shadow on a transparent surface?

Hi,

i’d like to overlay a blender generated file on a real picture and i’d like the scene to have a shadow that i also want to see on the photo.

But the shadow should be on a surface that is seen in the photo, so basically what i need is a shadow on a transparent surface.

Is something like that possible?

Best regards,
Torsten.

Yes,

Create a plane and assign a new material to it.
Press F5 and then click on the Shaders TAB (this tab is hidden by default).
Activate the Only Shadow button.
Place the plane where you want shadows to fall.
When you render, you will only get the shadow and the alpha will pass through. (assuming it is not obscured by other geometry in the scene).

You don’t have to rely on a simple plane either! You can sculpt geometry to match objects in footage and Blender will properly cast shadows on to them.