glEvalMesh

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.   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 con-
       nected 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⋅Δu+u1 );
	      glEnd();
where

	      Δu=(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⋅Δu+u1 is exactly u2.

In the two-dimensional case, glEvalMesh2, let

       Δu=(u2−u1)/n

       Δv=(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⋅Δu+u1, j⋅Δv+v1 );
	     glEvalCoord2( i⋅Δu+u1, (j+1)⋅Δv+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⋅Δu+u1, j⋅Δv+v1 );
	  glEnd();
       }

       for ( i = i1;  i <= i2; i += 1 ) {
	  glBegin( GL_LINE_STRIP );
	  for ( j = j1; j <= j1; j += 1 )
	     glEvalCoord2( i⋅Δu+u1, j⋅Δv+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⋅Δu+u1, j⋅Δv+v1 );
       glEnd();

In all three cases, the only absolute numeric requirements are that  if	 i=n,
then  the  value  computed  from  i⋅Δu+u1 is exactly u2, and if j=m, then the
value computed from j⋅Δv+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)