+-----------------------------+
       |                             |
       |                             |
       |                             |
       |             * (0,1/2)  <- (x1,y1)
       |            /                |
       |           /                 |
       |          /                  |
       | (-1/2,0)*                   |
       |    ^     \                  |
       | (xa,ya)   \                 |
       |            \                |
       |             * (0,-1/2) <- (x0,y0)
       |                             |
       |                             |
       |                             |
       +-----------------------------+