private static float calcScreenArea(BoundingBox bound, float distance, float screenWidth) {
        // Calc as if we are a BoundingSphere for now...
        float radiusSquare = bound.getXExtent() * bound.getXExtent()
                         + bound.getYExtent() * bound.getYExtent()
                         + bound.getZExtent() * bound.getZExtent();
        return ((radiusSquare * screenWidth * screenWidth) / (distance * distance * 4)) * FastMath.PI;
    }
Estimate the screen area of a bounding volume. If the volume isn't a
BoundingSphere, BoundingBox, or OrientedBoundingBox, 0 is returned.

@param bound The bounds to calculate the volume from.
@param distance The distance from camera to object.
@param screenWidth The width of the screen.
@return The area in pixels on the screen of the bounding volume.
jme3-core/src/main/java/com/jme3/util/AreaUtils.java