glEvalMesh

C Specification

void glEvalMesh1(
    GLenum	 mode,
    GLint	 i1,
    GLint	 i2)
void glEvalMesh2(
    GLenum	 mode,
    GLint	 i1,
    GLint	 i2,
    GLint	 j1,
    GLint	 j2)

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.

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

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·u + u1)
  glEnd();

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·u + u₁ is exactly u₂.

In the two-dimensional case, glEvalMesh2, let

u = (u₂ u₁) / n
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·u + u1, j·v + v1);
      glEvalCoord2(i·u + u1, (j + 1)·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·u + u1, j·v + v1);
    glEnd();
  }
  for (i = i1;  i  i2; i += 1) {
    glBegin(GL_LINE_STRIP);
    for (j = j1; j  j1; j += 1)
      glEvalCoord2(i·u + u1, j·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·u + u1,  j ·v + v1);
    }
  }
  glEnd();

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

See Also