c4d background with shadow matte

[Andreas_MonteNero]

hi,

i have to render some elements tracked into a film. i would like to have the 3d elements with al clean alpha channel containing the shadows. so i use a shadow matte material on the floor.

when i turn of the scene´s background object i loose the lighting of the background image and end up with much too dark shadows from the floor. when i leave the background turned on i did not find a way to get rid of it in the alpha channel. is there a way i can have the background contributing to the lighting of the scene but not to the alpha channel?

thank you much!

[Andreas_MonteNero]

sorry, i already found the perfect answer in this thread:

https://answers.arnoldrenderer.com/questions/10383/background-shadow-matte.html

i only haven´t understood the solution correctly with multiplying the alpha.

thanks anyway