I want to render a object with a background in MentalRay. The lightsource is a HDR-Image. For compositing I need the beautypass and the shadow separately. Normaly for that reason I use a standart light (f.e. directional) in combination with a "use backround shader" or the "render passes". But in MentalRay together with a HDR it doesn't work: When I use a "use background shader" the image for the shadow is just black without a alfa channel and when I use the render passes the beautypass looks good but the shadow has neither RGB-information nor alfa-information.
Whats my mistake and how can I solve it?