how to fill in pixels

I have just started using VisAD and after coding
up my first application, have run into two problems.

1) The image shows up as a set of thin lines, i.e. the pixels
are not filled in.  Perhaps I am using the wrong rendering
or data type ... ?
  My math type is:
     (radial, gate) --> (x,y,z, value)
 and I am displaying using the java 3D display
 with x as Display.XAxis, etc. and value as Display.RGBA

 I also tried using the FlowSpherical coordinate transform
 instead of explicitly providing x,y,z but I get the same result.
 I would like a textured image ... (I did the setTextureEnable
 without any effect).

2) I saw on the list that Don Murray suggested using a set of numbers
and explicitly assigning colors to get discrete colors for a
continuous field.  The problem is that my data are floating point 
values, and so I can not really come up with exact
numbers for a color table. 
So, I used a FlatField and the setFunction method.
How do I tell the FlatField that the data should be
NEAREST_NEIGHBOR and not WEIGHTED_AVERAGE?
 

thanks
Lak