Shadow tint on rendering for compositing problem

[miroslavfilipovic]

Greetings venerable people of this valuable community , I'm trying to solve this problem .....in general this image is rendered on black background ,in frame buffer looks correct, but cant save that shadow tint information for compositing purposes where I need to put background plate in post .

 

I'm get some better result when rendering on image background picture but then renderer must calculate unnecessary pixels on entire image ,even then in compositing i getting a lot weaker and brighter shadow when merge over unprocessed and not blured image

 

I know it is a way to tint shadow artificially, but I want it to be natural from HDRI sky as it looks in frame buffer.

 

searching with google I see a lot of compositing with wrong (different) tint in CG shadows which I want to avoid.

 

so please if somebody can give me advice

 

I do something wrong or missing something ?

 

thanks in advance and sorry for my bad english