Starting ffplay X-window without a window manager

I want what is essentially a kiosk, to display a video stream using ffplay, without a whole Desktop environment. I started with minimal CentOS7, and installed xterm and X11, per this simple guide: https://linuxconfig.org/how-to-run-x-applications-without-a-desktop-or-a-wm I've created a .xinitrc file:

#!/bin/bash
exec firefox

and when I execute startx, it opens an X11 window containing Firefox. But, when I replace the .xinitrc file with this one:

#!/bin/bash
exec ffplay udp://192.168.0.237:5444

I just get a blank screen, in spite of knowing that ffplay command is right. Though I can tell from ps -ef | grep ffplay that ffplay is indeed executing. I think it is just piping the output video someplace different from where Firefox did. I don't know how to tell where that is, nor how to force it to go to localhost:0.0. Note the CentOS7 box is a Hyper-V VM, which I am accessing via a Hyper-V console.