private void trackPixelUsage(IndexGif89Frame igf) throws IOException
      {   
        byte[] ci_pixels = (byte[]) igf.getPixelSource(); 
        int npixels = ci_pixels.length;   
        for (int i = 0; i < npixels; ++i)
          if (ci_pixels[i] >= ciCount)
            ciCount = ci_pixels[i] + 1;
      }