i420 转 nv21

  private static byte[] I420ToNv21(byte[] i420bytes, int width, int height) {
    byte[] nv21bytes = new byte[i420bytes.length];
    int y_len = width * height;
    int uv_len = y_len / 4;
    System.arraycopy(i420bytes, 0, nv21bytes, 0, y_len);
    for (int i =0; i < uv_len; i++) {
      byte u = i420bytes[y_len + i];
      byte v = i420bytes[y_len + uv_len + i];
      nv21bytes[y_len + i*2] = v;
      nv21bytes[y_len + i*2 +1] = u;
    }
    return nv21bytes;
  }