in case nobody already mentioned this, the use of a sharp-edged poser shadow in an hdri render may be intuitive, but is not in accord with the laws of optics IMVHO.  the whole idea behind hdri is that it should provide the sole illumination in the scene, leading to soft AO shadows.  however, in the absence of indirect lighting (by activation of the GIvariables using python script), hdri in FFRender is currently not up to specs as it is in more advanced renderers.