1991-2006 Silicon Graphics, Inc.
glEvalMesh 3G
glEvalMesh 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
    			      ·
    			      Δ
    		              u

    		          +

    		              u
    		              1

    		   );
    glEnd();

where

Δ u = u 2 - u 1 n

and n, u 1 , and u 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 computed from i · Δ u + u 1 is exactly u 2 .

In the two-dimensional case, `glEvalMesh2`, let .cp

Δ u = u 2 - u 1 n

Δ v = v 2 - v 1 m

where n, u 1 , u 2 , m, v 1 , and v 2 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
    				·
    			        Δ
    		                u

    		            +

    		                u
    		                1

    			    ,

    			        j
    				·
    			        Δ
    		                v

    		            +

    		                v
    		                1

    		     );
          glEvalCoord2(

    			        i
    				·
    			        Δ
    		                u

    		            +

    		                u
    		                1

    			    ,

    			                    j
    				            +
    			                    1

    				·
    			        Δ
    		                v

    		            +

    		                v
    		                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
    				·
    			        Δ
    		                u

    		            +

    		                u
    		                1

    			    ,

    			        j
    				·
    			        Δ
    		                v

    		            +

    		                v
    		                1

    		     );
       glEnd();
    }

    for ( i = i1;  i <= i2; i += 1 ) {
       glBegin( GL_LINE_STRIP );
       for ( j = j1; j <= j1; j += 1 )
          glEvalCoord2(

    			        i
    				·
    			        Δ
    		                u

    		            +

    		                u
    		                1

    			    ,

    			        j
    				·
    			        Δ
    		                v

    		            +

    		                v
    		                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
    				·
    			        Δ
    		                u

    		            +

    		                u
    		                1

    			    ,

    			        j
    				·
    			        Δ
    		                v

    		            +

    		                v
    		                1

    		     );
    glEnd();

In all three cases, the only absolute numeric requirements are that if i = n , then the value computed from i · Δ u + u 1 is exactly u 2 , and if j = m , then the value computed from j · Δ v + v 1 is exactly v 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, glEvalCoord, glEvalPoint, glMap1, glMap2, glMapGrid

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.