private void trackPixelUsage(IndexGif89Frame igf) {
		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;
	}