$ gcc v4l2grab.c -o v4l2grab -Wall -ljpeg -lv4l2 -DIO_READ -DIO_MMAP -DIO_USERPTR
/tmp/user/1000/ccIvliiI.o: In function `imageProcess':
v4l2grab.c:(.text+0x374): undefined reference to `YUV420toYUV444'
collect2: error: ld returned 1 exit status
$ apt list --installed | grep v4l
libv4l-0/bionic,now 1.14.2-1 amd64 [installed]
libv4l-dev/bionic,now 1.14.2-1 amd64 [installed]
libv4l2rds0/bionic,now 1.14.2-1 amd64 [installed]
libv4lconvert0/bionic,now 1.14.2-1 amd64 [installed]
v4l-conf/bionic,now 3.103-4build1 amd64 [installed]
v4l-utils/bionic,now 1.14.2-1 amd64 [installed]
v4l2loopback-dkms/bionic,bionic,now 0.10.0-1ubuntu1 all [installed]
v4l2loopback-source/bionic,bionic,now 0.10.0-1ubuntu1 all [installed]
v4l2loopback-utils/bionic,now 0.10.0-1ubuntu1 amd64 [installed]
v4l2ucp/bionic,now 2.0.2-4build1 amd64 [installed]