glEvalMesh1(3)

NAME

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

C SPECIFICATION

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

       delim $$

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  vari-
             able $i$.

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 vari-
              able $i$.

       j1, j2 Specify the first and last integer values for grid domain  vari-
              able $j$.

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$^cdot^DELTA u ~+~ u sub 1$ );
              glEnd();

where

              $ DELTA u ~=~ (u sub 2 ~-~ u sub 1 ) ^/^ n$

and  $n$,  $u  sub  1$,  and  $u  sub  2$ 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 com-
puted from $ i^cdot^DELTA u ~+~ u sub 1$ is exactly $u sub 2$.

In the two-dimensional case, glEvalMesh2, let

       $ DELTA u ~=~ mark ( u sub 2 ~-~ u sub 1 ) ^/^ n$

       $ DELTA v ~=~ lineup ( v sub 2 ~-~ v sub 1 ) ^/^ m$,

where $n$, $u sub 1$, $u sub 2$, $m$, $v sub 1$, and $v sub 2$ are  the  argu-

ments 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$^cdot^DELTA u ~+~ u sub 1$, j$^cdot^DELTA v ~+~ v sub 1$ );
             glEvalCoord2( i$^cdot^DELTA u ~+~ u sub 1$, (j+1)$^cdot^DELTA v ~+~ v sub 1$ );
          }
          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$^cdot^DELTA u ~+~ u sub 1$, j$^cdot^DELTA v ~+~ v sub 1$ );
          glEnd();
       }

       for ( i = i1;  i <= i2; i += 1 ) {
          glBegin( GL_LINE_STRIP );
          for ( j = j1; j <= j1; j += 1 )
             glEvalCoord2( i$^cdot^DELTA u ~+~ u sub 1$, j$^cdot^DELTA v ~+~ v sub 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$^cdot^DELTA u ~+~ u sub 1$, j$^cdot^DELTA v ~+~ v sub 1$ );
       glEnd();

In  all  three  cases,  the  only  absolute  numeric  requirements are that if
$i~=~n$, then the value computed from $i^cdot^DELTA u ~+~ u sub 1$ is  exactly
$u  sub 2$, and if $j~=~m$, then the value computed from $j ^cdot^ DELTA v ~+~
v sub 1$ is exactly $v sub 2$.

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

SEE ALSO

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

                                                                GLEVALMESH(3G)