The demo is an android project which covert camera preview data in NV21 format to I420 by libyuv.
The test.yuv file in yuv420p format will be generated under /sdcard/ path:
private static final String FILE_NAME = Environment.getExternalStorageDirectory().getAbsolutePath() + "/test.yuv";
You can use ffplay to play the test.yuv by the following command:
ffplay -v info -f rawvideo -pixel_format yuv420p -video_size 640x480 test.yuv