Abstract: The frame of our work is the efficient realistic rendering of scenes containing a huge amount of data for which an a priori knowledge is available. In this paper, we present a new model able to render forests of pine-trees efficiently in ray-tracing and free of aliasing. This model is based on three scales of shaders representing the geometry (i.e. needles) that is smaller than a pixel size. These shaders are computed by analytically integrating the illumination reflected by this geometry using the a priori knowledge. They include the effects of local illumination, shadows and opacity within the concerned volume of data.