Hi,

I need a plane at an altitude say h from earth’s surface. I am going to use this to clip a sphere. In normal cartesian coordination system, I am able to visualize it. But when it comes to a globe, I have few doubts:

1.) The units that i follow is in meters, so I draw a plane that is at a height of (earth’s radius + h) meters from earth’s core(0,0,0). Is this correct? or some conversion is required.

2.) I fail to understand whether the axes of globe(x,y,z) are fixed or not, i mean if they are fixed then let say z axis is normal at longitude 0 degrees, then if I am standing at lat lon (32.5, 75.2) then how can the equation of the plane tangent to the surface at this location be z=radius of earth at (32.5, 75.2). It should be in terms of x,y,z and not z alone. isn’t it?

To my help my api provides the conversion from latlon to cartesian coords(Vec4) as required by openGL.

```
Vec4 centerPoint = this.computePointFromPosition(dc,
this.location.getLatitude(), this.location.getLongitude(), altitudes[0], terrainConformant[0]);
```

I create a plane at this location:

```
DoubleBuffer eqn1 = BufferUtils.createDoubleBuffer(8).put(new double[] {centerPoint.getX(), centerPoint.getY(), centerPoint.getZ(), centerPoint.getW()}); //Some thing wrong with this Please check if this equation is correct
eqn1.flip();
gl.glClipPlane(GL.GL_CLIP_PLANE0, eqn1);
gl.glEnable(GL.GL_CLIP_PLANE0);
```

Here is the code, please check the equation of plane and let me know if some modification is required:

```
@Override
public void drawSphere(DrawContext dc)
{
double[] altitudes = this.getAltitudes(dc.getVerticalExaggeration());
boolean[] terrainConformant = this.isTerrainConforming();
int subdivisions = this.getSubdivisions();
if (this.isEnableLevelOfDetail())
{
DetailLevel level = this.computeDetailLevel(dc);
Object o = level.getValue(SUBDIVISIONS);
if (o != null && o instanceof Integer)
subdivisions = (Integer) o;
}
Vec4 centerPoint = this.computePointFromPosition(dc,
this.location.getLatitude(), this.location.getLongitude(), altitudes[0], terrainConformant[0]);
Matrix modelview = dc.getView().getModelviewMatrix();
modelview = modelview.multiply(Matrix.fromTranslation(centerPoint));
modelview = modelview.multiply(Matrix.fromScale(this.getRadius()));
double[] matrixArray = new double[16];
modelview.toArray(matrixArray, 0, false);
this.setExpiryTime(-1L); // Sphere geometry never expires.
GL gl = dc.getGL(); // GL initialization checks for GL2 compatibility.
gl.glPushAttrib(GL.GL_POLYGON_BIT | GL.GL_TRANSFORM_BIT);
try
{
gl.glEnable(GL.GL_CULL_FACE);
gl.glFrontFace(GL.GL_CCW);
// Were applying a scale transform on the modelview matrix, so the normal vectors must be re-normalized
// before lighting is computed. In this case we're scaling by a constant factor, so GL_RESCALE_NORMAL
// is sufficient and potentially less expensive than GL_NORMALIZE (or computing unique normal vectors
// for each value of radius). GL_RESCALE_NORMAL was introduced in OpenGL version 1.2.
gl.glEnable(GL.GL_RESCALE_NORMAL);
gl.glMatrixMode(GL.GL_MODELVIEW);
//clipping
//double eRad = canvas.getModel().getGlobe().getRadiusAt(location);//added this to clipping height to come to earth's surface from core, please check this
DoubleBuffer eqn1 = BufferUtils.createDoubleBuffer(8).put(new double[] {centerPoint.getX(), centerPoint.getY(), centerPoint.getZ(), centerPoint.getW()}); //Some thing wrong with this Please check if this equation is correct
eqn1.flip();
gl.glClipPlane(GL.GL_CLIP_PLANE0, eqn1);
gl.glEnable(GL.GL_CLIP_PLANE0);
gl.glPushMatrix();
try
{
gl.glLoadMatrixd(matrixArray, 0);
this.drawUnitSphere(dc, subdivisions);
}
finally
{
gl.glPopMatrix();
}
}
finally
{
gl.glPopAttrib();
}
}
@Override
public void drawUnitSphere(DrawContext dc, int subdivisions)
{
Object cacheKey = new Geometry.CacheKey(this.getClass(), "Sphere", subdivisions);
Geometry geom = (Geometry) this.getGeometryCache().getObject(cacheKey);
if (geom == null || this.isExpired(dc, geom))
{
if (geom == null)
geom = new Geometry();
this.makeSphere(1.0, subdivisions, geom);
this.updateExpiryCriteria(dc, geom);
this.getGeometryCache().add(cacheKey, geom);
}
this.getRenderer().drawGeometry(dc, geom);
}
@Override
public void makeSphere(double radius, int subdivisions, Geometry dest)
{
GeometryBuilder gb = this.getGeometryBuilder();
gb.setOrientation(GeometryBuilder.OUTSIDE);
GeometryBuilder.IndexedTriangleArray ita = gb.tessellateSphere((float) radius, subdivisions);
float[] normalArray = new float[3 * ita.getVertexCount()];
gb.makeIndexedTriangleArrayNormals(ita, normalArray);
dest.setElementData(GL.GL_TRIANGLES, ita.getIndexCount(), ita.getIndices());
dest.setVertexData(ita.getVertexCount(), ita.getVertices());
dest.setNormalData(ita.getVertexCount(), normalArray);
}
```