Real time soft shadow generation for arbitrary planar lights

Abstract

Shadows are crucial for the human perception of the 3D world. Hard shadows are produced when a typical 011 definition of shadows is applied: if the light source can be seen from a pixel, it is lit; otherwise, it is in shadow. However, in nature, most light sources have a measurable size and hence they can be partially occluded. This fuzzy definition of shadows introduces a new genre of shadow generation problem: soft shadow generation. Soft shadows are obviously very important for realistic image rendering. But true soft shadow calculation is a computation intensive problem which requires considering complicated receiver-occluderlight source relation in three dimensions. So, to give real time graphics a realistic touch, we must come up with approximation algorithms that produce almost physically correct shadow. Of the main two trends of shadow algorithms, shadow map approach suffers from sampling artifacts due to image space computation. Hence, to produce convincing shadows in real time, a shadow volume algorithm is needed that can generate soft shadows. We have developed an algorithm, under the shadow volume theme, to compute soft shadows for triangulated simple three dimensional objects. We have introduced a double silhouette concept; we are considering two silhouettes - umbra silhouette, responsible for the outline of the umbra region; and the penumbra silhouette, responsible for the outline of the penumbra region of the generated soft shadow. Our algorithm basically interpolates between these two extremes of the penumbra region. We have devised a way to break up the occluders into silhouette rings, and compute shadow contributions at any pixel based on only these structures. Our goal was to produce smooth shadow, so we devised an interestingly innovative way of combining contributions from overlapping projections of silhouette edges: 'edge shadows'. We have compared the quality of the shadow generated from our algorithm with that of a few well known recent algorithms. Our algorithm does not suffer from the artifacts previous . algorithms had. On top of that, since we compute shadows only depending on the silhouette edges, special cases like the idea of overlapping shadow and self shadow degenerated into the common case of generating soft shadow from silhouette edges.