// 获取bitmap
     Bitmap b = Bitmap.createBitmap(activity.getWindow().getDecorView().getWidth(),
                    activity.getWindow().getDecorView().getHeight(), Bitmap.Config.ARGB_8888);
            Canvas canvas = new Canvas(b);
            activity.getWindow().getDecorView().draw(canvas);

     
      int[] argb = new int[inputWidth * inputHeight];
       // Bitmap 获取 argb
        b.getPixels(argb, 0, inputWidth, 0, 0, inputWidth, inputHeight);
        byte[] yuv = new byte[inputWidth * inputHeight * 3 / 2];
        // argb 8888 转 i420
        conver_argb_to_i420(yuv, argb, inputWidth, inputHeight);

 // argb 8888 转 i420
 public static void conver_argb_to_i420(byte[] i420, int[] argb, int width, int height) {
        final int frameSize = width * height;

        int yIndex = 0;                   // Y start index
        int uIndex = frameSize;           // U statt index
        int vIndex = frameSize*5/4; // V start index: w*h*5/4

        int a, R, G, B, Y, U, V;
        int index = 0;
        for (int j = 0; j < height; j++) {
            for (int i = 0; i < width; i++) {
                a = (argb[index] & 0xff000000) >> 24; //  is not used obviously
                R = (argb[index] & 0xff0000) >> 16;
                G = (argb[index] & 0xff00) >> 8;
                B = (argb[index] & 0xff) >> 0;

                // well known RGB to YUV algorithm
                Y = ( (  66 * R + 129 * G +  25 * B + 128) >> 8) +  16;
                U = ( ( -38 * R -  74 * G + 112 * B + 128) >> 8) + 128;
                V = ( ( 112 * R -  94 * G -  18 * B + 128) >> 8) + 128;

                // I420(YUV420p) -> YYYYYYYY UU VV
                i420[yIndex++] = (byte) ((Y < 0) ? 0 : ((Y > 255) ? 255 : Y));
                if (j % 2 == 0 && i % 2 == 0) {
                    i420[uIndex++] = (byte)((U<0) ? 0 : ((U > 255) ? 255 : U));
                    i420[vIndex++] = (byte)((V<0) ? 0 : ((V > 255) ? 255 : V));
                }
                index ++;
            }
        }
    }