glEvalMesh1, glEvalMesh2

  • Article

The glEvalMesh1 and glEvalMesh2 functions compute a one- or two-dimensional grid of points or lines.

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

Parameters

  • mode
    In glEvalMesh1, specifies whether to compute a one-dimensional mesh of points or lines. The following symbolic constants are accepted: GL_POINT and GL_LINE.
  • i1, i2
    The first and last integer values for grid domain variable i.
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. The following symbolic constants are accepted: GL_POINT, GL_LINE, and GL_FILL.
  • i1, i2
    The first and last integer values for grid domain variable i.
  • j1, j2
    The first and last integer values for grid domain variable j.

Remarks

Use glMapGrid and glEvalMesh in tandem to efficiently generate and evaluate a series of evenly spaced map domain values. The glEvalMesh function 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. The mode parameter 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&Delta;u + u1)

}

glEnd( );

where

&Delta;u = (u2 u1) / n

and n, u1, and u2 are the arguments to the most recent glMapGrid1 function. The type parameter 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&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 function. Then, if mode is GL_FILL, glEvalMesh2 is equivalent to:

for (j = j1; j < j2; j += 1)

{

glBegin(GL_QUAD_STRIP);

for (i = i1; i <= i2; i += 1)

{

glEvalCoord2(i&Delta; u + u1 ( ) , j &Delta; v + v1);

glEvalCoord2(i&Delta; u + u1 ( ) , (j+1) &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&Delta; u + u1, j&Delta; v + v1);

}

glEnd( );

}

for (i = i1; i <= i2; i += 1)

{

glBegin(GL_LINE_STRIP);

for (j = j1; j <= j1; j += 1)

{

glEvalCoord2(i&Delta; u + u1, j&Delta; 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&Delta; u + u1, j&Delta; v + v1);

}

}

glEnd( );

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

The following functions retrieve information relating to glEvalMesh:

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

Error Codes

The following are the error codes generated and their conditions.

Error code Condition
GL_INVALID_ENUM mode was not an accepted value.
GL_INVALID_OPERATION glEvalMesh was called between a call to glBegin and the corresponding call to glEnd.

Requirements

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Gl.h.
**  Library:** Use Opengl32.lib.

See Also

glBegin, glEnd, glEvalCoord, glEvalPoint, glMap1, glMap2, glMapGrid