private int computeColorDepth(int colorcount) {
		/* color depth = log-base-2 of maximum number of simultaneous colors, i.e. */
		/* bits per color-index pixel     */
		if (colorcount <= 2)
			return 1;
		if (colorcount <= 4)
			return 2;
		if (colorcount <= 16)
			return 4;
		return 8;
	}