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20040810: IDV - linux - VisAD Arrays

>From: "Stuart Wier" <address@hidden>
>Organization: UNAVCO
>Keywords: 200408101823.i7AIN75R027284  VisAD shapes

Hi Stu-

>Institution: UNAVCO
>Package Version: 1.1b3
>Operating System: linux
>Hardware Information: intel
>Inquiry: In IDV class ShapeUtility (see code below) for defining a cube shape 
> you say the values of "coordinates" are "xyz coordinates of the vertices."  A
>  cube has 8 vertices, but the array has 144 numbers in it. Even if the vertic
> es apply more than once, ie 4 vertices on each of 6 sides, there are only 24 
> vertices and 72 xyz values.  What is the defining rule here?  Does vertex ord
> er matter? Likewise I can't figure out the normals; there are a lot of them a
> nd what is the association with faces or vertices?

Every two consecutive (x,y,z) pairs defines one side of one face of the 
cube.  That give 2(points)x4(sides)x3(values (xyz)/point) = 24 values for each 
face and for 6 faces, that makes 144 numbers.  This could probably be done with 
fewer points - I forget where the code came from.  You could play around to 
see if it works with one xyz point per vertex.

>
>
>else if (s.equals(CUBE)) {
>            shape             = new VisADQuadArray();
>            shape.coordinates = new float[] {
>                1.0f, 1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f,
>                -1.0f, -1.0f, -1.0f, -1.0f, -1.0f, -1.0f, -1.0f, 1.0f, -1.0f,
>                -1.0f, 1.0f, -1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
>                -1.0f, 1.0f, 1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, -1.0f,
>                -1.0f, 1.0f, -1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 1.0f, 1.0f, 1.0f,
>                1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 1.0f, -1.0f,
>                1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, -1.0f, 1.0f,
>                1.0f, -1.0f, 1.0f, 1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 1.0f, -1.0f,
>                1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, -1.0f, -1.0f, -1.0f,
>                -1.0f, -1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, -1.0f,
>                1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 1.0f,
>                -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f,
>                1.0f, -1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, -1.0f, 1.0f,
>                1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, -1.0f, -1.0f, -1.0f,
>                -1.0f, -1.0f, -1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f,
>                1.0f, -1.0f, 1.0f
>            };

The normals are perpendicular to each face.  So, for the first 24 values
the normal is the -1 Z axis, etc.

>            shape.normals = new float[144];
>            for (int i = 0; i < 24; i += 3) {
>                shape.normals[i]       = 0.0f;
>                shape.normals[i + 1]   = 0.0f;
>                shape.normals[i + 2]   = -1.0f;
>
>                shape.normals[i + 24]  = 0.0f;
>                shape.normals[i + 25]  = 0.0f;
>                shape.normals[i + 26]  = 1.0f;
>
>                shape.normals[i + 48]  = 1.0f;
>                shape.normals[i + 49]  = 0.0f;
>                shape.normals[i + 50]  = 0.0f;
>
>                shape.normals[i + 72]  = -1.0f;
>                shape.normals[i + 73]  = 0.0f;
>                shape.normals[i + 74]  = 0.0f;
>
>                shape.normals[i + 96]  = 0.0f;
>                shape.normals[i + 97]  = 1.0f;
>                shape.normals[i + 98]  = 0.0f;

Hope that helps.  Like I said, I just drew a picture and plotted
the first few points to figure out what was what.  I don't claim
to be an expert on 3D geometry. ;-)

Don Murray
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