Why do I get a grey display ?

Hi

I have two datasets, one is 350x700 another is 900x1500.
I use the alomost same VisAD code to display them in Lat-Lon 2D domain,
except the length of data samples is different,


      display = new DisplayImplJ3D("display", new
TwoDDisplayRendererJ3D());

      latMap = new ScalarMap(RealType.Latitude, Display.YAxis);
      lonMap = new ScalarMap(RealType.Longitude, Display.XAxis);
      display.addMap(latMap);
      display.addMap(lonMap);

      imageValue = RealType.getRealType("image");
      rgbMap = new ScalarMap(imageValue, Display.RGB);
      display.addMap(rgbMap);

      imageRef = new DataReferenceImpl("image");
      display.addReference(imageRef);

      .........

I could get a color display for dataset with less data samples but a
grey display for the large dataset.

The interest thing is:
when I click on the display (since  PointManipulationRendererJ3D has
been added to the above display), I could get the correct lat, lon and
data value at the point I have clicked.

Does anyone know why I end up with a grey display for the larger
dataset?

By the way, I am on SGI IRIX with
2  600 MHZ IP30 Processors
CPU: MIPS R14000 Processor Chip Revision: 2.4
FPU: MIPS R10010 Floating Point Chip Revision: 0.0
Main memory size: 1536 Mbytes

java version "1.3.1"
Java(TM) 2 Runtime Environment, Standard Edition (build
1.3.1-mtibuild-011130-22:40)
Classic VM (build 1.3.1-mtibuild-011130-22:40, native threads, mipsjit)

Java 3D 1.2.1 from SGI

Thanks for your great  help.