I''ve spent much time to understand the function castshow(), but I didn''t succeed. Why were pass1 and pass2 used? Why do we use glStencilOp(GL_KEEP, GL_KEEP, GL_INCR)(increase 1)in pass1,and glStencilOp(GL_KEEP, GL_KEEP, GL_DECR) (decrease 1)in pass2. Are there any tutorials on web about shadow?Or would you please expain this part for me. Thanks very much.