glEvalMesh.3gl

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 · du + u1)
	glEnd();

  where

       du = (u2-u1)/n

  and n, u1, and u2 are	the arguments to the most recent glMapGrid1 command.
  type is GL_POINT 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·du + u1 is exactly u2.

  In the two-dimensional case, glEvalMesh2, let

			 du = (u2-u1)/n

			 dv = (v2-v1)/m 

  where	n, u1, u2, m, v1, and v2 are the arguments to the most recent
  glMapGrid2 comand.  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·du + u1, j·dv + v1);
	  glEvalCoord2(i·du + u1, (j+1)·dv + 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·du + u1, j·dv + v1);
      glEnd(); 
  } 
  for (i = i1; i <= i2; i += 1) {
      glBegin(GL_LINE_STRIP);
      for (j = j1; j <= j1; j += 1)
	  glEvalCoord2(i·du + u1, j·dv + 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·du + u1, j·dv + v1);
      }	
  } 
  glEnd();

  In all three cases, the only absolute	numeric	requirements are that if
  i = n, then the value	computed from i·du + u1	is exactly u2, and if j	= m,
  then the value computed from j·dv + 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 called between a call to
  glBegin and the corresponding	call to	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, glEvalCoord,	glEvalPoint, glMap1, glMap2, glMapGrid

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Last Edited: Fri Dec 6 11:18:03 EST 1996 by AFV
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