internal static void HollowEllipsoidCallback ( Player player, Position[] marks, object tag ) {
    byte drawBlock = (byte)tag;
    if( drawBlock == (byte)Block.Undefined ) {
        drawBlock = (byte)player.lastUsedBlockType;
    }

    // find start/end coordinates
    int sx = Math.Min( marks[0].x, marks[1].x );
    int ex = Math.Max( marks[0].x, marks[1].x );
    int sy = Math.Min( marks[0].y, marks[1].y );
    int ey = Math.Max( marks[0].y, marks[1].y );
    int sh = Math.Min( marks[0].h, marks[1].h );
    int eh = Math.Max( marks[0].h, marks[1].h );

    // find axis lengths
    double rx = (ex - sx + 1) / 2d;
    double ry = (ey - sy + 1) / 2d;
    double rh = (eh - sh + 1) / 2d;

    double rx2 = 1 / (rx * rx);
    double ry2 = 1 / (ry * ry);
    double rh2 = 1 / (rh * rh);

    // find center points
    double cx = (ex + sx) / 2d;
    double cy = (ey + sy) / 2d;
    double ch = (eh + sh) / 2d;

    // rougher estimation than the non-hollow form, a voxelized surface is a bit funky
    int volume = (int)(4 / 3d * Math.PI * ((rx+.5)*(ry+.5)*(rh+.5) - (rx-.5)*(ry-.5)*(rh-.5)) * 0.85);
    if( !player.CanDraw( volume ) ) {
        player.MessageNow( "You are only allowed to run draw commands that affect up to {0} blocks. This one would affect {1} blocks.",
                           player.info.rank.DrawLimit,
                           volume );
        return;
    }

    player.undoBuffer.Clear();

    int blocks = 0, blocksDenied = 0;
    bool cannotUndo = false;

    for( int x = sx; x <= ex; x++ ) {
        for( int y = sy; y <= ey; y ++ ) {
            for( int h = sh; h <= eh; h++ ) {

                double dx = (x - cx);
                double dy = (y - cy);
                double dh = (h - ch);

                if( (dx*dx)*rx2 + (dy*dy)*ry2 + (dh*dh)*rh2 <= 1 ) {
                    // we touched the surface
                    // keep drilling until we hit an internal block
                    do {
                        DrawOneBlock( player, drawBlock, x, y, h, ref blocks, ref blocksDenied, ref cannotUndo );
                        DrawOneBlock( player, drawBlock, x, y, (int)(ch - dh), ref blocks, ref blocksDenied, ref cannotUndo );
                        dh = (++h - ch);
                    } while( h <= (int)ch &&
                            ((dx+1)*(dx+1) *rx2 + (dy*dy)       *ry2  + (dh*dh)       *rh2 > 1 ||
                             (dx-1)*(dx-1) *rx2 + (dy*dy)       *ry2  + (dh*dh)       *rh2 > 1 ||
                             (dx*dx)       *rx2 + (dy+1)*(dy+1) *ry2  + (dh*dh)       *rh2 > 1 ||
                             (dx*dx)       *rx2 + (dy-1)*(dy-1) *ry2  + (dh*dh)       *rh2 > 1 ||
                             (dx*dx)       *rx2 + (dy*dy)       *ry2  + (dh+1)*(dh+1) *rh2 > 1 ||
                             (dx*dx)       *rx2 + (dy*dy)       *ry2  + (dh-1)*(dh-1) *rh2 > 1) );
                    break;
                }
            }
        }
    }
    Logger.Log( "{0} drew a hollow ellipsoid containing {1} blocks of type {2} (on world {3})", LogType.UserActivity,
                player.name,
                blocks,
                (Block)drawBlock,
                player.world.name );
    DrawingFinished( player, "drawn", blocks, blocksDenied );
}