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Re: Elevation Grid

Neil Woodhouse wrote:
> 
> Hi,
> 
> I was glad to see that someone finally proposed a working group
> on Geographical & Cartographic information. As a Geologist i am
> interested in the representation of geographical information in 
> 3D in the first instance as a teaching aid.

hear hear

> So can i kick off a little discussion and ask what is the members
>  opinion of VRML spec 6.17-Elevation Grid; can this be improved.

Nothing.  The VRML 97 spec is already approved by ISO.

Uh, just kidding!  Improvements are always possible.  What did you
have in mind?  


http://www.vrml.org/Specifications/VRML97/part1/nodesRef.html#ElevationGrid

6.17 ElevationGrid

ElevationGrid { 
  eventIn      MFFloat  set_height
  exposedField SFNode   color             NULL
  exposedField SFNode   normal            NULL
  exposedField SFNode   texCoord          NULL
  field        MFFloat  height            []      # (-,)
  field        SFBool   ccw               TRUE
  field        SFBool   colorPerVertex    TRUE
  field        SFFloat  creaseAngle       0       # [0,]
  field        SFBool   normalPerVertex   TRUE
  field        SFBool   solid             TRUE
  field        SFInt32  xDimension        0       # [0,)
  field        SFFloat  xSpacing          1.0     # (0,)
  field        SFInt32  zDimension        0       # [0,)
  field        SFFloat  zSpacing          1.0     # (0,)
}

The ElevationGrid node specifies a uniform rectangular 
grid of varying height in the Y=0 plane of the local
coordinate system. The geometry is described by a scalar 
array of height values that specify the height of a
surface above each point of the grid.

The xDimension and zDimension fields indicate the number 
of elements of the grid height array in the X and Z
directions. Both xDimension and zDimension shall be 
greater than or equal to zero. If either the xDimension or
the zDimension is less than two, the ElevationGrid contains 
no quadrilaterals. The vertex locations for the
rectangles are defined by the height field and the xSpacing 
and zSpacing fields:

     The height field is an xDimension by zDimension array 
     of scalar values representing the height above the
     grid for each vertex. 
     The xSpacing and zSpacing fields indicate the distance 
     between vertices in the X and Z directions
     respectively, and shall be greater than zero. 

Thus, the vertex corresponding to the point P[i, j] on the grid is 
placed at:

    P[i,j].x = xSpacing × i
    P[i,j].y = height[ i + j × xDimension]
    P[i,j].z = zSpacing × j

    where 0 <= i < xDimension and 0 <= j < zDimension,
    and P[0,0] is height[0] units above/below the origin of the local
    coordinate system

The set_height eventIn allows the height MFFloat field to be 
changed to support animated ElevationGrid nodes.

The color field specifies per-vertex or per-quadrilateral
colours for the ElevationGrid node depending on the
value of colorPerVertex. If the color field is NULL, 
the ElevationGrid node is rendered with the overall attributes
of the Shape node enclosing the ElevationGrid node
(see 4.14, Lighting model).

The colorPerVertex field determines whether colours 
specified in the color field are applied to each vertex or
each quadrilateral of the ElevationGrid node. If 
colorPerVertex is FALSE and the color field is not NULL, the
color field shall specify a Color node containing at 
least (xDimension-1)×(zDimension-1) colours; one for each
quadrilateral, ordered as follows:

    QuadColor[i,j] = Color[ i + j × (xDimension-1)]

    where 0 <= i < xDimension-1 and 0 <= j < zDimension-1,
    and QuadColor[i,j] is the colour for the quadrilateral defined
        by height[i+j×xDimension], height[(i+1)+j×xDimension],
        height[(i+1)+(j+1)×xDimension] and height[i+(j+1)×xDimension]

If colorPerVertex is TRUE and the color field is not NULL, 
the color field shall specify a Color node containing
at least xDimension × zDimension colours, one for each vertex, 
ordered as follows:

    VertexColor[i,j] = Color[ i + j × xDimension]

    where 0 <= i < xDimension and 0 <= j < zDimension,
    and VertexColor[i,j] is the colour for the vertex defined by
        height[i+j×xDimension]

The normal field specifies per-vertex or per-quadrilateral normals 
for the ElevationGrid node. If the normal field
is NULL, the browser shall automatically generate normals, 
using the creaseAngle field to determine if and how
normals are smoothed across the surface (see 4.6.3.5, 
Crease angle field).

The normalPerVertex field determines whether normals are 
applied to each vertex or each quadrilateral of the
ElevationGrid node depending on the value of normalPerVertex. 
If normalPerVertex is FALSE and the normal
node is not NULL, the normal field shall specify a 
Normal node containing at least
(xDimension-1)×(zDimension-1) normals; one for each 
quadrilateral, ordered as follows:

    QuadNormal[i,j] = Normal[ i + j × (xDimension-1)]

    where 0 <= i < xDimension-1 and 0 <= j < zDimension-1,
    and QuadNormal[i,j] is the normal for the quadrilateral defined
        by height[i+j×xDimension], height[(i+1)+j×xDimension],
        height[(i+1)+(j+1)×xDimension] and height[i+(j+1)×xDimension]

If normalPerVertex is TRUE and the normal field is not NULL, 
the normal field shall specify a Normal node
containing at least xDimension × zDimension normals; one 
for each vertex, ordered as follows:

    VertexNormal[i,j] = Normal[ i + j × xDimension]

    where 0 <= i < xDimension and 0 <= j < zDimension,
    and VertexNormal[i,j] is the normal for the vertex defined
        by height[i+j×xDimension]

The texCoord field specifies per-vertex texture coordinates 
for the ElevationGrid node. If texCoord is NULL,
default texture coordinates are applied to the geometry. 
The default texture coordinates range from (0,0) at the
first vertex to (1,1) at the last vertex. The S texture 
coordinate is aligned with the positive X-axis, and the T
texture coordinate with positive Z-axis. If texCoord is 
not NULL, it shall specify a TextureCoordinate node
containing at least (xDimension)×(zDimension) texture 
coordinates; one for each vertex, ordered as follows:

    VertexTexCoord[i,j] = TextureCoordinate[ i + j × xDimension]

    where 0 <= i < xDimension and 0 <= j < zDimension,
    and VertexTexCoord[i,j] is the texture coordinate for the vertex
        defined by height[i+j×xDimension]

The ccw, solid, and creaseAngle fields are described in 
4.6.3, Shapes and geometry.

By default, the quadrilaterals are defined with a 
counterclockwise ordering. Hence, the Y-component of the
normal is positive. Setting the ccw field to FALSE 
reverses the normal direction. Backface culling is enabled
when the solid field is TRUE.

See Figure 6.5 for a depiction of the ElevationGrid node.


all the best, Don
-- 
Don Brutzman  Naval Postgraduate School, Code UW/Br Root 200  work 408.656.2149
              Monterey California 93943-5000 USA              fax  408.656.3679
Virtual worlds/underwater robots/Internet http://www.stl.nps.navy.mil/~brutzman

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