: given two different billboard sets with alpha textures intended to create particle special effects (such as point lights and smoke puffs), rendering them correctly is tedious.
The issue arising in this scenario is that there's no way to use depth writing and make certain billboards obey depth information as they appear in front of others that are clearly closer to the camera. I've described the problem on the Ogre forums several times without any suggestions being given (since the application I'm writing uses their engine).
What could be done then? sort all individual billboards from different billboard sets to avoid writing the depth and still have nice alpha blended results? If yes, please do point out some resources to start with in the frames of the aforementioned Ogre engine. Any other suggestions are welcome!