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GLEVALMESH(3G)                                                                                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.  Sym-bolic Symbolic
             bolic 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 poly-gons. polygons.
              gons.  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 evenlyspaced
       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 deter-mines determines
       mines 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 dot DELTA u + u1 );
              glEnd();
where

              DELTA(u) = (u2 - u1 ) / n


and  n,  u1,  and  u2 are the arguments to the most recent glMapGrid1 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 dot DELTA(u) +  u1  is
exactly u2.

In the two-dimensional case, glEvalMesh2, let

       DELTA(u) = ( u2 - u1 ) / n

       DELTA(v) = ( v2 - v1 ) / m,

where  n,  u1,  u2, m, v1, and v2 are the arguments to the most recent 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 dot DELTA(u) + u1, j dot DELTA(v) + v1 );
             glEvalCoord2( i dot DELTA(u) + u1, (j+1) dot DELTA(v) + v1 );
          }
          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 dot DELTA(u) + u1, j dot DELTA(v) + v1 );
          glEnd();
       }

       for ( i = i1;  i <= i2; i += 1 ) {
          glBegin( GL_LINE_STRIP );
          for ( j = j1; j <= j1; j += 1 )
             glEvalCoord2( i dot DELTA(u) + u1, j dot v + v1 );
          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 dot u + u1, j dot DELTA(v) + v1 );
       glEnd();

In all three cases, the only absolute numeric requirements are that if i = n, then the value computed from i
dot  DELTA(u)  + u1 is exactly u2, and if j = m, then the value computed from j dot DELTA(v) + v1 is exactly
v2.

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

SEE ALSO
       glBegin(3G), glEvalCoord(3G), glEvalPoint(3G), glMap1(3G), glMap2(3G), glMapGrid(3G)




                                                                                              GLEVALMESH(3G)