vector fields - locarithmic scale

Hello together

Once more I need solutions to my problems. Hope somebody can help me.

1. I want to visualize a wind vector field. As range samples I have got an
array containing the numeric values for direction (degree) and speed
(m/s), which I put into a FlatField object. (As usual the wind speed is
represented by the vector length). As is done from
BarbRendererJ2D.main() I've mapped the direction RealType to
Display.Flow1Azimuth and the speed RealType to
Display.Flow1Radial. Unfortunately I' m getting no result but the
following NullPointerException ..               

        at visad.ShadowType.assembleFlow(Compiled Code)
        at visad.java2d.ShadowTypeJ2D.assembleFlow(Compiled Code)
        at visad.ShadowFunctionOrSetType.doTransform(Compiled Code)
        at visad.java2d.ShadowFunctionOrSetType.doTransform(Compiled Code)
        at visad.java2d.DefaultRendererJ2D.doTransform(Compiled Code)
        at visad.java2d.RendererJ2D.doAction(Compiled Code)
        at visad.DisplayImpl.doAction(Compiled Code)
        at visad.java2d.DisplayImplJ2D.doAction(Compiled Code)
        at Code)
        at visad.util.ThreadPool$ Code)

I've checked my code several times but could not find the fault.

2. For each cell a vector object should be rendered. The problem is that
depending on spatial resolution or "point of view" position the vector
objects overlap. On the basis of this facts I want to implement
something like an "LoD" method. Depending on how close I've zoomed to the 
scene, only parts of the vector objects should be displayed. (For example
vectors could be rendered as averaged values of the neighbour
vectors). Has someone an idea how to do this ? And where is
the zooming-"stuff" defined.

3. How can I map RealType components to LOCARITHMIC scaled axis ? 

Thank you for your help