取ARGB颜色的分度值

        private static int getR32(int c) { return (c >>  0) & 0xFF; }
        // access the red component from a premultiplied color
        private static int getG32(int c) { return (c >>  8) & 0xFF; }
        // access the red component from a premultiplied color
        private static int getB32(int c) { return (c >> 16) & 0xFF; }
        // access the red component from a premultiplied color
        private static int getA32(int c) { return (c >> 24) & 0xFF; }