A DVD-ROM stores a plurality of shadow volume data corresponding to respective
attitudes of an object which casts a shadow. A CPU sets a shadow volume corresponding
to the attitude of the object by performing interpolation based on the shadow volume
data as necessary. Based on the thus-set shadow volume, a GPU determines a shadow
region using a stencil buffer. Based on the determined results, the GPU updates
luminance information of each pixel stored in a color buffer. According to this
configuration, when a game system uses the shadow volume to draw a shadow, it is
possible to reduce the processing load caused by setting the shadow volume while
drawing a more realistic shadow corresponding to the attitude of the object which
casts the shadow.