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                                               317
SECTION 4.6                                                                            Patterns




            fd =0                            fd =1                       fd =2

               va                              va                   va            vd

                                                                                  One new
                                                                                   vertex

     vb                  vc             vb               vc    vb            vc

                              vd
                                                     One new
          Three new                                   vertex
                                               vd
           vertices

                    ve             vf




     FIGURE 4.17 Connecting triangles in a free-form Gouraud-shaded triangle mesh


Complex shapes can be created by using the edge flags to control the edge on
which subsequent triangles are formed. Figure 4.18 shows two simple examples.
Mesh 1 begins with triangle 1 and uses the following edge flags to draw each suc-
ceeding triangle:
 1 ( fa = fb = fc = 0 )                        7 ( fi = 2 )
 2 ( fd = 1 )                                  8 ( fj = 2 )
 3 ( fe = 1 )                                  9 ( fk = 2 )
 4 ( ff = 1 )                                 10 ( f l = 1 )
 5 ( fg = 1 )                                 11 ( f m = 1 )
 6 ( fh = 1 )

Mesh 2 again begins with triangle 1 and uses the following edge flags:
1 ( fa = fb = fc = 0 )                       4 ( ff = 2 )
2 ( fd = 1 )                                 5 ( fg = 2 )
3 ( fe = 2 )                                 6 ( fh = 2 )

The stream must provide vertex data for a whole number of triangles with appro-
priate edge flags; otherwise, an error occurs.

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