Hi Idear,
Please refer to a tutorial on which the question is based, or use the “Cineversity Q&A”. Thanks!
I have no problems reading through your English, but please don’t use the short versions of the terms like sry for sorry. We have no 140 letters limitations here. It would make it a little bit easier on me that way. Thanks for considering.
So, to your problem, please post in the forum which I have mentioned above, and provide a little scene file. Such a scene file delivers (hopefully) any needed information the answer to your question. (I assume that you have checked the Alpha option in the renderings, and use a format that supports alpha channels)
All the best
Sassi