I'm using this system to extract images from a video stream so I can add the images to my live tweets, without needing to also hold a camera.
The ffmpeg command extracts i-frames so I should be able to capture slide changes, etc.
My setup is an old iPhone 4 on a tripod plus a zoom lens, running Larix Broadcaster - a free app with no restrictions on broadcasting time. You don't need to use an iPhone; you just need a device that can push a video stream to an rtmp url; a Raspberry Pi with a camera would work just as well.
The Larix app sends an rtmp stream to an instance of nginx running on my laptop that's been compiled with the nginx rtmp streaming module. The nginx config is really basic and just rebroadcasts this stream.
If you want to try this yourself, use my build-nginx script to get a custom nginx setup:
build-nginx -m https://github.com/arut/nginx-rtmp-module.git@v1.2.0
I then point ffmpeg at the nginx stream and it extracts the images into a folder named after the speaker; all this is done through a simple Makefile.
The last component is the networking. I need the the iPhone to stream nicely to my laptop, and I don't want to rely on the venue wifi, and I can't use mobile data, because the rtmp stream needs to connect to my laptop's IP, so instead I've got a Netgear N300 wifi range extender which both my laptop and the iPhone 4 connect to.
I can configure the Netgear to use the venue wifi or use another phone's hotspot for the actual internet connection.
You'll need to know what RTMP URL to stream your video to. On the laptop where you'll be running nginx, use the Makefile to show it to you:
make url
or if you know which network adapter you want to use:
ETH=eth1 make url
This will output a url like rtmp://10.10.10.1/live/tweets enter this into the Larix app (or whatever streaming video capture software you're using).
Now you know the url, and presuming that you've started nginx and are now streaming video to it from your camera or device, you'll want to start grabbing frames.
Just call:
make
or again, if you're using a different adapter,
ETH=eth1 make
This will create a folder called something like stills/1510321049 where the numerical directory is just the epoch seconds timestamp for when you ran the command.
If you'd like a nicer name, call it like this:
SPEAKER=jay make
This will instead make a folder called stills/jay containing the video stills.