right now I render the scene with a depth buffer from the lights position and aim, though it's actually a directional lights. So what I do is that i make the difference in the aim and the position large so that the 'rays' almost become parallel, so it seems like a directional light. The only problem with this is that the shadows become very edgy, as the size of the depth rendering map is quite small compared to the area that needs to be covered with shadows (the cameras position is very far away from the shadows position), to make it detailed. So what I though could be possible was to change the size of the render target with the device to a larger and then render the texture, and then change the size back and then use the texture, but wouldn't that be quite slow?
So how should this be done? As it is done in previous games, like e.g. Battlefield 3: (look at the building or the trees shadows on the grass)