glEvalMesh(3G)

NAME
     glEvalMesh1, glEvalMesh2 - compute a one- or two-dimensional grid of
     points or lines


C SPECIFICATION
     void glEvalMesh1( GLenum mode,
                       GLint i1,
                       GLint i2 )


PARAMETERS
     mode  In glEvalMesh1, specifies whether to compute a one-dimensional mesh
           of points or lines.  Symbolic constants GL_POINT and GL_LINE are
           accepted.

     i1, i2
           Specify the first and last integer values for grid domain variable
           i.

C SPECIFICATION
     void glEvalMesh2( GLenum mode,
                       GLint i1,
                       GLint i2,
                       GLint j1,
                       GLint j2 )


PARAMETERS
     mode In glEvalMesh2, specifies whether to compute a two-dimensional mesh
          of points, lines, or polygons.  Symbolic constants GL_POINT,
          GL_LINE, and GL_FILL are accepted.

     i1, i2
          Specify the first and last integer values for grid domain variable
          i.

     j1, j2
          Specify the first and last integer values for grid domain variable
          j.

DESCRIPTION
     glMapGrid and glEvalMesh are used in tandem to efficiently generate and
     evaluate a series of evenly-spaced map domain values.  glEvalMesh steps
     through the integer domain of a one- or two-dimensional grid, whose range
     is the domain of the evaluation maps specified by glMap1 and glMap2.
     mode determines whether the resulting vertices are connected as points,
     lines, or filled polygons.

     In the one-dimensional case, glEvalMesh1, the mesh is generated as if the
     following code fragment were executed:

          glBegin( type );
          for ( i = i1; i <= i2; i += 1 )
                            .
             glEvalCoord1( i Wu + u  );
                                     1
          glEnd();

     where

          Wu = (u  - u )/n
                   2    1


     and n, u , and u  are the arguments to the most recent glMapGrid1
             1       2
     command.  type is GL_POINTS if mode is GL_POINT, or GL_LINES if mode is
     GL_LINE.

     The one absolute numeric requirement is that if i = n, then the value
                    .
     computed from i Wu + u  is exactly u .
                             1             2

     In the two-dimensional case, glEvalMesh2, let

          Wu = (u  - u )/n
                   2    1

          Wv = (v  - v )/m,
                   2    1

     where n, u , u , m, v , and v  are the arguments to the most recent
               1   2      1       2
     glMapGrid2 command.  Then, if mode is GL_FILL, the glEvalMesh2 command is
     equivalent to:


          for ( j = j1; j < j2; j += 1 ) {
             glBegin( GL_QUAD_STRIP );
             for ( i = i1; i <= i2; i += 1 ) {
                               .            .
                glEvalCoord2( i Wu + u , j Wv + v  );
                                        1            1
                               .                .
                glEvalCoord2( i Wu + u , (j+1) Wv + v  );
                                        1                1
             }
             glEnd();
          }


     If mode is GL_LINE, then a call to glEvalMesh2 is equivalent to:


          for ( j = j1; j <= j2; j += 1 ) {
             glBegin( GL_LINE_STRIP );
             for ( i = i1; i <= i2; i += 1 )
                               .            .
                glEvalCoord2( i Wu + u , j Wv + v  );
                                        1            1
             glEnd();
          }

          for ( i = i1;  i <= i2; i += 1 ) {
             glBegin( GL_LINE_STRIP );
             for ( j = j1; j <= j1; j += 1 )
                               .            .
                glEvalCoord2( i Wu + u , j Wv + v  );
                                        1            1
             glEnd();
          }


     And finally, if mode is GL_POINT, then a call to glEvalMesh2 is
     equivalent to:


          glBegin( GL_POINTS );
          for ( j = j1; j <= j2; j += 1 )
             for ( i = i1; i <= i2; i += 1 )
                               .            .
                glEvalCoord2( i Wu + u , j Wv + v  );
                                        1            1
          glEnd();


     In all three cases, the only absolute numeric requirements are that if
                                          .
     i = n, then the value computed from i Wu + u  is exactly u , and if
                                                   1             2
                                          .
     j = m, then the value computed from j Wv + v  is exactly v .
                                                   1             2

ERRORS
     GL_INVALID_ENUM is generated if mode is not an accepted value.

     GL_INVALID_OPERATION is generated if glEvalMesh is executed between the
     execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED GETS
     glGet with argument GL_MAP1_GRID_DOMAIN
     glGet with argument GL_MAP2_GRID_DOMAIN
     glGet with argument GL_MAP1_GRID_SEGMENTS
     glGet with argument GL_MAP2_GRID_SEGMENTS


MACHINE DEPENDENCIES
     RealityEngine, RealityEngine2, and VTX systems do not handle 1D maps for
     colors correctly.


SEE ALSO
     glBegin, glEvalCoord, glEvalPoint, glMap1, glMap2, glMapGrid